Science

Atomic structure

Atoms consist of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] surrounding a nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] that contains protons [protons: Sub-atomic particles with a positive charge and a relative mass of 1. ] and neutrons [neutrons: Uncharged sub-atomic particles, with a mass of 1 relative to a proton. ].

Neutrons are neutral, but protons and electrons are electrically charged. Protons have a relative charge of +1, while electrons have a relative charge of -1.

The number of protons in an atom is called its atomic number. In the periodic table atoms are arranged in atomic number order.

Electrons are arranged in energy levels or shells, and different energy levels can hold different numbers of electrons. The electronic structure of an atom is a description of how the electrons are arranged, which can be shown in a diagram or by numbers. There is a link between the position of an element in the periodic table and its electronic structure.

The structure of an atomAlthough the word 'atom' comes from the Greek for indivisible, we now know that atoms are not the smallest particles of matter. Atoms are made from smaller subatomic particles.

At the centre of an atom is a nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] containing protons [protons: Sub-atomic particles with a positive charge and a relative mass of 1. ] and neutrons [neutrons: Uncharged sub-atomic particles, with a mass of 1 relative to a proton. ]. Electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] are arranged around the nucleus in energy levels or shells. Make sure you can label a simple diagram of an atom like this one.

Both protons and electrons have an electrical charge. Both have the same size of electrical charge, but the proton is positive and the electron negative. The neutron is neutral.

The electrical charge of particlesparticle relative chargeproton +1neutron 0electron-1The total number of electrons in an atom is always the same as the number of protons in the nucleus. This means atoms have no overall electrical charge.

The number of protons in an atom is called its atomic number [atomic number: The number of protons in the nucleus of an atom. Also called the proton number. ] - also called the proton number. Atoms are arranged in the periodic table in order of increasing atomic number. You

may need to re-visit the section in AQA GCSE Science on the periodic table to check you recall it.

Energy levels and shellsElectrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] are arranged in different shells around the nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ]. The innermost shell - or lowest energy level - is filled first. Each succeeding shell can only hold a certain number of electrons before it becomes full. The innermost shell can hold a maximum of two electrons, the second shell a maximum of eight, and so on. The table gives the maximum capacity of the first four shells - which is as much as you need to know at GCSE.

Maximum capacity of the first four shellsenergy level or shellmaximum number of electronsfirst 2second 8third 8fourth 18A lithium atom, for example, has three electrons. Two are in the first energy level, and one in the second.

A carbon atom has six electrons. Two are in the first energy level, and four in the second energy level.

Arrangement of electrons in a lithium atom

Arrangement of electrons in a carbon atom

Electronic structure 1The electronic structure of an atom is a description of how the electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] are arranged. For your exam, you need to be able to describe the electronic structure of the first 20 elements in the periodic table. You may need to re-visit the section in AQA GCSE Science on the periodic table for this.

The first 20 elements in the periodic table run from hydrogen to calcium. Their electronic structures can be shown either as diagrams or numbers. You need to know how to do both.

Take lithium, for example. The drawing shows each energy level as a circle around the nucleus, with each electron represented by a dot. In the exam, do not worry about colouring in the electrons. Just make them clear and ensure they are in the right place. Sometimes you will be asked to use a cross rather than a dot. The numerical method is to write the chemical symbol (Li ) followed by the number of electrons in each energy level, innermost first, Li 2,1.

Electronic structure of lithium

ElementNumeric format

ElectronsPeriodic table group

 

Li 2,1

Lithium atoms have three electrons. Two of these fit into the first energy level, with the third in the second energy level.

Group 1

Electronic structure 2Below are some more electronic structures. Remember - you need to learn the electronic structures of the first 20 elements.

The number of electrons in the highest occupied energy level of each atom is the same as the element's group number.

Electronic structures of elements

ElementNumeric format

ElectronsPeriodic table group

 

F 2,7

Fluorine atoms have nine electrons. Two of these fit into the first energy level. The remaining seven fit into the second energy level.

Group 7

 

Ne 2,8

Neon atoms have ten electrons. Two of these fit into the first energy level. The remaining eight electrons fit into the second energy level. Because its highest occupied energy level is full, neon is stable [stable: Atoms are stable if their outer shell contains its maximum number of electrons. ] and unreactive.

Group 0 - that is, the eighth group

ElementNumeric format

ElectronsPeriodic table group

 

Na 2,8,1

Sodium atoms have 11 electrons. Two of these fit into the first energy level, eight into the second energy level. The last one fits into the third energy level.

Group 1

 

Ca 2,8,8,2

Calcium atoms have 20 electrons. Two of these fit into the first energy level, eight into the second energy level, another eight into the third energy level. The last two fit into the fourth energy level.

Group 2

Electronic structure and the periodic tableAs you have seen, there is a link between an atom's electronic structure and its position in the periodic table. You can work out an atom's electronic structure from its place in the periodic table.

Periodic table related to electronic structure

The diagram shows a section of the periodic table, with the elements arranged as usual in the order of their atomic

number, from 2 to 20. The red numbers below each chemical symbol show its electronic structure.

Moving across each period, you can see that the number of occupied energy levels is the same as the period number.

As you go across each period from left to right, an energy level gradually becomes filled with electrons. The highest occupied energy level contains just one electron on the left-hand side of the table. It is filled by the time you get to the right-hand side.

Moving down each group, you can see that the number of electrons in the highest occupied energy level is the same as the group number.

Each element in a group therefore has the same number of electrons in its highest occupied energy level. Group 0 is a partial exception to this rule. Although it comes after Group 7, it is not called Group 8 - and it contains helium, which has only two electrons in its outer shell.

Working out an element's electronic structureHere is how to use the periodic table to work out an electronic structure:

1. Find the element in the periodic table. Work out which period it is in, and draw that number of circles around the nucleus.

2. Work out which group the element is in and draw that number of electrons in the outer circle - with eight for Group 0 elements - except helium.

3. Fill the other circles with electrons. Remember - two in the first, eight in the second and third, and 18 in the fourth.

4. Finally, count your electrons and check that they equal the atomic number.

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Science

Ionic bonding

Ions are electrically charged particles formed when atoms [atoms: An atom is the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons. ] lose or gain electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ]. They have the same electronic structures as noble gases.

Metal atoms form positive ions, while non-metal atoms form negative ions. The strong electrostatic [electrostatic: An electrostatic force is generated by differences in electric charge (ie positive and negative) between two particles. It can also refer to electricity at rest. ] forces of attraction between oppositely charged ions are called ionic bonds.

Ionic compounds [ionic compound: An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). The ions swap electrons to achieve a full outer shell. ] have high melting and boiling points.

How ions formIons are electrically charged particles formed when atoms lose or gain electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ]. This loss or gain leaves a complete highest energy level, so the electronic structure of an ion is the same as that of a noble gas - such as a helium, neon or argon.

Metal atoms and non-metal atoms go in opposite directions when they ionise:

Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions.

Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions.

Positively charged sodium and aluminium ions

Negatively charged oxide and chloride ions

How many charges?There is a quick way to work out what the charge on an ion should be:

the number of charges on an ion formed by a metal is equal to the group number of the metal

the number of charges on an ion formed by a non-metal is equal to the group number minus eight

hydrogen forms H+ ions

 Group 1

Group 2

Group 3

Group 4

Group 5

Group 6

Group 7

Group 0

Example element

Na Mg Al C N O Cl He

Charge 1+ 2+ 3+ Note 1 3- 2- 1- Note 2Symbol of ion Na+ Mg2+ Al3+ Note 1 N3- O2- Cl- Note 2Note 1: carbon and silicon in Group 4 usually form covalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ] by sharing electrons.

Note 2: the elements in Group 0 do not react with other elements to form ions.

