Download - Chemical Equation & Reaction

Class 10th

Chemical Reaction and Equation Chemistry

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Q#1 Define symbol. What information is conveyed by symbol?

Ans. A symbol is a brief representation of the name of the element. It consists of either the first letter or the

first letter along with another significant letter of the name of the element.

A symbol conveys the following information:-

1. The name of the element (qualitative significance)

2. One atom of the element (quantitative significance)

3. Atomic mass of the element (quantitative significance)

The concept of symbol was first of all introduced by a Swedish Scientist Berzelius.

Q#2 What is a chemical formula and write the significance of formula.

Ans. The chemical formula of a compound is a brief representation of a substance in terms of symbols of

various elements present in it. For example, formula H2O represents water.

Significance of H2O:-

1. H2O represents water. 2. H2O represents one mole of water. 3. H2O represents one molecule of water. 4. H2O shows that it is made of hydrogen and oxygen. 5. It shows it contains 2-atoms of hydrogen and one atom of oxygen. 6. It shows valency of oxygen is two and the valency of hydrogen is one. 7. It shows molecular mass of water as 18 a.m.u. 8. It indicates that 18 parts of water has 2 parts of hydrogen and 16 parts of oxygen by weight.

Q#3 What is a chemical equation? Give the various steps involved in writing a chemical equation.

Ans. The chemical equation is a statement that describes a chemical change in terms of symbols and

formulae of the reactants and products.

The substances which take part in a chemical reaction are called reactants and are written on the left

hand side in a chemical equation while those substances which form during chemical reaction are called

products and are always written on the right hand side in a chemical equation.

Writing chemical equation:- The different steps of writing an equation are listed below:-

1. The reactants are always written on the left side of an arrow. 2. The products are written on the right hand side of an arrow. 3. The formulae of reactants and products are connected by plus sign (+). 4. The sign of arrow shows the direction in which the reaction is predominant.

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Q#4 What are the essentials of chemical equation?

Ans. Following are the essentials of chemical equations

1. It must represent a true chemical change 2. It must be balanced. 3. It should be molecular.

Q#5 What are the limitation of chemical equation?

Ans. The chemical equation suffers from certain limitations. It does not convey information about the

following facts:-

1. The physical state of the reactants and products. 2. Concentration of the reactants. 3. Time used in the completion of a reaction. 4. Speed of the reaction. 5. Whether the reaction is exothermic or endothermic. 6. Whether the reaction is reversible or irreversible. 7. Essential condition for reaction like temperature, pressure or use of any catalyst. 8. Changes such as precipitation change in colour, evolution of heat, light, light or sound energy

occurring during the chemical reaction.

Q#6 Write pre-knowledge before writing a balanced chemical equation.

Ans. Following knowledge is necessary before one starts writing a balanced chemical equation.

1. Whether or not a chemical reaction takes place between two chemical substances. 2. One must know all the reactants and products formed during a chemical change. 3. One must know the correct symbols and formulae of all the reactants and products.

Q#7 What is the significance of chemical equation.

Ans. To understand the significance of chemical equation, we consider the equation

2H2 +O2 2H2O

The above equations conveys that

1. Hydrogen gas combines with oxygen to form water. 2. Two moles of H react with one mole of oxygen to form two moles of water. 3. 2 molecules of hydrogen react with one molecule of oxygen to form two molecules of water. 4. 4 gms of hydrogen react with 32 gms of oxygen to form 36 gms of water.

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Q#8 What are the various ways in which a chemical equation can be made more informative? Give

examples to illustrate your answer.

Ans. The chemical equations can be made more informative in three ways:

i) By indicating the physical state of the reactants and the products.

ii) By indicating the heat changes taking place in the reaction.

iii) By indicating the conditions under which the reactions take place.

i) By indicating the physical state of the reactants and the products:-

Solid state is indicated by the symbol (s)

Liquid state is indicated by the symbol (l)

Aqueous solution is indicated by the symbol (aq)

Gaseous state is indicated by the symbol (g)

The physical state of the reactants and products are shown by putting the symbols just after their

formulae in the equation given below:

CaCO3 (s) + 2HCl (aq) CaCl2 (aq) + H2O (l) + CO2 (g)

Calcium carbonate Hydrochloric acid Calcium chloride Water Carbon dioxide

ii) By indicating the heat changes taking place in the reaction.

There are two types of reactions on the basis of heat changes involved, exothermic reactions and

endothermic reactions.

An exothermic reaction is indicating by writing +Heat or +Heat energy or +Energy on the products side of an equation as shown below:

C(s) + O2 (g) CO2 (g) + Heat

Carbon Oxygen Carbon dioxide

Similarly, an endothermic reaction is indicated by writing +Heat on the reactants side of an equation as shown in the equation given below

CaCO3(s) + Heat CaO(s) + CO2

Calcium carbonate Calcium oxide Carbon dioxide

iii) By indicating the conditions under which the reactions take place.

If heat is required for a reaction to take place, then sign delta () is put over the arrow of the equation. If a catalyst is required for a reaction to take place, then the symbol or formula of a catalyst is written

above or below the arrow sign in the equation.

