Chemical reaction

A chemical reaction causes chemical change in the

substances. There are 2 types of changes take place in matter.

They are physical change and chemical change.

Chemical reaction

A chemical change that takes place by addition,

decomposition or displacement of the atoms or molecules of the

matter is called chemical reaction.

Physical change Chemical change

i. It is a temporary change in which

no new substance is formed.

i. It is a permanent change in which

new substances are formed.

ii. It is reversible change. ii. It is irreversible change.

iii. There is only change in the

physical states of substance.

iii. There is change in both

physical and chemical

composition of the substances.

Day 1

A chemical reaction involves both reactants and products.

The substances that undergo a chemical change during a

chemical reaction are called reactants. They are written in the left

side of the arrow in a chemical equation.

The substances that are formed as a result of chemical

change in the reactants are called products. They are written in

the right side of the arrow in a chemical equation.

Chemical equation

The symbolic representation of an actual chemical reaction

in terms of symbol and molecular formula is called chemical

equation.

Zn + 2HCl ZnCl2 + H2

Essentials of chemical equation

Chemical equation must satisfy the following conditions:

(i) It must represent a true chemical reaction.

For example , the following equation is not experimentally

possible. Cu + H2SO4 CuSO4 + H2

(ii) It must be balanced.

The number of atoms of each element in reactants must be

equal to the number of atoms of the same element in the product.

Such reaction is called balanced chemical equation. The equation

which is not balanced is called skeleton or unbalanced equation.

Zn + HCl ZnCl2 + H2 (Unbalanced)

Zn + 2HCl ZnCl2 + H2 (Balanced)

(iii) It must be molecular.

Metals are written in atomic form and other compounds are

written in molecular form. Molecules of H, N, O, F, Cl, Br and I

should be written as H2, N2, O2, F2, Cl2, Br2 and I2 respectively

(when written separately).

Limitations of a chemical equation

i. It does not tell us about the conditions such as temperature,

pressure, light, catalyst, etc required for a chemical reaction.

ii. It does not tell about the physical state of reactants and

products.

iii. It does not tell whether heat is absorbed or evolved in the

reaction.

iv. It does not tell about the concentration of reactants and

products.

v. It does not tell about the rate of reaction.

vi. It does not tell about the mechanism of reaction.

Types of chemical reaction

1. Combination reaction

2. Decomposition reaction

3. Displacement reaction

4. Acid base reaction

Chemical reaction

Day 2

Combination or synthesis reaction

The chemical reaction in which two or more reactants

combine together to form a single product is called combination

reaction.

A + B AB

Sodium + Chlorine Sodium chloride

2Na + Cl2 2NaCl

Iron + Oxygen Ferric oxide

4Fe + 3 O2 2Fe2O3

Decomposition or analysis reaction

The chemical reaction in which a reactant decomposes to

give two or more products is called decomposition reaction.

AB A + B

Calcium carbonate Calcium oxide + Carbon dioxide

CaCO3 CaO + CO2

Potassium chlorate Potassium chloride + Oxygen

2KClO3 2KCl + 3O2

Displacement reaction

The chemical reaction in which an atom or radical in a molecule

is replaced by another atom or radical to form new products is called

displacement reaction. There are two types of displacement reaction.

They are single and double displacement reaction.

Single displacement reaction

The chemical reaction in which an atom in a molecule is

replaced by another atom is called single displacement reaction.

A + BC AC + B

Calcium + Sulphuric acid Calcium sulphate + Hydrogen

Ca + H2SO4 CaSO4 + H2

Double displacement reaction

The chemical reaction in which atoms or radicals of the reactants

are exchanged to form new substances is called double displacement

reaction.

AB + CD AD + CB

Sodium chloride + Silver nitrate Sodium nitrate + Silver chloride

NaCl + AgNO3 NaNO3 + AgCl

Acid base reaction

The chemical reaction in which acid combines with base to

form salt and water is called acid base reaction. It is also called

neutralization reaction. This is because acid loses acidity and

base loses basicity and form neutral substances.

Acid + Base Salt + Water

Hydrochloric + Sodium Sodium + Water

acid hydroxide chloride

HCl + NaOH NaCl + H2O

Sulphuric acid + Ferrous oxide Ferrous sulphte + Water

H2SO4 + FeO FeSO4 + H2O

Effect of temperature in the rate of chemical reaction

The increase in temperature of the reactants, increases the

kinetic energy of the molecules and the rate of collision between

the reactants increases. As a result, the rate of chemical reaction

increases.