Metal ions

You need to be able to show the electronic structure of some common metal ions, using diagrams like these:

Lithium, Li

Lithium is in Group 1. It has one electron [electron: An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ] in its highest energy level. When this electron is lost, a lithium ion Li+ is formed.

Sodium, Na

Sodium is also in Group 1. It has one electron in its highest energy level. When this electron is lost, a sodium ion Na+ is formed.

Neon atom

Note that a sodium ion has the same electronic structure as a neon atom (Ne).

But be careful - a sodium ion is not a neon atom. This is because the nucleus [nucleus: The central part of an atom.

It contains protons and neutrons, and has most of the mass of the atom. ] of a sodium ion is the nucleus of a sodium atom and has 11 protons - but the nucleus of a neon atom has only 10.

Magnesium, Mg

Magnesium is in Group 2. It has two electrons in its highest energy level. When these electrons are lost, a magnesium ion Mg2+ is formed.

A magnesium ion has the same electronic structure as a neon atom (Ne).

Calcium, Ca

Calcium is also in Group 2. It has two electrons in its highest energy level. When these electrons are lost, a calcium ion Ca2+ is formed.

A calcium ion has the same electronic structure as an argon atom (Ar).

Non-metal ionsYou need to be able to show the electronic structure of some common non-metal ions, using diagrams like these:

Fluorine, F

Fluorine is in Group 7. It has seven electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] in its highest energy level. It gains an electron from another atom in reactions, forming a fluoride ion, F-.

Note that the atom is called fluorine, but the ion is called fluoride.

Neon atom

Note that a fluoride ion has the same electronic structure as a neon atom (Ne).

Once again, a fluoride ion is not a neon atom, because the nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] of a fluoride ion is the nucleus of a fluorine atom, with 9 protons, and not of a neon atom, with 10.

Chlorine, Cl

Chlorine is in Group 7. It has seven electrons in its highest energy level. It gains an electron from another atom in reactions, forming a chloride ion, Cl-.

Oxygen, O

Oxygen is in Group 6. It has six electrons in its highest energy level. It gains two electrons from one or two other atoms in reactions, forming an oxide ion, O2-.

Ionic compounds and ionic bondingWhen metals react with non-metals, electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] are transferred from the metal atoms to the non-metal atoms, forming ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons. ]. The resulting compound is called an ionic compound [ionic compound: An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron): The ions swap electrons to achieve a full outer shell. ].

Consider reactions between metals and non-metals, for example,

sodium + chlorine → sodium chloride magnesium + oxygen → magnesium oxide calcium + chlorine → calcium chloride

In each of these reactions, the metal atoms give electrons to the non-metal atoms. The metal atoms become positive ions and the non-metal atoms become negative ions.

There is a strong electrostatic [electrostatic: An electrostatic force is generated by differences in electric charge (ie positive and negative) between two particles. It can also refer to electricity at rest. ] force of attraction between these oppositely charged ions, called an ionic bond. The animation shows ionic bonds being formed in sodium chloride, magnesium oxide and calcium chloride.

There are many ionic bonds in an ionic compound such as sodium chloride, arranged in giant lattice [lattice: A lattice is a regular grid-like arrangement of atoms in a material. ] structures. Ionic compounds have high melting and boiling points.

Dot-and-cross diagramsYou need to be able to draw dot-and-cross diagrams to show the ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons. ] in some common ionic compounds [ionic compound: An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). The ions swap electrons to achieve a full outer shell. ].

Sodium chloride, NaCl

Sodium ions have the formula Na+, while chloride ions have the formula Cl-. You need to show one sodium ion and one chloride ion. In the exam, make sure the dots and crosses are clear, but do not worry about colouring them.

Magnesium oxide, MgO

Magnesium ions have the formula Mg2+, while oxide ions have the formula O2-. You need to show one magnesium ion and one oxide ion.

Calcium chloride, CaCl2

Calcium ions have the formula Ca2+. Chloride ions have the formula Cl-.

You need to show two chloride ions, because two chloride ions are needed to balance the charge on a calcium ion.

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Science

Covalent bonding

A covalent bond is a strong bond between two non-metal atoms [atoms: An atom is the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons. ]. It consists of a shared pair of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ]. A covalent bond can be represented by a straight line or dot-and-cross diagram.

Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds

Sharing electronsYou will need to understand what covalent bonding is, and to remember some of the properties of molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ] that are formed in this way.

A covalent bond forms when two non-metal atoms [atoms: An atom is the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons. ] share a pair of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ]. The electrons involved are in the highest occupied energy levels - or outer shells - of the atoms. An atom that shares one or more of its electrons will complete its highest occupied energy level.

Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.

Example - the animation shows a covalent bond being formed between a hydrogen atom and a chlorine atom, to form hydrogen chloride.

After bonding, the chlorine atom is now in contact with eight electrons in its highest energy level - so it is stable [stable: Atoms are stable if their outer shell contains its maximum number of electrons. ]. The hydrogen atom is now in contact with two electrons in its highest energy level - so the hydrogen is also stable.

How many bonds?Atoms may form multiple covalent bonds - that is, share not just one pair of electrons but two or more pairs. Atoms of different elements will form either one, two, three or four covalent bonds with other atoms.

There is a quick way to work out how many covalent bonds an element will form. The number of covalent bonds is equal to eight minus the group number (you can brush up on group numbers by reading through the section in AQA GCSE Science on the periodic table). The table below gives more detail on this rule:

 Group 4 Group 5 Group 6 Group 7

example carbon nitrogen oxygen chlorinenumber of bonds8 - 4 = 48 - 5 = 38 - 6 = 28 - 7 = 1Hydrogen forms one covalent bond. The noble gases in Group 0 do not form any.

Representing covalent bondsCovalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ] can be represented in several different ways.

Straight lines and models

Straight lines are the most common way to represent covalent bonds, with each line representing a shared pair of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ]. 2-D or 3-D molecular models are especially useful for showing the relationship between atoms in multiple covalent bonds. Below are some examples of straight lines and images of 3-D models.

Models for covalent bondselement formulachemical structureball-and-stick model

hydrogenH2

water H2O

ammoniaNH3

methane CH4

Double and triple bondsNote that molecules can have a double covalent bond - meaning they have two shared pairs of electrons - or a triple covalent bond - three shared pairs of electrons. A double covalent bond is shown by a double line, and a triple bond by a triple line.

A molecule of oxygen (O2) consists of two oxygen atoms held together by a double bond, like this:

A molecule of nitrogen (N2) has two nitrogen atoms held together by a triple bond, like this:

Dot-and-cross diagramsDot-and-cross diagrams are another way to represent covalent bonds. The shared electron from one atom is

shown as a dot, while the shared electron from the other atom is shown as a cross.

When drawing dot-and-cross diagrams for covalent bonds, you only need to show the electrons in the highest occupied energy level, as only these are involved.

The animations show covalent bonds represented by both straight lines and dot-and-cross diagrams:

Above - covalent bonding between hydrogen atoms to form a molecule [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. It is the smallest part of a substance that displays the properties of the substance. ] of hydrogen gas (H2)

Below - covalent bonding between oxygen atoms to form a molecule of oxygen gas (O2). The lowest energy level is not shown.

Dot-and-cross diagrams - elementsFor your exam, you need to be able to draw dot-and-cross diagrams representing the covalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ] in the molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ] of some common gaseous elements [elements: All atoms of an element have the same atomic number, the same number of protons and electrons, and so the same chemical properties. ]. Remember: you only need to show the electrons in the highest energy level.

Hydrogen, H2

Hydrogen atoms can each form one covalent bond. One pair of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] is shared in a hydrogen molecule (H2).

Chlorine, Cl2

Chlorine atoms can each form one covalent bond. One pair of electrons is shared in a chlorine molecule (Cl2).

Oxygen, O2

Oxygen atoms can each form two covalent bonds. Two pairs of electrons are shared in an oxygen molecule (O2) - a double bond [double bond: A double bond is a covalent bond resulting from the sharing of four electrons (two pairs) between two atoms. ].