2KClO3(s) 2KCl(s) + 3O2 (g)

Potassium chlorate MnO

2 Potassium chloride Oxygen

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Also the conditions of temperature and pressure at which the reaction takes place can also be indicated

in an equation by writing their values above or below the arrow sign in the equation.

CO (g) + 2H2 (g) 300 atm; 3000C CH3OH (l)

Carbon dioxide Hydrogen ZnO + Cr2O3 Methanol

Q#9 What is balanced and unbalanced chemical equation? Give some examples.

Ans. Balanced equation:- A chemical equation in which no. of atoms of each element is same on the side of

reactants and products.

Example.

2Mg + O2 2MgO

Magnesium Oxygen Magnesium oxide

2KNO3 2KNO2 + O2

Potassium nitrate Potassium nitrite

Unbalanced or skeleton equation:- A chemical equation in which number of atoms of various

elements towards the side of reactants is not equal to number of atoms of various elements towards the

side of products is called an unbalanced equation.

Example

KNO3 KNO2 + O2

This equation is wrong as atoms can neither be created nor destroyed by chemical reactions.

Q#10 What do you understand by frequency number in a chemical equation?

Ans. The frequency of occurrence of various elements in an equation is called frequency number. E.g. In

KNO3 KNO2 + O2

frequency of K=2, frequency of Oxygen =3, Frequency of N=2

Rules for balancing a chemical equation

1. Write frequency numbers of all the elements in chemical equation. 2. Start balancing equation from that element which has least frequency no. 3. Other element should be balanced in the order of increasing frequency no. 4. If two or more elements have same frequency no. then balance metallic element first 5. If there are two or more metallic elements with same frequency number, balance the metal with

highest at. no. first. Then balance the next metallic elements with low atomic nos.

6. If there are two or more non-metallic elements, with same frequency number, balance the non-metallic element with highest atomic number first. Other non-metallic element should be balanced

in the decreasing order of atomic numbers.

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Examples of finding frequency number:-

Ca(HCO3)2 + Ca (OH)2 CaCO3 + H2O

f.no. ca=3 f.no of H=3 , f.no of C=2, f.no. of O=4

i. Element Ca H C O

Frequency number 3 3 2 4

Order of balancing each element 2nd

3rd

1st 4

th

Thus the equation is balanced as Ca (HCO3)2 + Ca (OH) 2 2CaCO3 + 2H2O

ii Al2 (CO3)3 + HNO3 Al (NO3)3 +H2O +CO2

Element Al C O H N

Frequency number 2 2 5 2 2

Order of balancing 1st 3

rd 5

th 4

th 2

nd

Thus the equation is balanced as Al2 (CO3)3 + 6HNO3 2Al (NO3)3 + 3CO2 + 3H2O

iii. K2Cr2O7 + H2SO4 K2SO4 +Cr2(SO4)3 +H2O +O2

Element K Cr O H S

Frequency No. 2 2 2 6 2 3

Order of balancing 2nd

1st 5

th 3

rd 4

th

Thus the eqn. is balanced as

2K2Cr2O7 + 8H2SO4 2K2SO4 + 2Cr2(SO4)3 + 8H2O +3O2

Q#11 What is the information conveyed by a balanced chemical equation

Ans A balanced chemical equation is given below

CuO + CO Cu + CO2

It gives us the following informations:-

i. It tells us about the substances (reactants and products) ii. It tells us about the symbols and formulae of all substances involved in a chemical reaction iii. It tells us about the number of atoms or molecules of all substances involved in a chemical

reaction.

iv. It tells us about the weights of all substances involved in a chemical reaction.

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Q#12 What do you understand by chemical reaction? What are the characteristics of the chemical

reaction?(g)

Ans. A chemical reaction is a chemical change in which new chemical substances with new properties are

formed under a set of certain specified and controlled conditions. During chemical reactions,

rearrangements of atoms take place due to the breaking of old bonds and making of new bonds between

the reacting molecules and the product molecules.

The substances which take part in a chemical reaction are called reactants.

The new substances produced in a chemical reaction are called products.

Some examples of chemical reactions are as follows:

1. Calcium carbonate reacts with dilute hydrochloric acid to form calcium chloride, carbon dioxide and water.

CaCO3 + 2HCl CaCl2 + H2O + CO2

Calcium carbonate Hydrochloric acid Calcium chloride Water Carbon dioxide

2. 2H2 + O2 electric spark 2H2O

Hydrogen Oxygen or burning

Water

3. 2Mg + O2 heat 2MgO

Magnesium Oxygen Magnesium oxide (White water)

Chemical reactions require different set of conditions such as, temperature, pressure, presence of

light and presence of catalysts.

Characteristics of chemical reaction:-

Some typical characteristics of chemical reactions are:

1. During the course of a chemical reaction, concentration of the reactants decreases and that of products increases with time.

2. The rate of a chemical reaction depends upon the concentration of the reactants, temperature, pressure and the presence of the catalyst.

3. During the course of a chemical reaction, energy in the form of heat, light or sound is either absorbed or evolved.

The important characteristics of chemical reactions are:

1. Evolution of gas:- Some chemical reactions are characterized by the evolution of gas. For example, when calcium carbonate reacts with dilute hydrochloric acid, then carbon dioxide gas is evolved.

