CHEMICAL REACTIONS

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Physical Changes

Physical Changes

Physical changes are changes in physical properties such as volume or density, or changes in state such as changes from solid to liquid or from liquid to gas.

• Physical changes do not involve the production of new substances.

• Physical changes are often readily reversible.

• When compared with chemical changes, only small energy changes are involved in physical processes.

Following are the examples of physical changes:Changes of state

Changes in shape and size

Dissolve a solute in a solvent

Filtration

Chemical Changes

Chemical Change

Chemical changes are those in which new substances with different compositions and properties are formed.

• In a chemical change reactants are converted into products.

• Chemical changes are often very difficult to reverse.

Examples:• The combustion of petrol or diesel in car engines and methane in Bunsen burner are

examples of chemical changes. In both cases, new substances (carbon dioxide and water) are produced, and heat and light energy are released.

• When an electric current is passed through water, two new substances, hydrogen gas and oxygen gas, are produced.

Chemical Changes

Which of the following processes is a chemical change?

Physical and Chemical Change

Energy Changes Associated with Physical and Chemical Change

In general, the energy changes associated with physical processes such as boiling, melting and dissolving are much smaller than those associated with chemical changes.

BoilingWhen liquid water is boiled the water molecules gain sufficient kinetic energy to escape from the liquid surface and enter the air as a vapour. The molecules of water vapour are identical to those in the liquid water. This process is an example of a physical change. Energy measurements show that 44 kJ of energy is required to boil 18 g of water at 100oC. This physical change is easy to reverse by cooling the water vapour.

ElectrolysisWhen water is electrolysed the covalent bonds holding the atoms together are broken and new products form. This chemical change requires much more energy than the physical change involved in boiling the water. Measurements show that 286 kJ of energy is required to electrically decompose 18 g of water. This is much greater than the energy required for boiling. This chemical change is very difficult to reverse compared with the physical change involved in boiling.

Electron Dot Diagram

Representation of some ionic compounds by Lewis dot formulae or electron dot diagram

Formation of magnesium oxideElectronic configuration of Magnesium (12) → 2, 8, 2Electronic configuration of Oxygen (8) → 2, 6

Formation of calcium fluorideElectronic configuration of Calcium (20) → 2, 8, 8, 2 Electronic configuration of Fluorine (9) → 2, 7

Few More Examples of Electron Dot Diagram

Hydrogen(H2)

Oxygen(O2)

Nitrogen(N2)

Water(H2O)

Methane(CH4)

Chemical Formulae and Compounds

Compound: Compounds consist of two or more elements that are chemically combined. The formulas of compounds contain the symbols of the elements involved, and the subscripts indicate the relative numbers of atoms of each element in the compound.

Since atoms of elements combine to form molecules, it should be possible to represent the molecule in terms of symbols of the constituent atoms. This symbolic expression for a molecule is called a Formula.

Example of Formulae of Few Elements and CompoundsNitrogen, Chlorine, Carbon Dioxide, Carbon Monoxide

Chemical Formulae and Compounds

Compound Formula Compound Formula

Hydrochloric acid HCl Carbon monoxide CO

Sulphuric acid H2SO4 Carbon dioxide CO2

Sulphurous acid H2SO3 Silica (sand) SiO2

Nitric acid HNO3 Caustic soda(Sodium hydroxide)

NaOH

Nitrous acid HNO2 Caustic potash(Potassium hydroxide)

KOH

Phosphoric acid H3PO4 Washing soda(Sodium carbonate)

Na2CO3

Boric acid H3BO3 Baking soda(Sodium bicarbonate)

NaHCO3

Sulphur dioxide SO2 Limestone or marble(Calcium carbonate)

CaCO3

Sulphur trioxide SO3 Water H2O

Nitrous oxide N2O Sulphuretted hydrogen(Hydrogen Sulphide)

H2S

Nitric oxide NO Ammonia NH3

Nitrogen trioxide N2O3 Phosphine PH3

Nitrogen dioxide NO2 Methane CH4

Nitrogen pentoxide N2O5    

Chemical Formulae and Compounds

VALANCY Consider the following molecules: HCl - Hydrogen chloride (Hydrochloric acid) H2O (Water) NH3 (Ammonia) CH4 (Methane)

Looking at these molecules, we find that while one chlorine atom combines with only one hydrogen atom, one oxygen atom combines with two, one nitrogen atom combines with three while one carbon atom combines with four hydrogen atoms, i.e., the atoms of chlorine, oxygen, nitrogen and carbon show different combining capacities.

This combining capacity of an atom or a radical is called its valency. It is measured in terms of hydrogen atoms or oxygen atoms. Valency of an atom or a radical is the number of hydrogen atoms or double the number of oxygen atoms which will combine with it.Examples: HCl, H2SO4, SO2 , H2S, NH3

Chemical Formulae and Compounds

HOW TO WRITE A FORMULA? In the formation of a chemical compound, combination takes place between the positive and the negative radicals in such a way that the product of valency and the number of radicals is the same for both the radicals. Thus to write the formula of a compound we proceed as follows:(i) Write the symbols of the two radicals side by side with valencies at the top, the positive radical to the left and negative radical to the right.(ii) Cross the valencies after removing the common factor, if any. The numbers are to be placed to the lower right of each symbol. The compound radical must be enclosed within a bracket and the number placed outside the bracket to the lower right.

Example 1: For writing the formula of sodium sulphate, proceeding as above we have:(i) Writing symbols side by side with valencies at the top.Na1+SO4

2-

(ii) There being nothing common in 1 and 2, crossing the valencies we get the desired formula:Na2SO4

Example 2: Similarly for writing the formula of barium carbonate, we have the two steps:(i) Writing the symbols side by side:Ba2+CO3

2-

(ii) Removing the common factor 2, and crossing the valencies we get the desired formula:BaCO3

Chemical Formulae and Compounds

Identify the correct formulae for the following compounds.(a) chromium(III) oxide (b) potassium nitrate(c) magnesium carbonate (d) mercury(II) chloride

Chemical Formulae and Compounds

TYPES OF COMPOUNDSCompounds are classified in following three groups on basis of their bonding and structure:

1. Covalent Molecular Compounds2. Covalent Network Compounds3. Ionic Compounds

Chemical Formulae and Compounds

Covalent Molecular CompoundsIn covalent molecular compounds the formula represents the number of atoms of each element in one molecule of the compound. Covalent molecular compounds are formed by covalent bonding between different atoms. Covalent bond is a bond formed when two atoms share one or more electron pairs. Each atom contributes equal number of electron(s) towards the bond formation.Examples:NH3 – ammonia

CH4 – methane or carbon tetrahydride

CCl4 – tetrachloromethane (carbon tetrachloride)

CO2 – carbon dioxide

H2O – dihydrogen oxide (water)

Chemical Formulae and Compounds

Covalent Network CompoundsCovalent network compounds do not occur as simple molecules. For example, silicon dioxide contains silicon and oxygen atoms covalently bonded together in an extended three-dimensional network structure as shown in the figure below.

Chemical Formulae and Compounds

Ionic CompoundsIonic compounds consist of oppositely charged ions held together by electrostatic attraction to form a crystal lattice. The electrostatic attraction between the oppositely charged ions is called ionic bonding. The ions in an ionic solid are arranged in a regular three-dimensional lattice. The structure of sodium chloride is shown in the figure below.