Dot-and-cross diagrams - compoundsYou will also need to be able to draw dot-and-cross diagrams representing the covalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ] in the

molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ] of some common compounds [compounds: Substances formed by the chemical union (involving bond formation) of two or more elements. ]:

Hydrogen chloride, HCl

Hydrogen atoms and chlorine atoms can each form one covalent bond. One pair of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] is shared in a hydrogen chloride molecule (HCl).

Water, H2O

Hydrogen atoms can each form one covalent bond, while oxygen atoms can each form two covalent bonds. Two pairs of electrons are shared in a water molecule (H2O).

Ammonia, NH3

Hydrogen atoms can each form one covalent bond, while and nitrogen atoms can each form three covalent bonds.

Three pairs of electrons are shared in an ammonia molecule (NH3).

Methane, CH4

Hydrogen atoms can each form one covalent bond, while carbon atoms can each form four covalent bonds. Four pairs of electrons are shared in a methane molecule (CH4).

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Science

Different substances and their properties

Ionic substances form giant ionic lattices [lattice: A lattice is a regular grid-like arrangement of atoms in a material. ] containing oppositely charged ions. They have high melting and boiling points, and conduct [conduct: To allow electricity, heat or other energy forms to pass through. ] electricity when melted or dissolved in water.

Simple molecular substances consist of molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ] in which the atoms are joined by strong covalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ]. Their molecules are held together by weak forces, so these substances have low melting and boiling points. They do not conduct electricity.

Giant covalent structures contain many atoms joined together by covalent bonds to form a giant lattice. They have high melting and boiling points. Graphite and diamond have different properties because they have different structures. Graphite conducts heat and electricity well because it also has free electrons.

Nanoparticles are 1-100 nm in size, typically the size of small molecules - far too small to see with a microscope. They have remarkable properties that are different to the same substance in bulk form.

Metals form giant structures containing free electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ], making them good conductors of heat and electricity.

Ionic compoundsIonic bonds form when a metal reacts with a non-metal. Metals form positive ions; non-metals form negative ions.

Ionic bonds are the electrostatic [electrostatic: An electrostatic force is generated by differences in electric charge (ie positive and negative) between two particles. It can also refer to electricity at rest. ] forces of attraction between oppositely charged ions.

Positively charged Na ions and negatively charged Cl ions

The oppositely charged ions are arranged in a regular way to form giant ionic lattices [lattice: A lattice is a regular grid-like arrangement of atoms in a material. ]. Ionic compounds [compounds: Substances formed by the chemical union (involving bond formation) of two or more elements. ] often form crystals as a result. The illustration shows part of a sodium chloride (NaCl) ionic lattice.

Properties of ionic compounds High melting and boiling points - Ionic bonds are very

strong - a lot of energy is needed to break them. So ionic compounds have high melting and boiling points.

Conductive when liquid - Ions are charged particles, but ionic compounds can only conduct [conduct: To allow electricity, heat or other energy forms to pass through. ] electricity if their ions are free to move. Ionic compounds do not conduct electricity when they are solid - only when dissolved in water or melted.

Properties of ionic compounds

Ionic compound

Properties

Sodium chloride, NaCl

High melting point: 800ºC

Non-conductive in its solid state, but when dissolved in water or molten [molten: Molten means reduced to liquid form by heating. It is mainly used to describe rock, glass or metal. ] NaCl will conduct electricity.

Magnesium oxide, MgO

Higher melting point than sodium chloride: around 2,800ºC. This is because its Mg2+ and O2- ions have a greater number of charges, so they form stronger ionic bonds than the Na+ and Cl- ions in sodium chloride.

Because magnesium oxide stays solid at such high temperatures, it remains non-conductive. It is used for high-temperature electrical insulation.

Covalent compounds - simple moleculesCovalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ] form between non-metal atoms. Each bond consists of a shared pair of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ], and is very strong. Covalently bonded substances fall into two main types:

1. simple molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ] and

2. giant covalent structures.

Simple molecules

A molecule of carbon dioxide

These contain only a few atoms held together by strong covalent bonds. An example is carbon dioxide (CO2), the

molecules of which contain one atom [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ] of carbon bonded with two atoms of oxygen.

Properties of simple molecular substances

Low melting and boiling points - This is because the weak intermolecular forces break down easily.

Non-conductive - Substances with a simple molecular structure do not conduct [conduct: To allow electricity, heat or other energy forms to pass through. ] electricity. This is because they do not have any free electrons or an overall electric charge.

Higher Tier onlyHydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points.

The animation shows how the weak intermolecular forces between water molecules break down during boiling or melting:

Covalent bonding - giant covalent structuresGiant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ]. The atoms are usually arranged into giant regular lattices [lattice: A lattice is a regular grid-like

arrangement of atoms in a material. ] - extremely strong structures because of the many bonds involved. The graphic shows the molecular structure of diamond and graphite: two allotropes [allotropes: Allotropes are structurally different forms of an element. They differ in the way the atoms bond with each other and arrange themselves into a structure. Because of their different structures, allotropes have different physical and chemical properties. ] of carbon, and of silica (silicon dioxide).

From left to right - graphite, diamond, silica

Properties of giant covalent structures

Very high melting points - Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600ºC.

Variable conductivity - Diamond does not conduct [conduct: To allow electricity, heat or other energy forms to pass through. ] electricity. Graphite contains free electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ], so it does conduct electricity. Silicon is semi-conductive - that is, midway between non-conductive and conductive.

GraphiteGraphite is a form of carbon in which the carbon atoms form layers. These layers can slide over each other, so graphite is much softer than diamond. It is used in pencils, and as a lubricant [lubricant: A substance used to reduce

the friction between two solid surfaces. ]. Each carbon atom in a layer is joined to only three other carbon atoms. Graphite conducts electricity.

DiamondDiamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. As a result, diamond is very hard and has a high melting point. It does not conduct electricity.

SilicaSilica, which is found in sand, has a similar structure to diamond. It is also hard and has a high melting point, but contains silicon and oxygen atoms, instead of carbon atoms.

The fact that it is a semi-conductor makes it immensely useful in the electronics industry: most transistors are made of silica.

Buckminsterfullerene

Structure of a buckminsterfullerene molecule - a large ball of 60 atoms

Buckminsterfullerene is yet another allotrope of carbon. It is actually not a giant covalent structure, but a giant molecule in which the carbon atoms form pentagons and hexagons - in a similar way to a leather football. It is used in lubricants.

NanoparticlesMeasurements

The table shows some of the units used to measure length. As you go down the table, each unit is 1,000 times smaller than the one above it.

Units used to measure lengthUnit name Unit symbolMeaninggigametre Gm one billion metresmegametreMm one million metreskilometre km one thousand metresmetre m one metremillimetre mm one thousandth of a metremicrometreµm one millionth of a metrenanometre nm one billionth of a metre

Nanotubes like this could be used to make tiny mechanical devices, molecular computers or extremely strong materials

Nanoparticles range in size from about 100 nm down to about 1 nm. They are typically the size of small molecules, and far too small to see with a microscope.

Working with nanoparticles is called nanotechnology.

Uses of nanoparticlesNanoparticles have a very large surface area compared with their volume. So they are often able to react very quickly. This makes them useful as catalysts to speed up reactions. For example, they can be used in self-cleaning ovens and windows.

Nanoparticles also have different properties to the same substance in normal-sized pieces. For example, titanium dioxide is a white solid used in house paint and certain

sweet-coated chocolates. Titanium dioxide nanoparticles are so small they do not reflect visible light, so cannot be seen. They are used in sunblock creams to block harmful ultraviolet light without appearing white on the skin.

Nanoscience may lead to the development of:

new catalysts new coatings new computers stronger and lighter building materials sensors that detect individual substances in tiny

amounts.