CaCO3 + 2HCl CaCl2 + CO2 + H2O

Similarly, zinc granules react with dilute hydrochloric acid produce hydrogen gas.

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Zn + 2HCl ZnCl2 + H2

2. Change in colour:- Some reactions are characterized by a change in colour. For example when green coloured copper carbonate is heated strongly, a black residue of copper oxide is left.

CuCO3 heat

CuO + CO2

Copper carbonate Copper oxide

(Green) (Black)

When citric acid reacts with potassium permanganate solution, then the purple colour of potassium

permanganate solution disappears.

3. Formation of Precipitate:- Some reaction are characterized by the formation of a precipitate. e.g. when a solution of silver nitrate(colourless) is mixed with a solution of sodium chloride (colourless), a

white precipitate of silver chloride is formed.

AgNO3 + NaCl AgCl + NaNO3

Silver nitrate Sodium chloride Silver chloride (white) Sodium nitrate

Similarly, when dilute sulphuric acid is added to a solution of barium chloride, a white precipitate of

barium phosphate is formed.

BaCl2 + H2SO4 BaSO4 + 2HCl

Barium chloride sulphuric acid Barium sulphate

When hydrogen sulphide gas is passed into the blue solution of copper sulphate, a black precipitate of

copper sulphide is formed.

CuSO4 + H2S CuS + H2SO4

Copper sulphate (Blue) Hydrogen sulphide Copper sulphide (Black) Sulphuric acid

4. Production of energy like heat energy:- Some chemical reaction are characterized by a change in temperature for example,

When water is added to quicklime (calcium oxide), heat is evolved and the water becomes very hot.

CaO + H2O Ca (OH)2 + Heat

Calcium oxide Water Calcium hydroxide

Similarly, when barium hydroxide and ammonium chloride are mixed in a test tube, then barium

chloride, ammonia and water are formed. A lot of heat energy is absorbed during the reaction and the

test tube becomes very cold due to fall in temperature.

Ba(OH)2 + 2NH4Cl BaCl2 + 2NH3 + H2O

5. Change is state:- Some chemical reactions are characterized by the change in state. To show the change in state, s(for solid), l(liquid) , g(for gas) and aq(for aqueous) are written after their symbols or

formulae. Fro example,

Electrolysis of water gives hydrogen and oxygen gases.

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2H2O (l) 2H2 (g) + O2 (g)

Water Hydrogen Oxygen

Similarly, when candle wax is burned, then water and carbon dioxide is formed

Q#13 What are the various types of chemical reactions? Explain by giving examples.

Ans. Types of chemical reactions Chemical reactions are categorized on the basis of chemical changes which occur in them. Chemical

reactions are of various types such as combination reaction, decomposition reaction, displacement

reaction, double displacement reaction and oxidation and reduction reactions (redox reaction)

1. Combination reaction: - In this type of chemical reaction, two or more elements or compounds join together through chemical bonds to form a single substance. A reaction of this type may be represented

generally as

A + B AB

Pictorially, this can be represented as

+

A B AB

Examples:-

1. Nitrogen reacts with hydrogen at high pressure in the presence of iron catalyst o form ammonia. This is called Habers process.

N2 + 3H2 Iron 2NH3

Nitrogen Hydrogen Ammonia

2. Magnesium reacts with oxygen on heating to form magnesium oxide.

2Mg + O2 Heat

2MgO

Magnesium Oxygen Magnesium oxide

3. When iron is heated with sulphur, iron sulphide is formed.

Fe(s) + S(s) Heat

FeS(s)

Iron sulphur Iron sulphide

4. CO2 + H2O H2CO3

Carbon dioxide Water Carbonic acid

5. CaO + H2O Ca(OH)2

Calcium oxide Water Calcium hydroxide

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Decomposition reaction:- It is a chemical change during which a compound breaks down to give two or more

substances. This reaction may be represented generally as

AB A + B

Pictorially, this can be represented as

+

AB A B

Some examples of decomposition reaction are as follows:-

1. Mercuric oxide on heating above 3000C decomposes to form its respective metal and oxygen gas.

2HgO heat 2Hg + O2

Mercuric oxide Mercury Oxygen

2. When copper carbonate is heated, it decomposes into copper oxide and carbon dioxide.

CuCO3 heat CuO + CO2

Copper carbonate Copper oxide

3. 2NaCl Electric current 2Na(s) + Cl2(g)

Sodium chloride

Decomposition reaction may be further classified as.

a. Thermal decomposition:- This is also known as pyrolysis . In this, a chemical compound is broken down by heat

Example,

10000C

CaCO3(s) CaO(s) + CO2 (g)

Calcium Carbonate Calcium Oxide Carbon dioxide

2HgO 3000 C 2Hg + O2

Mercuric oxide Mercuric Oxygen

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2KMnO4 heat

K2Mno4 + MnO2 + O2

Potassium permanganate Potassium manganate Manganese dioxide

b. Electrolytic decomposition:- This is also called electrolysis .This is a process in which a chemical compound, in the molten state or in aqueous solution, is decomposed by the passage of electricity.