The solution turns in pink colour when potassium

permagnet solution is mixed in a mixture containing oxalic acid

and dilute sulphuric acid. The pink colour will be disappeared

quickly when the container is heated in a burner.

Rate of chemical reaction

The change in the concentration of reactants to form

products per unit time is called rate of chemical reaction.

Any five factors that affect the rate of chemical

reaction may be temperature, pressure, light, surface area

and catalyst.

Chemical reaction

Day 3

Activity to show the effect of temperature in the rate of chemical

reaction

Materials required: Oxalic acid, sulphuric acid, water, potassium

permagnet (KMnO4), two beakers, glass rod, spirit lamp, match stick

Procedure: Put few oxalic acid crystals in two different beakers.

Put about 10 ml dilute sulphuric acid in each beaker. Put about 5

ml potassium permagnet solution in each of them and stir the

substances with the help of a glass rod for an instant. The solution

in both the beakers turn in pink colour. Now, heat one beaker up

to about 60oC– 80oC with the help of a spirit lamp.

Observation: The pink colour disappears quickly from the hot

beaker. The pink colour disappears after a long time from the cold

beaker .

Conclusion: The rate of chemical reaction increases with the increase of temperature.

Effect of pressure in the rate of chemical reaction

The rate of chemical reaction increases with the increase of

pressure.

High pressure

Nitrogen + Hydrogen Ammonia

High pressure

N2 + 3H2 2NH3

Effect of light in the rate of chemical reaction

Green plants require sun light (solar energy) to prepare

their food in the presence of water and carbon dioxide.

Sun light

Carbon dioxide + Water Glucose + Oxygen

Chlorophyll

6CO2 + 6H2O C6H12O6 + 6O2

Let us take an example, there are two test tubes containing

equal volume of dilute sulphuric acid. If equal mass of zinc pieces

and zinc powder is kept separately in the test tubes, the rate of

chemical reaction is faster in the test tube containing zinc powder

than the zinc pieces. This is because equal mass of zinc powder

have more surface area than the zinc pieces and the collision

between the reactants is faster in the test tube containing zinc

powder.

Effect of surface area in the rate of chemical reaction

When the reactants are larger in size, their surface area is

more. There is great chance of collision between them and the

rate of reaction is fast. When the reactants are smaller in size,

their surface area is less. There is less chance of collision

between them and the rate of reaction is slow. Therefore, the rate

of chemical reaction is affected by the surface area of the

reactants.

Activity to show the surface area affect the rate of chemical

reaction

Materials required: Two beakers, water, glass rod, top pan

balance, spoon, measuring cylinder, two watch glasses

Chemicals required: Zinc powder, zinc pieces, dilute hydrochloric

acid

Procedure: Put 20/20 ml dilute hydrochloric acid in two beakers.

Measure 2gm zinc powder and 2 gm zinc pieces separately by

using top pan balance and then kept them in two different watch

glasses. Now, put zinc powder in one of the beaker and zinc

pieces in another beaker simultaneously.

Observation: The gas is evolved faster in the beaker containing

zinc powder than other beaker.

Conclusion: The rate of chemical reaction increases with the

increase of surface area of the reactants.

Catalyst

The chemical substance which alters the rate of chemical

reaction without undergoing any chemical change in it is called

catalyst.

There are two types of catalyst. They are positive and

negative catalyst.

The catalyst which increases the rate of chemical reaction

is called positive catalyst. Manganese dioxide [MnO2] acts as

positive catalyst in the decomposition of potassium chlorate. Iron

[Fe] acts as positive catalyst to manufacture ammonia gas from

nitrogen and hydrogen gas.

The catalyst which decreases the rate of chemical reaction

is called negative catalyst. Glycerin [C3H5(OH)3] acts as negative

catalyst in the decomposition of hydrogen peroxide. Gypsum acts

as negative catalyst in cement.

Characteristics of catalyst

Any two characteristics of catalyst are as follows:

i. The mass and chemical properties of the catalyst remain same.

ii. It does not initiate the rate of chemical reaction but alters the

rate of undergoing chemical reaction.

Endothermic and exothermic reaction

The chemical reaction in which heat is required (absorbed)

is called endothermic reaction.

Heat

Calcium carbonate Calcium oxide + Carbon dioxide

CaCO3 CaO + CO2

The chemical reaction in which heat is evolved or produced

is called exothermic reaction.

Carbon + Oxygen Carbon dioxide + Heat

C + O2 CO2 + heat