Metallic bonding - higherGiant structures with free electronsMetals form giant structures in which electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] in the outer shells of the metal atoms [atoms: An atom is the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons. ] are free to move. The metallic bond is the force of attraction between these free electrons and metal ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons. ]. Metallic bonds are strong, so metals can maintain a regular structure and usually have high melting and boiling points.

Atomic structure of a metal

Metals are good conductors [conductor: An electrical conductor is a material which allows an electrical current to pass through it easily. It has a low resistance. A thermal conductor allows thermal energy to be transferred through it easily. ] of electricity and heat, because the free electrons carry a charge or heat energy through the metal. The free electrons allow metal atoms to slide over each other, so metals are malleable [malleable: the ability of a material to permanently deform, or lose its shape, in all directions without cracking. ] and ductile [ductile: able to deform, usually by stretching along its length. ].

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Science

Chemical calculations - both Tiers

The mass number of an atom is its total number of protons and neutrons.

The relative formula mass of a compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ] is found by adding together the relative atomic masses of all the atoms in the formula [formula: A formula is a combination of symbols that indicates the chemical composition of a substance. ] of the compound.

Mass number

Structure of an atom

Each atom consists of a nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] containing protons and neutrons, with electrons arranged around it.

Protons [protons: Sub-atomic particles with a positive charge and a relative mass of 1. ] and neutrons [neutrons: Uncharged sub-atomic particles, with a mass of 1 relative to a proton. ] both have a relative mass of 1 unit.

Electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] have a very small mass compared to protons and neutrons. Generally when working out the mass of

atoms and molecules we can ignore the mass of the electrons.

Notice that most of the mass of an atom is found in its nucleus:

the mass number of an atom is the total number of protons and neutrons it contains

the atomic number (also called the proton number) is the number of protons it contains.

The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger.

The full chemical symbol for an element [element: A substance made of one type of atom only. ] shows its mass number at the top, and atomic number at the bottom. Here is the full symbol for carbon.

It tells us that a carbon atom has six protons. It will also have six electrons, because the number of protons and electrons in an atom is the same.

The symbol also tells us that the total number of protons and neutrons in a carbon atom is 12. Note that you can work out the number of neutrons from the mass number and atomic number. In this example, it is 12 - 6 = 6 neutrons.

IsotopesIsotopes are atoms of an element with the normal number of protons and electrons, but different numbers of neutrons. Isotopes have the same atomic number, but different mass numbers.

The different isotopes of an element have identical chemical properties. However, some isotopes are radioactive [radioactive: A substance that emits radiation is said to be radioactive. ].

Isotopes of hydrogenMost hydrogen atoms consist of just one proton [proton: A proton is a small particle with a positive charge found in the nucleus of the atom. ] and one electron [electron: An electron is a very small negatively-charged particle found in an atom in the space surrounding the nucleus. ], but some also have one or two neutrons [neutrons: Uncharged sub-atomic particles, with a mass of 1 relative to a proton. ].

Isotopes of hydrogenIsotope protonselectronsneutrons

1 1 1 - 1 = 0

1 1 2 - 1 = 1

1 1 3 - 1 = 2

Isotopes of chlorineChlorine atoms contain 17 protons and 17 electrons. About 75 per cent of chlorine atoms have 18 neutrons, while about 25 per cent have 20 neutrons.

Isotopes of chlorineIsotope protonselectronsneutrons

17 17 35 - 17 = 18

17 17 37 - 17 = 20

Relative atomic massDifferent atoms have different masses. Atoms have such a small mass [mass: production method for making hundreds of identical products, usually on a production line. Also called repetitive flow production. ] it is more convenient to

know their masses compared to each other. Carbon is taken as the standard atom and has a relative atomic mass (Ar) of 12.

Atoms with an Ar of less than this have a smaller mass than a carbon atom. Atoms with an Ar which is more than this have a larger mass than a carbon atom.

The table below shows some Ar values (you do not need to remember them: you will be given them in the exam if you need them to answer a question).

Ar values of elementsElementAr

H 1C 12O 16Mg 24Cl 35.5These values tell you that magnesium atoms are twice as heavy as carbon atoms, and 24 times heavier than hydrogen atoms; while hydrogen atoms are 12 times lighter than carbon atoms. They also allow you to work out that three oxygen atoms weigh the same as two magnesium atoms.

Chlorine's Ar of 35.5 is an average of the masses of the different isotopes [isotopes: Atoms of an element with the same number of protons and electrons but different numbers of neutrons. ] of chlorine.

Relative formula massFor your exam, you will need to know what relative formula mass is - and be able to work out the relative formula mass of a compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ] when given its formula.

To find the relative formula mass (Mr) of a compound, you just add together the Ar values for all the atoms in its formula.

Examples Example 1: Find the Mr of carbon monoxide (CO). The Ar of carbon is 12 and the Ar of oxygen is 16. So the Mr of carbon monoxide is 12 + 16 = 28.

Example 2:Find the Mr of sodium oxide (Na2O). The Ar of sodium is 23 and the Ar of oxygen is 16. So the Mr of sodium oxide is (23 x 2) + 16 = 62.

The relative formula mass of a substance, shown in grams, is called one mole of that substance. So one mole of carbon monoxide has a mass of 28g, and one mole of sodium oxide has a mass of 62g.

The table shows some more examples of relative formula mass calculations, using the relative atomic mass values given at the bottom of the page.

Relative formula mass calculationsCompound Formula Calculation Mr Water H2O 1 + 1 + 16 = 18Sodium hydroxide

NaOH 23 + 16 + 1 = 40

Magnesium hydroxide

Mg(OH)224 + 16 + 16 + 1 + 1 = (remember that there are two of each atom inside the brackets)

58

Ar of H = 1 Ar of O = 16 Ar of Na = 23 Ar of Mg = 24

Percentage compositionPercentage composition is just a way to describe what proportions of the different elements [elements: All atoms of an element have the same atomic number, the same number of protons and electrons, and so the same chemical properties. ] there are in a compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ].

If you have the formula of a compound, you should be able to work out the percentage by mass of an element in it.

ExampleThe formula for sodium hydroxide is NaOH. It contains three different elements: Na, O and H. But the percentage by mass of each element is not simply 33.3 per cent, because each element has a different relative atomic mass. You need to use the Ar values to work out the percentages. Here is how to do it:

Question What is the percentage by mass of oxygen (O) in sodium hydroxide (NaOH)?

1. First, work out the relative formula mass of the compound, using the Ar values for each element. In the case of sodium hydroxide, these are Na = 23, O = 16, H = 1. (You will be given these numbers in the exam.)

2. Next, divide the Ar of oxygen by the Mr of NaOH, and multiply by 100 to get a percentage.

Answer 1. Mr of NaOH is 23 + 16 + 1 = 40 2. (16 ÷ 40 ) × 100 = 0.4 × 100 = 40%

So the percentage by mass of oxygen in sodium hydroxide is 40%.

Remember, if there is more than one atom of the element in the compound, you need to multiply your answer by the number of atoms. If your answer is more than 100%, you have gone wrong!

Conservation of massMass is never lost or gained in chemical reactions. We say that mass is always conserved. In other words, the total mass [mass: production method for making hundreds of

identical products, usually on a production line. Also called repetitive flow production. ] of products [product: A product is a substance formed in a chemical reaction. ] at the end of the reaction is equal to the total mass of the reactants [reactants: substances present at the start of a chemical reaction ] at the beginning.

This fact allows you to work out the mass of one substance in a reaction if the masses of the other substances are known. For example,

In practice, it is not always possible to get all of the calculated amount of product from a reaction:

reversible reactions may not go to completion some product may be lost when it is removed from the

reaction mixture some of the reactants may react in an unexpected way

Atom economyThe atom economy of a process tells you the proportion of atoms in the reactants that become part of a useful product. Sustainable development means meeting our needs without damaging the chances of future generations meeting their needs. Processes with high atom economies are more efficient and produce less waste. They are important to sustainable development.