Example,

1. 2NaCl (l) Electric

2Na + cl2

Fused sodium chloride Current

sodium metal chlorine gas.

2. 2H2O Electric

2H2 + O2

Current

Photosynthetic decomposition: - This is also known as photosynthesis. This is the breaking up of a

compound brought about by light.

Example

2HCLO Sunlight

2HCl + O2

Hypochlorous acid U.V

Hydrochloric acid Oxygen

2H2O2

Sunlight 2H2O + O2

Hydrogen peroxide. Water Oxygen

AgBr Sunlight

Ag + Br

Silver Bromide Silver Bromine

This reaction is used in photography

Uses of decomposition reaction

1. Thermal decomposition reactions are used in the extraction of less active metals from their ores .

ZnCO3 Heat

ZnO + CO2

Zinc carbonate Zinc oxide Carbon dioxide

ZnO + C heat

Zn + CO

Zinc oxide Coke reduction

Zinc metal Carbon monoxide

2. Electrolytic decomposition is used in the extraction of more active metals from their fused ionic compounds.

2NaCl Electric current

2Na + Cl2

Fused sodium chloride Sodium metal Chlorine gas

at cathode at anode

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Decomposition reactions in our body:- Food which we eat consists of highly complex organic compounds

such as carbohydrates, fats and proteins. These compounds are broken down in our digestive system. For

example, the carbohydrates and fats break down to form simple substances like glucose and fructose.

Similarly, the proteins break down to amino acids.

Displacement reaction or substitution reaction:- When a more active element displaces a less active element

from its aqueous ionic compound, the reaction which takes place is called displacement or substitution reaction.

It may be represented as

A + BC AC + B

This can be pictorially represented as.

+ + +

A BC AC B

The ability of an element to displace another is determined by its relative position in the reactivity series. A

more electropositive metal displaces a less electropositive metal from its aqueous solution. Similarly, a more

electronegative element displaces a less electronegative element.

Some of the examples are given below.

1. When a piece of iron is placed in copper sulphate solution, then ferrous sulphate solution and copper are formed.

CuSO4 + Fe FeSO4 + Cu

Copper sulphate (blue) Iron Ferrious sulphate (green) Copper

In this Fe displaces Cu from copper sulphate solution. The blue colour of CuSO4 solution fades away and a

pale green. Solution of ferrous sulphate is formed. Reddish brown copper metal gets deposited on the iron

piece.

2. CuSO4 + Mg MgSO4 + Cu

Copper sulphate Magnesium Magnesium sulphate Copper

(Blue) (colourless)

Here magnesium displaces copper from the copper sulphate solution.

3. 2AgNO3 + Cu Cu(NO3)2 + 2Ag

Silver nitrate Copper nitrate Silver

(Colourless solution) (Blue solution)

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4. Zn (s) + 2HCl(dil) ZnCl2(aq) + H2(g)

Zinc hydrochloric acid Zinc chloride Hydrogen

Activity Series:- The activity series is an arrangement of elements in the decreasing order of the

electropositive character. The activity series of metals is given in below

Activity Series

K

Na

Ba

Ca

Mg

Al

Zn

Cr

Fe

Co

Ni

Sn

Pb

H

Cu

Hg

Ag

Au

Pt

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Double Displacement Reactions:-

A chemical reaction in which two ionic compounds in their aqueous solutions react by exchanging their ions/

radicals to form two new compounds is called double displacement reaction. It is also called partner exchange

reaction. It can be represented as

+ - + - +- +-

AB + CD AD + CB

Solution Solution

This may be pictorially represented as

+ +

A B C D AD CB

These reactions are of two types:

Precipitation reactions: - In these reactions, two compounds react in their aqueous state to form an insoluble

product which appears in the form of a precipitate.

Some examples of precipitation reaction are given below:

1. Aqueous sodium chloride and silver nitrate react to form a white precipitate of silver chloride.

NaCl + AgNO3 AgCl + NaNO3

Sodium chloride Silver nitrate Silver chloride Sodium nitrate

(White ppt)

2. Aqueous copper sulphate and sodium hydroxide react to form a blue precipitate of copper hydroxide

CuSO4 + 2NaOH Cu(OH)2 + Na2SO4

Copper sulphate Sodium hydroxide Copper hydroxide SodiumSulphate

(sky blue)

3. BaCl2(aq) + CuSo4(aq) BaSO4 + CuCl2(aq)

Barium chloride Copper sulphate Barim sulphate Copper chloride

(White ppt.)

Neutralization reactions:- In these reactions and acid reacts with a base ; by exchanging their radicals, to

form salt and water only.

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Some examples are given below:

1. Sulphuric acid reacts with sodium hydroxide solution to form sodium sulphate(salt ) and water. H2SO4 + 2NaOH Na2SO4 + 2H2O

Sulphuric acid Sodium hydroxide Sodium sulphate water

2. HCl + KOH KCl + H2O Hydrochloric acid Potasium hydroxide Potassium chloride water

Reversible reactions:- These reactions which do not go completely from the reactants towards the products

because the products too react among themselves to regenerate the reactants.

After sometimes, a reversible reaction comes to a state of equilibrium in which the forward and backward

reactions take place at the same speed and the amounts of the amounts of the compounds remain unchanged.