SummaryWatch this illustrated podcast for a summary of chemical calculations at foundation level.

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Science

Chemical calculations - higher

You should be able to calculate the masses of reactants and products from balanced equations, and the formula of a compound from information about reacting masses.

Concentrations and volumesConcentrationsYou will not be asked to calculate any concentrations of solutions in GCSE Additional Science, but you need to know certain things about concentration.

The concentration of a solution is measured in moles per cubic decimetre, mol/dm3. The greater the concentration, the more dissolved particles there are in the solution:

Equal volumes of different solutions that have the same concentration contain the same number of particles of dissolved solute. For example, a solution at 2 mol/dm3

is twice as concentrated as a solution at 1 mol/dm3.

Volumes of gasesYou will not be asked to calculate any volumes of gases in GCSE Additional Science, but you need to know certain things about them.

Equal volumes of gases at the same temperature and pressure contain the same number of molecules. For example:

50cm3 of hydrogen contains twice as many molecules as 25cm3 of oxygen at the same temperature and pressure.

One mole of any gas at room temperature and normal pressure occupies about 24 dm3.

Empirical formulas

You can use information about reacting masses to calculate the formula of a compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ]. Here is an example:

Question Suppose 3.2g of sulfur reacts with oxygen to produce 6.4g of sulfur oxide. What is the formula of the oxide?

Use the fact that the Ar of sulfur is 32 and the Ar of oxygen is 16.

Answer Find the mass of each element. Conservation of

mass tells us that the mass of oxygen = the mass of sulfur oxide - the mass of sulfur.

The mass of oxygen reacted = 6.4 - 3.2 = 3.2g So we have 3.2g of sulfur and 3.2g of oxygen. Now divide the mass of each element by its Ar

value. sulfur: 3.2 ÷ 32 = 0.1 oxygen: 3.2 ÷ 16 = 0.2 Finally, find the ratio of the elements. You can do this by dividing the results by the

smallest of the numbers to give you the number of atoms of each element in the compound.

In this case the smallest value is 0.1, so divide both results by that.

S = 0.1 ÷ 0.1 = 1 O = 0.2 ÷ 0.1 = 2 (If one of the numbers ends in 0.5, multiply all the

numbers by 2 - this is because you cannot have half-atoms in a compound.)

So the ratio of sulfur to oxygen is 1:2

The number of atoms tells you that the formula for sulfur oxide is SO2

Here is the calculation again in tabular form to help you remember the steps:

Steps to calculation the formula of a compoundstepaction S O1 find masses 3.23.22 look up given Ar values32 163 divide masses by Ar 0.10.24 find the ratio 1 2Result: the formula for the oxide = SO2 

Reacting massesIf you have a balanced equation for a reaction, you can calculate the masses of reactants [reactants: substances present at the start of a chemical reaction ] and products [product: A product is a substance formed in a chemical reaction. ].

Sample questionLook at this equation: CaCO3(s)    →    CaO(s) + CO2(g)

If we have 50g of CaCO3, how much CaO can we make?

First, work out the Mr values for the two compounds:

Mr of CaCO3 is 40 + 12 + 16 + 16 + 16 = 100 Mr of CaO is 40 + 16 = 56

This means that 100g of CaCO3 would yield 56g of CaO in this reaction. In the question we are told we have only half of that amount of CaCO3, 50g. So we will get half the amount of CaO, 28g.

So the mass of CaO we can make = 28g

Notice in this that 22g of CO2 would also be produced, as 50 - 28 = 22

Percentage yieldIn practice, it is not always possible to get the calculated amount of product in a reaction:

reversible reactions may not go to completion some product may be lost when it is removed from the

reaction mixture

some of the reactants may react in an unexpected way

The yield of a reaction is the actual mass of product obtained. The percentage yield can be calculated:

For example, the maximum theoretical mass of product in a certain reaction is 20g, but only 15g is actually obtained.

percentage yield = 15⁄20 × 100 = 75%

The maximum theoretical mass of product itself can be calculated using a reacting mass calculation, such as the one on the previous page.

Atom economyThe atom economy of a chemical reaction is a measure of the amount of starting materials that become useful products. Inefficient, wasteful processes have low atom economies. Efficient processes have high atom economies, and are important for sustainable development, as they use fewer natural resources and create less waste.

The atom economy of a reaction can be calculated:

Note that, because the total mass of products equals the total mass of reactants, you can put that into the bottom of the fraction in the calculation like this:

For example, what is the atom economy for making hydrogen by reacting coal with steam?

Write the balanced equation:

C(s) + 2H2O(g)    →    CO2(g) + 2H2(g)

Write out the Ar and Mr values underneath:

C(s) + 2H2O(g)    →    CO2(g) + 2H2(g)

12       2 × 18                44           2 × 2

Remember that the Ar or Mr in grams is one mole, so:

total mass of products = 44 + 4 = 48g (note that this is the same as the reactants: 12 + 36 = 48g)

mass of desired product (H2) = 4g

% atom economy = 4⁄48 × 100 = 8.3%

This process has a low atom economy and is therefore an inefficient way to make hydrogen. It also uses a non-renewable resource: coal.

SummaryThis illustrated podcast explains how to work out a balanced equation for a chemical reaction.

This illustrated podcast shows how to work with moles in chemical calculations.

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Science

Rates of reaction

The rate of a reaction can be measured by the rate at which a reactant [reactant: substances present at the start of a chemical reaction ] is used up, or the rate at which a product [product: A product is a substance formed in a chemical reaction. ] is formed.

The temperature, concentration, pressure of reacting gases, surface area of reacting solids, and the use of

catalysts, are all factors which affect the rate of a reaction.

Chemical reactions can only happen if reactant particles collide with enough energy. The more frequently particles collide, and the greater the proportion of collisions with enough energy, the greater the rate of reaction.

Measuring ratesDifferent reactions can happen at different rates. Reactions that happen slowly have a low rate of reaction. Reactions that happen quickly have a high rate of reaction. For example, the chemical weathering of rocks is a very slow reaction: it has a low rate of reaction. Explosions are very fast reactions: they have a high rate of reaction.

Reactants and productsThere are two ways to measure the rate of a reaction:

1. measure the rate at which a reactant [reactant: substances present at the start of a chemical reaction ] is used up

2. measure the rate at which a product [product: A product is a substance formed in a chemical reaction. ] is formed

The method chosen depends on the reaction being studied. Sometimes it is easier to measure the change in the amount of a reactant that has been used up; sometimes it is easier to measure the change in the amount of product that has been produced.

Things to measureThe measurement itself depends on the nature of the reactant or product:

the mass of a substance - solid, liquid or gas - is measured with a balance

the volume of a gas is usually measured with a gas syringe, or sometimes an upside down measuring cylinder or burette

It is usual to record the mass or total volume at regular intervals and plot a graph. The readings go on the vertical axis, and the time goes on the horizontal axis.

For example, if 24cm3 of hydrogen gas is produced in two minutes, the mean rate of reaction = 24 ÷ 2 = 12cm3 hydrogen / min.

Factors affecting the rateYou will be expected to remember the factors that affect the rate of reactions, and to plot or interpret graphs from rate experiments.

How to increase the rate of a reactionThe rate of a reaction increases if:

the temperature is increased the concentration [concentration: The concentration of

a solution tells us how much of a substance is dissolved in water. Concentration is measured in moles per dm3. The higher the concentration, the more particles of the substance are present. ] of a dissolved reactant [reactant: substances present at the start of a chemical reaction ] is increased

the pressure of a reacting gas is increased solid reactants are broken into smaller pieces a catalyst [catalyst: A catalyst changes the rate of a

chemical reaction without being changed by the reaction itself. ] is used

Rate of reaction and changing conditions

The graph above summarises the differences in the rate of reaction at different temperatures, concentrations and size of pieces. The steeper the line, the greater the rate of reaction. Reactions are usually fastest at the beginning, when the concentration of reactants is greatest. When the line becomes horizontal, the reaction has stopped.

Collisions and reactionsYou will be expected to explain, in terms of particles and their collisions, why changing the conditions of a reaction changes its rate.

CollisionsFor a chemical reaction to occur, the reactant [reactant: substances present at the start of a chemical reaction ] particles must collide. Collisions with too little energy do not produce a reaction.

The collision must have enough energy for the particles to react. The minimum energy needed for particles to react is called the activation energy.

Changing concentration or pressureIf the concentration of a dissolved reactant is increased, or the pressure of a reacting gas is increased:

there are more reactant particles in the same volume there is a greater chance of the particles colliding

the rate of reaction increases

Changing particle sizeIf a solid reactant is broken into small pieces or ground into a powder:

its surface area is increased more particles are exposed to the other reactant there is a greater chance of the particles colliding the rate of reaction increases

Changing the temperatureIf the temperature is increased:

the reactant particles move more quickly more particles have the activation energy or greater the particles collide more often, and more of the

collisions result in a reaction the rate of reaction increases

Using a catalystCatalysts increase the rate of reaction without being used up. They do this by lowering the activation energy needed. With a catalyst, more collisions result in a reaction, so the rate of reaction increases. Different reactions need different catalysts.

Catalysts are important in industry because they reduce costs.

Science

Energy changes and reversible reactions

Exothermic [exothermic: Heat energy is released in an exothermic reaction. We know this because the surroundings get warm. ] reactions transfer energy to the surroundings. Endothermic [endothermic: In an endothermic reaction, energy is taken in from the surroundings. The surroundings then have less energy

than they started with, so the temperature falls. ] reactions take in energy from the surroundings.

Reversible reactions are where the products [product: A product is a substance formed in a chemical reaction. ] can react to remake the original reactants [reactants: substances present at the start of a chemical reaction ]. If the forward reaction is exothermic, the reverse reaction is endothermic.

Exothermic and endothermic reactionsWhen a chemical reaction occurs, energy is transferred to, or from, the surroundings - and there is often a temperature change. For example, when a bonfire burns it transfers heat energy to the surroundings. Objects near a bonfire become warmer. The temperature rise can be measured with a thermometer.

Exothermic reactionsThese are reactions that transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. Some examples of exothermic reactions are:

burning neutralisation reactions between acids and alkalis the reaction between water and calcium oxide

Endothermic reactionsThese are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can also be detected using a thermometer. Some examples of endothermic reactions are:

electrolysis [electrolysis: Electrolysis is the decomposition (separation or break-down) of a compound using an electric current. ]

the reaction between ethanoic acid and sodium carbonate

the thermal decomposition [thermal decomposition : A reaction in which substances are broken down by heat. ] of calcium carbonate in a blast furnace

The animation shows an exothermic reaction between sodium hydroxide and hydrochloric acid, and an endothermic reaction between sodium carbonate and ethanoic acid.

Reversible reactionsMany reactions, such as burning fuel, are irreversible - they go to completion and cannot be reversed easily. Reversible reactions are different. In a reversible reaction, the products [product: A product is a substance formed in a chemical reaction. ] can react to produce the original reactants [reactants: substances present at the start of a chemical reaction ] again.

When writing chemical equations for reversible reactions, we do not use the usual one-way arrow. Instead, we use two arrows, each with just half an arrowhead - the top one pointing right, and the bottom one pointing left. For example,

ammonium chloride ammonia + hydrogen chloride

The equation shows that ammonium chloride (a white solid) can break down to form ammonia and hydrogen chloride. It also shows that ammonia and hydrogen chloride (colourless gases) can react to form ammonium chloride again.

The animation below shows a reversible reaction involving white anhydrous copper(II) sulfate and blue hydrated copper(II) sulfate, the equation for which is:

anhydrous copper(II) sulfate + water hydrated copper(II) sulfate

The reaction between anhydrous copper(II) sulfate and water is used as a test for water. The white solid turns blue in the presence of water.

Ammonia and the Haber processAmmoniaAmmonia (NH3) is a compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ] of nitrogen and hydrogen. It is a colourless gas with a choking smell, and a weak alkali which is very soluble in water.

Ammonia is used to make fertilisers, explosives, dyes, household cleaners and nylon. It is also the most important raw material in the manufacture of nitric acid.

Ammonia is manufactured by combining nitrogen and hydrogen in an important industrial process called the Haber process.

An ammonia production plant. Photo courtesy of WMC Resources Ltd

The Haber processThe raw materials for this process are hydrogen and nitrogen. Hydrogen is obtained by reacting natural gas - methane - with steam, or through the cracking of oil. Nitrogen is obtained by burning hydrogen in air. Air is 80 per cent nitrogen; nearly all the rest is oxygen. When hydrogen is burned in air, the oxygen combines with the hydrogen, leaving nitrogen behind.

Nitrogen and hydrogen will react together under these conditions:

a high temperature - about 450ºC a high pressure - about 200 atmospheres (200 times

normal pressure) an iron catalyst

The reaction is reversible.

nitrogen + hydrogen ammonia

N2(g)       + 3H2(g) 2NH3(g)

The (g) indicates that the substance is a gas.

The flow chart shows the main stages in the Haber process. The reaction is reversible, and some nitrogen and hydrogen remain mixed with the ammonia. The reaction mixture is cooled so that the ammonia liquefies and can be removed. The remaining nitrogen and hydrogen are recycled.

The Haber process for making ammonia

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Reversible reactions - higher

When reversible reactions reach equilibrium the forward and reverse reactions are still happening but at the same rate, so the concentrations of reactants and products do not change.

The balance point can be affected by temperature, and also by pressure for gasses in equilibrium.

What is a chemical equilibrium?If a chemical reaction happens in a container where one or more of the reactants [reactants: substances present at the start of a chemical reaction ] or products [product: A product is a substance formed in a chemical reaction. ] can escape, you have an open system. If a chemical reaction happens in a container where none of the reactants or products can escape, you have a closed system. Reversible reactions [Reversible reactions: Reversible reactions are chemical reactions which can go both ways. The direction of the reaction depends on the condition of the reactants. ] that happen in a closed system eventually reach an equilibrium.

Chemical equilibriumIn a chemical equilibrium, the concentrations of reactants and products do not change. But the forward and reverse reactions have not stopped - they are still going on at the same rate as each other.

Imagine walking the wrong way on an escalator - at the same speed as the escalator, but in the opposite direction. Your legs would still be walking forwards, and the escalator would continue to move backwards. However, the net result would be that you stay in exactly the same place. This is what happens in an equilibrium.

Other factorsIf we remove the products from an equilibrium mixture, more reactants are converted into products. If a catalyst [catalyst: A catalyst changes the rate of a chemical reaction without being changed by the reaction itself. ] is used, the reaction reaches equilibrium much sooner, because the catalyst speeds up the forward and reverse reactions by the same amount. The concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst.

Effect of temperature on an equilibrium For the Higher Tier, you need to know what happens to the amount of product in an equilibrium mixture if the temperature is changed.

If the forward reaction is exothermic [exothermic: Heat energy is released in an exothermic reaction. We know this because the surroundings get warm. ] and the temperature is increased, the yield of products is decreased. If the temperature is decreased, the yield of products is increased.

If the forward reaction is endothermic [endothermic: In an endothermic reaction, energy is taken in from the surroundings. The surroundings then have less energy than they started with, so the temperature falls. ] and the temperature is increased, the yield of products is increased. If the temperature is decreased, the yield of products is decreased.