Examples,

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

N2(g) + O2(g) 2 NO(g)

2SO2 + O2 2 SO3

Q#14 What is oxidation and reduction reaction? Explain with examples.

Ans. Oxidation and reduction reactions:- According to the earlier concept, the oxidation of a substance

takes place when:

1. There is addition of oxygen to a substance.

2. There is addition of a non metal to a substance. 3. There is removal of hydrogen from a substance. 4. There is removal of metal from a substance.

Oxidising agents:- The substance is an oxidizing agent if :

1. It supplies oxygen from oxidation.

2. It supplies a non metal from oxidation. 3. It removes hydrogen from another substance. 4. It removes metal from another substance.

Reduction:- The reduction of a substance takes place when;

1. There is addition of hydrogen to a substance. 2. There is addition of metal to a substance.

3. There is removal of oxygen from a substance. 4. There is removal of non metal from a substance.

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Reducing agent:- The substances is a reducing agent if;

1. It supplies hydrogen for reduction. 2. It supplies metal for reduction. 3. It removes oxygen from another substance. 4. It removes non metal from another substance.

Example;

1. When hydrogen gas is passed over heated copper oxide, the copper metal and water are formed.

Reduced

CuO + H2 Cu + H2O

Oxidised

In this, removal of oxygen takes place from copper oxide. So, copper oxide is reduced to

copper. Similarly, addition of oxygen takes place in hydrogen. So, hydrogen is oxidized to water.

In the above reaction, hydrogen removes oxygen from copper oxide; therefore, hydrogen is a

reducing agent. Moreover, copper oxide is the supplier of oxygen; therefore copper oxide is an

oxidizing agent.

Oxidized (removal of hydrogen)

2. H2S + Br2 2HBr + S Hydroigen sulphide Bromine Hydro bromic acid Sulphur

Reduced (addition of hydrogen)

1. When chlorine gas is passed through potassium iodide solution, it forms potassium chloride and iodine.

2KI + Cl2 2KCl + I2

Potassium iodide Chlorine Potassium chloride Iodine

In the above reaction, potassium metal is removed from potassium iodide; therefore, potassium iodide is

oxidized to iodine. Also, the addition of potassium metal takes place in chlorine. Therefore, chlorine is

reduced to potassium chloride.

Morever, potassium iodide supplies metallic element potassium, therefore it is a reducing agent.

Chlorine removes metallic element potassium from potassium iodide; therefore it is an oxidizing agent.

Electronic concept of oxidation and reduction:-

Oxidation:- When an atom or an ion looses an electron /electrons, the oxidation is said to take place.

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Examples:-

1. Na - e Na+ Sodium metal is oxidized to sodium ion.

2. Fe2+ e Fe3+

Ferrous ion is oxidized to ferric ion.

Reduction:- when an atom or an ion gains an electron/electrons, the reduction is said to taken place.

Examples:-

1. Cl + e Cl

Chlorine atom is reduced to chlorine ion.

2. Fe3+ + e Fe

2+

Ferric ion is reduced to ferrous ion.

Redox Reaction:- A Chemical reaction in which loss of electrons and gain of electrons takes place

simultaneously is called redox reaction.

Example:-

Sodium metal reacts with chlorine to form sodium chloride.

2Na + Cl2 2NaCl

Sodium chlorine sodium chloride.

The reaction can be written as,

2Na 2e 2N+

Cl2 + 2e

2Cl

In this sodium metal is oxidized to sodium ion and chlorine is reduced to chlorine ion.

Q#15 What are the effects of oxidation reactions in everyday life?

Ans. There are two common effects of oxidation reactions which we observe in our daily life are:

i) Corrosion of metals ii) Rancidity of food.

Corrosion :- Is the process in which metals are eaten up gradually by the action of air, moisture or a

chemical on their surface.

Rusting of iron metal is the most common form of corrosion. In this, iron metal is oxidized by the

oxygen of air in the presence of moisture to form hydrated iron (III) oxide called rust.

4Fe + 3O2 + 2.x.H2O 2Fe2O3.xH2O

Iron Oxygen Water Rust

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(Iron (III) oxide

The no. of water molecules (x) in the rust varies. Rust is a soft and porous substance which gradually

falls off from the surface of an iron object and hence weakens and eats up the whole iron object.

Rancidity:- The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant

smell and taste is called rancidity. Rancidity spoils the food material such as potato chips, fried items

etc. prepared in fats and oils which have been kept for a considerable time and makes them unfit for

eating.

The rancidity of food can be prevented or retarded in the following ways:

1. Rancidity can be prevented by adding antioxidants like BHA (Butylated Hydroxy-Anisole) and BHT (Butylated Hydroxy-toluene) to foods containing fats and oils so that fats and oils do not get oxidized

easily.

2. Rancidity can be prevented by packaging fats and oils containing foods in nitrogen gas. This prevents the food from being oxidized and hence do not turn rancid.

3. Rancidity can be retarded by keeping food in a refrigerator which slows down the oxidation of fats and oils due to low temperature.

4. Rancidity can be retarded by storing the food in air-tight containers where exposure of oxygen is very much reduced.

5. Rancidity can be retarded by storing foods away from light.

Text Book Questions

Q#1 Why should a magnesium ribbon be cleaned before burning in air?

Ans. Magnesium ribbon forms a coating of basic magnesium carbonate on its surface by the slow action of

moist air. This layer hinders the burning of magnesium. Hence it is cleaned by rubbing with a sand

paper before burning in air.

Q#2 Write the balanced equation for the following chemical reaction.

a. Hydrogen + Chlorine Hydrogen chlorine

b. Barium chloride + Aluminium sulphate Barium sulphate + Aluminium chloride

c. Sodium + Water Sodium hydroxide + Hydrogen

Ans. i) H2 + Cl2 2HCl

ii) 3BaCl2 + Al2(SO4)3 3BaSO4 + 2AlCl3

iii) 2Na + 2H2O 2NaOH + H2

Q#3 Write a balanced chemical equation with state symbols for the following reactions:

i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium

sulphate and the solution of sodium chloride.

ii) Sodium hydroxide solution in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

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Ans. i) BaCl2(aq) + Na2SO4(aq) BaSO4(s) + 2NaCl(aq)

ii) NaOH(aq) + HCl (aq) NaCl(aq) + H2O(l)

Q#4 A solution of a substance X is used for white washing.

i) Name the substance X and write its formula.

ii) Write the reaction of the substance X named (i) above with water.

Ans. i) The substance X is quicklime and its formula is CaO.

ii) CaO(s) + H2O(l) Ca(OH)2(aq)

Q#5 Why the amount of gas is collected in one of test tubes in Activity 6.7 is double of the amount collected

in the other? Name this gas.

Ans. Since the electrolysis of water produces 2 volumes of hydrogen gas and 1 volume of oxygen gas i.e. in

the ratio of 2 : 1.

2H2O(l) Electrolysis

2H2(g) + O2(g)

Thus one of the gases collected in one of the test tubes is double than the other gas and the gas is

hydrogen.

Q#6 Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Ans. Iron displaces copper from copper sulphate solution as shown by the equation given below;

CuSO4(aq) + Fe(s) FeSO4(aq) + Cu(s)

Blue Light green

The deep blue colour of copper sulphate solution fades due to the formation of light green solution of

iron sulphate.

Q#7 Give an example of a double displacement reaction other than the one given in Activity 6.10.

Ans. AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

Silver nitrate Sodium Chloride Silver chloride Sodium nitrate

Q#8 Identify the substances that are oxidized and the substances that are reduced in the following reactions:

(i) 4Na(s) + O2(g) 2Na2O(s)

(ii) CuO (s) + H2(s) Cu(s) + H2O(l)

Ans. i) Na has gained oxygen to form Na2O. But addition of oxygen is called oxidation, so sodium (Na)

is oxidized to sodium oxide (Na2O).

Now, O2 is changing into Na2O by the addition of sodium metal. But addition of metal is reduction, so

oxygen (O2) is reduced to Na2O.

ii) CuO(s) + H2(g) Cu(s) + H2O(l)

Copper oxide Hydrogen Copper Water

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(Black) (Red)

Copper oxide is reduced to copper as CuO has lost oxygen to form Cu. However, H2 has gained oxygen

to form H2O, therefore H2 is oxidized to water.

Exercise

Q#1 Which of the statements about the reaction below are incorrect?

2PbO(s) + C(s) 2Pb(s) + CO2 (g)

a) Lead is getting reduced.

b) Carbon dioxide is getting oxidized.

c) Carbon is getting oxidized.

d) Lead oxide is getting reduced.

(I) (a) and (b)

(II) (a) and (c)

(III) (a) (b) and (c)

(IV) All

Ans. i) (a) and (b)

Pbo has lost oxygen to form pb. Therefore, lead dioxide is getting reduced.

Carbon has gained oxygen to form CO2; therefore carbon is getting oxidized to CO2.

Q#2 Fe2O3 + 2Al Al2O3 + 2Fe

The above reaction is an example of a

a) Combination reaction

b) Double displacement reaction.

c) Decomposition reaction.

d) Displacement reaction.

Ans. Displacement reaction

Q#3 What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

a) Hydrogen gas and iron chloride are produced

.

b) Chlorine gas and Iron hydroxide are produced.

c) No reaction takes place.

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d) Iron salt and water are produced.

Ans. The following reaction takes place

Fe + 2HCl FeCl2 + H2

Thus, hydrogen and iron chloride are produced.

Q#4 What is a balanced chemical equation? Why should chemical equations be balanced?

Ans. A chemical equation in which number of atoms of each element is same on the side of reactants and

products is called balanced equation.

Example, 2Mg + O2 2MgO

According to the law of conservation of mass, the total mass of the elements present in the reactants has

to be equal to the total mass of the elements present in the products. Hence, we should balance a

skeletal chemical equation.