Haber process - effect of temperature change on yield of ammonia

This is important to industrial processes such as the Haber process. The forward reaction, which makes ammonia for fertilisers, is exothermic. So the yield of ammonia is better at lower temperatures. You might think that chemical engineers would choose a low temperature for the Haber process - but the reaction runs much faster at higher temperatures. So a compromise temperature is chosen - one that is high enough to get a reasonable rate of reaction, but not so high that the yield of ammonia is low.

The Haber process - choosing a temperatureA compromise temperatureThe forward reaction in the Haber process is exothermic [exothermic: Heat energy is released in an exothermic reaction. We know this because the surroundings get warm. ]. This means that if the temperature is increased, the position of equilibrium moves in the direction of the reverse reaction, and less ammonia is formed.

The effect of varying the temperature in the Haber processYou might think that a low temperature would be a good choice for the Haber process: if the forward reaction is exothermic, the yield of product at equilibrium

[equilibrium: If the rate of the forward reaction and the rate of the back reaction in a reversible reaction are equal, the reaction is in equilibrium. ] is increased at lower temperatures. However, if the temperature is too low the rate of reaction will be too low. This would make the process uneconomical. So a compromise temperature is chosen: low enough to get a good yield of ammonia but high enough to obtain a reasonable rate of reaction.

An iron catalystThe presence of a catalyst [catalyst: A catalyst changes the rate of a chemical reaction without being changed by the reaction itself. ] does not affect the position of the equilibrium, but it does increase the rate of the reaction. This means the ammonia is produced in a shorter time, reducing the cost of the process. Iron is a cheap catalyst.

Effect of pressure on an equilibriumFor the Higher Tier, you also need to know what happens to the amount of product [product: A product is a substance formed in a chemical reaction. ] in an equilibrium mixture of gases if the pressure is changed.

Count the molecules in the equationChanging the pressure has little effect on an equilibrium mixture without gases - but can have a big effect on an equilibrium mixture containing gases. If the pressure is increased, the position of equilibrium moves in the direction of the fewest molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ].

Look again at the Haber process, which makes ammonia (NH3):

N2(g) + 3H2(g) 2NH3(g)

On the left, there are 1 + 3 = 4 molecules of gas. On the right, there are two molecules of gas. If the pressure is increased, the position of equilibrium will move to the right and more ammonia will be made. If the pressure is reduced, the position of equilibrium will move to the left and less ammonia will be made.

Look at the graph. You can see that for any given temperature the yield of ammonia increases as the pressure increases. You can also see that, for any given pressure, the yield goes down as the temperature increases. This is because the forward reaction is exothermic [exothermic: Heat energy is released in an exothermic reaction. We know this because the surroundings get warm. ].

The Haber process: choosing a pressureYou will be expected to explain the choice of a high pressure for the Haber process.

Look at the symbol equation for the reaction in the Haber process:

N2(g) + 3H2(g) 2NH3(g)

There are 1 + 3 = 4 molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ] of gas on the left of the equation, only two molecules of gas on the right. In an equilibrium [equilibrium: If the rate of the forward reaction and the rate of the back reaction in a reversible reaction are equal, the reaction is in equilibrium. ] involving gases, an increase in pressure favours the reaction which produces the smallest number of molecules. In this case, an increase in pressure favours the forward reaction, and more ammonia is produced.

There is a limit to the pressure that can be used industrially, because very high pressures require very strong and expensive equipment. This means a compromise pressure is chosen - high enough to get a good yield of ammonia, but not so high that it would add too much to the costs of the process. The pressure chosen is usually about 200 atmospheres - equivalent to about half the pressure of the water around the wreck of the RMS Titanic.

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Electrolysis

Electrolysis is the process by which ionic substances are broken down into simpler substances using electricity. During electrolysis, metals and gases may form at the electrodes.

ElectrolysisTo understand electrolysis, you need to know what an ionic substance is.

Ionic substances form when a metal reacts with a non-metal. They contain charged particles called ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons. ]. For example, sodium chloride forms when sodium reacts with chlorine. It contains positively charged sodium ions and negatively charged chloride ions. Ionic substances can be broken down by electricity.

Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.

For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or molten [molten: Molten means reduced to liquid form by heating. It is mainly used to describe rock, glass or metal. ] (melted). For example, if electricity is passed through copper chloride solution, the copper chloride is broken down to form copper metal and chlorine gas.

Electrolysis

Here is what happens during electrolysis:

Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced [reduction: Reduction is a reaction in which oxygen is removed from a substance. Reduction also means a gain in electrons. ].

Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised [oxidation: Oxidation is a reaction in which oxygen combines with a substance. Oxidation also means a loss of electrons. ].

Predicting the products of electrolysisIonic substances in solution break down into elements during electrolysis. Different elements are released depending on the particular ionic substance.

At the negative electrodeAt the negative electrode, positively charged ions gain electrons. This is reduction, and you say that the ions have been reduced.

Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series:

the metal will be produced if it is less reactive than hydrogen

hydrogen will be produced if the metal is more reactive than hydrogen

The reactivity series of metal - carbon and hydrogen are not metals, but they are shown for comparison

So the electrolysis of copper chloride solution produces copper at the negative electrode. But the electrolysis of sodium chloride solution produces hydrogen.

At the positive electrodeAt the positive electrode, negatively charged ions lose electrons. This is oxidation, and you say that the ions have been oxidised. The table summarises some of the elements you should expect to get during electrolysis.

negative ion in solutionelement given off at positive electrodechloride, Cl– chlorine, Cl2bromide, Br– bromine, Br2

iodide, I– iodine, I2

sulfate, oxygen, O2

Putting it togetherThe table shows some common ionic compounds, and the elements released when their solutions are electrolysed.

ionic substance in solution

element at the negative electrode

element at the positive electrode

copper chloride, CuCl2 copper chlorinecopper sulfate, CuSO4 copper oxygensodium chloride, NaCl hydrogen chlorinehydrochloric acid, HCl hydrogen chlorinesulfuric acid, H2SO4 hydrogen oxygenPurification of copper

Copper is a good conductor of electricity, and is used extensively to make electrical wiring and components. The extraction of copper from copper ore [ore: An ore is a rock containing enough quantities of a mineral that it is profitable to extract it. ] is done by reduction [reduction: Reduction is a reaction in which oxygen is removed from a substance. Reduction also means a gain in electrons. ] with carbon. However, the copper produced is not pure enough for use as a conductor, so it is purified using electrolysis [electrolysis: Electrolysis is the decomposition (separation or break-down) of a compound using an electric current. ].

Electrolysis of copper

In this process, the positive electrode (the anode [anode: The positive electrode during electrolysis. ]) is made of the impure copper which is to be purified. The negative electrode (the cathode [cathode: A cathode is the electrode (electrical conductor) attached to the negative terminal of a battery. ]) is a bar of pure copper. The two electrodes are placed in a solution of copper(II) sulfate.

The animation shows what happens when electrolysis begins. Copper ions leave the anode and are attracted to the cathode, where they are deposited as copper atoms. The pure copper cathode increases greatly in size, while the anode dwindles away. The impurities left behind at the anode form a sludge beneath it.

The chlor-alkali industry

Brine is concentrated sodium chloride solution. If an electric current is passed through it, hydrogen gas forms at the negative electrode [electrode: Electrodes are conductors used to establish electrical contact with a circuit. The electrode attached to the negative terminal of a battery is called a negative electrode, or cathode. The electrode attached to the positive terminal of a battery is the positive electrode, or anode. ] and chlorine gas forms at the positive electrode. A solution of sodium hydroxide forms.

You might have expected sodium metal to be deposited at the negative electrode. But sodium is too reactive for this to happen, so hydrogen is given off instead.