Q#5 Translate the following statements into chemical equations and then balance them:

a) Hydrogen gas combines with nitrogen to form ammonia.

b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Ans. a) N2 + 3H2 2NH3

b) H2S + 3O2 2H2O + 2SO2

c) 3 BaCl2 + Al2(SO4)3 2AlCl3 + 3BaSO4

d) 2K + 2H2O 2KOH + H2

Q#6 Balance the following chemical equations:

a) HNO3 + Ca(OH)2 Ca(NO3)2 + H2O

b) NaOH + H2SO4 Na2SO4 + H2O

c) NaCl + AgNO3 AgCl + NaNO3

d) BaCl2 + H2SO4 BaSO4 + HCl

Ans. a) 2HNO3 + Ca(OH)2 Ca(NO3)2 + 2H2O

b) 2NaOH + H2SO4 Na2SO4 + 2H2O

c) NaCl + AgNO3 AgCl + NaNO3

d) BaCl2 + H2SO4 BaSO4 + 2HCl

Q#7 Write the balanced chemical equations for the following reactions:

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a) Calcium hydroxide + Carbon dioxide Calcium carbonate + Water

b) Zinc + Silver nitrate 4Zinc nitrate + Silver

c) Aluminium + Copper chloride Aluminium chloride + Copper

d) Barium chloride + Potassium sulphate Barium sulphate + Potassium chloride

Ans. a) Ca(OH)2 + CO2 CaCO3 + H2O

b) Zn + 2AgNO3 Zn(NO3)2 + 2Ag

c) 2Al + 3CuCl2 2AlCl3 + 3Cu

d) BaCl2 + K2SO4 BaSO4 + 2KCl

Q#8 Write the balanced chemical equation for the following and identify the type of reaction in each case:

a) Potassium bromide (aq) + Barium iodide?(aq) Potassium iodide (aq) + Barium bromide(s)

b) Zinc carbonate (s) Zinc oxide (s) + Carbon dioxide (g)

c) Hydrogen (g) + Chlorine (g) Hydrogen chloride(g)

d) Magnesium (s) + Hydrochloric acid(aq) Magnesium chloride (aq) + Hydrogen (g)

Ans. a) 2KBr(aq) + BaI2(aq) 2KI(aq) + BaBr2(aq)

It is double displacement reaction

b) ZnCO3(s) ZnO(s) + CO2(g)

It is decomposition reaction

c) H2(g) + Cl2(g) 2HCl(g)

It is a combination reaction.

d) Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

Displacement reaction

Q#9 What does one mean by exothermic and endothermic reactions? Give examples.

Ans. Exothermic reaction:- Those reactions which take place with the evolution of heat are called

exothermic reactions. In these reactions, the total heat energy absorbed by the reactants to break old

bonds is less than the total heat energy released by the products on the formation of new bonds.

Example,

The burning of coal in air to form CO2 is exothermic reaction

i. C + O2 CO2 + Heat

ii. N2 + 3H2 2NH3 + Heat

iii. 2Mg + O2 2MgO + Heat

iv. CH4 + O2 CO2 + H2O + Heat

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Endothermic reactions:- Those reactions which take place with the absorption of heat are called

endothermic reactions. In these reactions, the total heat energy absorbed by the reactants is more than

the total heat energy released by the products.

Examples:

Formation of nitric oxide (NO) by the combination of N2 and O2 at 30000C is an endothermic reaction.

i. N2 + O2 + Heat 2NO

ii. CaCO3 + Heat CaO + CO2

iii. 2KClO3 + Heat 2KCl + 3O2

iv. C + 2S + Heat CS2

Q#10 Why respiration is considered an exothermic reaction? Give examples.

Ans. During respiration, glucose combines with oxygen in the cells of our body to form carbon dioxide and

water along with the production of energy.

C6H12O6 (aq) + 6O2(g) 6CO2(g) + 6H2O(l) + E

Since energy is produced during this process, therefore it is an exothermic reaction.

Q#11 Why decomposition reaction is called the opposite of combination reaction?

Ans. In decomposition reaction, a single compound splits to give two or simpler substances whereas in a

combination reaction two or more simpler substances combine together to form a single compound.

Hence, they are opposite to each other.

Examples of decomposition reactions:

i) CaCO3(s) Heat

CaO(s) + CO2(g)

ii) 2HgO(s) Heat

2Hg(l) + O2(g)

Examples of combination reaction:

i) 2Mg(s) + O2(g) 2MgO(s)

ii) C(s) + O2(g) CO2(g)

Q#12 Write one equation each for decomposition reaction where energy is supplied in the form of heat, light

or electricity.

Ans. i) Decomposition reaction involving absorption of heat.

2HgO heat

2Hg(l) + O2(heat)

ii) Decomposition reaction involving absorption of light.

2AgCl Sunlight

2Ag(s) + Cl2(g)

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iii) Decomposition reaction involving absorption of electricity.

2H2O(l) Electric 2H2(g) + O2(g)

Current

Q#13 What is the difference between displacement and double displacement reaction? Write equations

for these reactions.

Ans. In a displacement reaction, a more active element displaces a less active element from its aqueous ionic

compound, where as in a double displacement reaction, two ionic compounds in their aqueous solutions

react by exchanging their ions/ radicals to form two new compounds.

Example for displacement reaction

Mg(s) + CuSO4(aq) MgSO4(aq) + Cu(s)

Magnesium Copper sulphate Magnesium sulphate Copper

(Blue) (Colourless)

Example for double displacement reaction

AgNO3(aq) + NaCl(aq) AgCl + NaNO3(aq)

Silver nitrate Sodium chloride Silver chloride sodium nitrate

(White ppt.)

Q#14 The refining of silver, the recovery of silver from silver nitrate solution involved displacement by

copper metal. Write down the reaction involved.

Ans. 2AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag(s)

Q#15 What do you mean by a precipitation reaction? Explain by giving examples.

Ans. Precipitation reactions: - In these reactions, two compounds react in their aqueous state to form an

insoluble product which appears in the form of a precipitate.

Some examples of precipitation reaction are given below:

1. Aqueous sodium chloride and silver nitrate react to form a white precipitate of silver chloride. NaCl + AgNO3 AgCl + NaNO3

Sodium chloride Silver nitrate Silver chloride Sodium nitrate

(White ppt)

2. Aqueous copper sulphate and sodium hydroxide react to form a blue precipitate of copper hydroxide CuSO4 + 2NaOH Cu(OH)2 + Na2SO4

Copper sulphate Sodium hydroxide Copper hydroxide SodiumSulphate

(sky blue)

3. BaCl2(aq) + CuSo4(aq) BaSO4 + CuCl2(aq) Barium chloride Copper sulphate Barim sulphate Copper chloride

(White ppt.)

Q#16 Explain the following in terms of gain or loss of oxygen with two examples each.

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a) Oxidation

b) Reduction

Ans. Oxidation:- Oxidation is a process which involves addition of oxygen

Examples

i) Burning of carbon in the presence of oxygen

C(s) + O2(g) Oxidation

CO2(g)

ii) Burning of magnesium in the presence of oxygen

2Mg(s) + O2(g) Burning

2MgO(s)

Reduction:- Reduction is a process that involves removal of oxygen.

Examples:

i) Heating zinc oxide with carbon

ZnO(s) + C(s) Heat

Zn(s) + CO (g)

Reduced (Removal of oxygen)

ii) Copper oxide is heated with hydrogen gas

CuO(s) + H2(g) Heat

Cu + H2O

Here copper oxide is reduced to copper.

Q#17 A shiny brown coloured element X on heating in air becomes black in colour. Name the element X and the black coloured compound formed.

Ans. The element X is copper and the black coloured compound formed is copper oxide (CuO) .Tthe reaction is given below:

2Cu(s) + O2(g) Heat

2CuO(s)

Q#18 Why do we apply paint on iron articles?

Ans. We apply paint on iron articles because air and moisture do not come in contact with the iron objects

and hence no rusting takes place.

Q#19 Oil and fat containing food items are flushed with nitrogen. Why?

Ans. Oil and fat containing food items are flushed with nitrogen gas to prevent from being oxidized and

hence the food items do not turn rancid.

Q#20 Explain the following terms with one example each.

a) Corrosion

b) Rancidity

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Ans. Corrosion: - Is the process in which metals are eaten up gradually by the action of air, moisture or a

chemical on their surface.

Rusting of iron metal is the most common form of corrosion. In this, iron metal is oxidized by

the oxygen of air in the presence of moisture to form hydrated iron (III) oxide called rust.

4Fe + 3O2 + 2.x.H2O 2Fe2O3.xH2O

Iron Oxygen Water Rust

(Iron (III) oxide

The no. of water molecules (x) in the rust varies. Rust is a soft and porous substance which gradually falls off

from the surface of an iron object and hence weakens and eats up the whole iron object.

Rancidity:- The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell

and taste is called rancidity. Rancidity spoils the food material such as potato chips, fried items etc. prepared in

fats and oils which have been kept for a considerable time and makes them unfit for eating.

The rancidity of food can be prevented or retarded in the following ways:

1. Rancidity can be prevented by adding antioxidants like BHA (Butylated Hydroxy-Anisole) and BHT (Butylated Hydroxy-toluene) to foods containing fats and oils so that fats and oils do not get oxidized

easily.

2. Rancidity can be prevented by packaging fats and oils containing foods in nitrogen gas. This prevents the food from being oxidized and hence do not turn rancid.

3. Rancidity can be retarded by keeping food in a refrigerator which slows down the oxidation of fats and oils due to low temperature.

4. Rancidity can be retarded by storing the food in air-tight containers where exposure of oxygen is very much reduced.

5. Rancidity can be retarded by storing foods away from light.

Q List of common metals in the increasing order of their atomic numbers/ atomic masses.

Metals Atomic Number Atomic Mass

Na 11 23

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Mg

Al

K

Ca

Cr

Mn

Fe

Ni

Cu

Zn

Ag

Ba

Au

Hg

Pb

12

13

19

20

24

25

26

28

29

30

47

56

79

80

82

24

27

39

40

52

55

56

59

64

65

108

137

197

201

207

Q List of common non- metals in the increasing order of their atomic numbers/ atomic masses.

Non-metals Atomic Number Atomic Mass

H

C

N

O

F

Si

P

S

Cl

Br

I

1

6

7

8

9

14

15

16

17

35

53

1

12

14

16

19

28

31

32

35

80

127

SOURCE;http://khalidnext.webs.com

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