Electrolysis

Electrolysis of sodium chloride solution

These three products - hydrogen, chlorine and sodium hydroxide - have important uses in the chemical industry:

Hydrogen

making ammonia making margarine

Chlorine

killing bacteria in drinking water killing bacteria in swimming pools

making bleach

making disinfectants

making hydrochloric acid

making PVC

making CFC's - limited production

Sodium hydroxide

making soap making paper

making ceramics

Electrolysis - higher

A half-equation shows you what happens at one of the electrodes during electrolysis. Electrons are shown as e-. A half-equation is balanced by adding, or taking away, a number of electrons equal to the total number of charges on the ions in the equation.

At the negative electrode

Positive ions gain electrons at the negative electrode, so are reduced.

In aluminium extraction: Al3+ + 3e- → Al In copper purification: Cu2+ + 2e- → Cu

Electrolysis of sodium chloride solution: 2H+ + 2e- → H2

At the positive electrode

Negative ions or neutral atoms lose electrons at the positive electrode and are oxidised. For example, chlorine is produced during the electrolysis of sodium chloride solution:

2Cl- - 2e- → Cl2

This half-equation can be rewritten as 2Cl- → Cl2 + 2e-

In aluminium extraction: 2O2- → O2 + 4e- In copper purification: Cu → Cu2++ 2e-

The simulation below shows what happens during the purification of copper by electrolysis.

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Science

Acids, bases and salts

Acids [acids: Corrosive substances which have a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions. ] have a pH of less than 7. Bases [bases: Substances with a pH higher than 7, and which have a high concentration of hydroxyl ions. Bases react with acids to form a salt and water (called neutralisation). Metal hydroxides, oxides and carbonates are all bases. ] have a pH of more than 7. When bases are dissolved in water, they are known as alkalis [alkalis: Bases which are soluble in water. ].

Salts [salts: class of chemical compounds, mostly metallic oxides. Examples are sodium chloride, potassium chloride, and magnesium sulphate ] are made when an acid reacts with a base, carbonate or metal. The name of the salt formed depends on the metal in the base and the acid used. For example, salts made using hydrochloric acid are called chlorides.

Acids and basesYou will have already learnt quite a lot about acids and bases in Key Stage 3 Science. If you feel a bit rusty on this topic, have a quick look at KS3 Bitesize Chemistry Acids and bases. Here are the bare bones of what you need to know:

AcidsSubstances with a pH of less than 7 are acids. The stronger the acid, the lower the pH number. Acids turn blue litmus paper red. They turn universal indicator red if they are strong, and orange or yellow if they are weak.

Bases

Substances that can react with acids and neutralise them to make a salt and water are called bases. They are usually metal oxides or metal hydroxides. For example, copper oxide and sodium hydroxide are bases.

AlkalisBases that dissolve in water are called alkalis. Copper oxide is not an alkali because it does not dissolve in water. Sodium hydroxide is an alkali because it does dissolve in water.

Alkaline solutions have a pH of more than 7. The stronger the alkali, the higher the pH number. Alkalis turn red litmus paper blue. They turn universal indicator dark blue or purple if they are strong, and blue-green if they are weak.

Neutral solutionsNeutral solutions have a pH of 7. They do not change the colour of litmus paper, but they turn universal indicator green. Water is neutral.

Reactions of acidsYou need to be able to describe the reactions of acids with bases, carbonates and metals. You should be able to work out the particular salt formed in the reaction.

Acids and basesWhen acids react with bases, a salt [salt: A compound formed by neutralisation of an acid by a base (eg a metal oxide) - the result of hydrogen atoms in the acid being replaced by metal atoms or positive ions. Sodium chloride - common salt - is one such compound. ] and water are made. This reaction is called neutralisation. In general:

acid + metal oxide → salt + water

acid + metal hydroxide → salt + water

Remember that most bases do not dissolve in water. But if a base can dissolve in water, it is also called an alkali.

CarbonatesWhen acids react with carbonates, such as calcium carbonate (found in chalk, limestone and marble), a salt, water and carbon dioxide are made. In general:

acid + metal carbonate → salt + water + carbon dioxide

Notice that an extra product - carbon dioxide - is made. It causes bubbling during the reaction, and can be detected using limewater. You usually see this reaction if you study the effects of acid rain on rocks and building materials.

Reactive metalsAcids will react with reactive metals, such as magnesium and zinc, to make a salt [salt: A compound formed by neutralisation of an acid by a base (eg a metal oxide) - the result of hydrogen atoms in the acid being replaced by metal atoms or positive ions. Sodium chloride - common salt - is one such compound. ] and hydrogen. In general:

acid + metal → salt + hydrogen

The hydrogen causes bubbling during the reaction, and can be detected using a lighted splint. You usually see this reaction if you study the reactivity series of metals.

Acids, alkalis and neutralisation - higherWhen atoms [atoms: An atom is the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons. ] or groups of atoms lose or gain electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ], charged particles called ions are formed. Ions can be either positively or negatively charged.

For the Higher Tier, you need to know which ions are produced by acids, and which are produced by alkalis. You

will also need to know the ionic equation for neutralisation [neutralisation: Neutralisation is the reaction between an acid and a base to form a salt plus water. ].

AcidsWhen acids dissolve in water they produce hydrogen ions, H+. For example, looking at hydrochloric acid:

HCl(aq) → H+(aq) + Cl-(aq)

Remember that (aq) means the substance is in solution.

AlkalisWhen alkalis dissolve in water they produce hydroxide ions, OH-. For example, looking at sodium hydroxide:

NaOH(aq) → Na+(aq) + OH-(aq)

Ammonia is slightly different. This is the equation for ammonia in solution:

NH3(aq) + H2O(l) → (aq) + OH-(aq)

Be careful to write OH- and not Oh-.

Neutralisation reactionWhen the H+ ions from an acid react with the OH- ions from an alkali, a neutralisation reaction occurs to form water. This is the equation for the reaction:

H+(aq) + OH-(aq) → H2O(l)

If you look at the equations above for sodium hydroxide and hydrochloric acid, you will see that there are Na+ ions and Cl- ions left over. These form sodium chloride, NaCl.

Salt preparationYou need to be able to work out which particular salt [salt: A compound formed by neutralisation of an acid by a base (eg a metal oxide) - the result of hydrogen atoms in the acid being replaced by metal atoms or positive ions. Sodium chloride - common salt - is one such compound. ]

is made in a reaction. You may be asked to describe how to make a salt.

Naming saltsThe name of a salt has two parts. The first part comes from the metal in the base [base: A substance with a pH higher than 7, and which has a high concentration of hydroxyl ions. Bases react with acids to form a salt and water (called neutralisation). Metal hydroxides, oxides and carbonates are all bases. ] or carbonate, or the metal itself if a reactive metal like magnesium or zinc is used.

The second part of the name comes from the acid used to make it. The names of salts made from hydrochloric acid end in -chloride, while the names of salts made from sulfuric acid end in -sulfate.

Formation of saltsMetal   Acid   Saltsodium hydroxidereacts withhydrochloric acidto make sodium chloridecopper oxide reacts withhydrochloric acidto make copper chloridesodium hydroxidereacts withsulfuric acid to makesodium sulfatezinc oxide reacts withsulfuric acid to make zinc sulfateAmmonia forms ammonium salts when it reacts with acids. Therefore:

ammonia reacts with hydrochloric acid to make ammonium chloride

Making saltsIf the base dissolves in water, you need to add just enough acid to make a neutral solution - check a small sample with universal indicator paper - then evaporate [evaporate: Evaporation is a change in state in which a liquid becomes a gas (vapour); molecules near the surface of a liquid may leave the liquid to become a vapour. ] the water. You get larger crystals if you evaporate the water slowly.

Copper oxide, and other transition metal oxides or hydroxides, do not dissolve in water. If the base does not dissolve in water, you need an extra step. You add the base to the acid until no more will dissolve and you have

some base left over (called an excess). You filter the mixture to remove the excess base, then evaporate the water in the filtrate [filtrate: Filtrate is fluid that has passed through a filter. ] to leave the salt behind.

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