What is a chemical reaction?

Chemical Reaction: The transformation of chemical substance into

another chemical substance is known as Chemical Reaction. For
example: Rusting of iron, the setting of milk into curd, digestion of
food, respiration, etc.

In a chemical reaction, a new substance is formed which is completely

different in properties from the original substance, so in a chemical
reaction, a chemical change takes place.

Only a rearrangement of atoms takes place in a chemical reaction.

⦁ The substances which take part in a chemical reaction are called

reactants.

⦁ The new substances produced as a result of a chemical reaction are

called products.

Example: The burning of magnesium in the air to form magnesium oxide is

an example of a chemical reaction.

2Mg(s) + O2(g) △→ 2MgO(s)

This is done to remove the protective layer of magnesium oxide from the
surface of the magnesium ribbon.

Reactant: Substances which take part in a chemical reaction are called

reactants.

Product: New substance formed after a chemical reaction is called a

product.

Example: MgO.

Characteristics of Chemical Reactions:

(i) Evolution of gas: The chemical reaction between zinc and dilute
sulphuric acid is characterised by the evolution of hydrogen gas.
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) ↑

(ii) Change in Colour: The chemical reaction between citric acid and
purple coloured potassium permanganate solution is characterised by a
change in colour from purple to colourless.

The chemical reaction between sulphur dioxide gas and acidified potassium
dichromate solution is characterized by a change in colour from orange to
green.

(iii) Change in state of substance: The combustion reaction of candle wax is

characterised by a change in state from solid to liquid and gas (because the
wax is a solid, water formed by the combustion of wax is a liquid at room
temperature whereas, carbon dioxide produced by the combustion of wax
is a gas). There are some chemical reactions which can show more than one
characteristics.

(iv) Change in temperature: The chemical reaction between quick lime

water to form slaked lime is characterized by a change in temperature
(which is a rise in temperature).

The chemical reaction between zinc granules and dilute sulphuric acid is
also characterised by a change in temperature (which is a rise in
temperature).

(v) Formation of precipitate: The chemical reaction between sulphuric acid

and barium chloride solution is characterised by the formation of a white
precipitate of barium sulphate.

BaCl2(aq) + H2SO4(aq) → BaSO4(s) (ppt) + 2HCl(aq)

What is a chemical Equation?

Chemical Equation: Representation of chemical reaction using symbols

and formulae of the substances is called Chemical Equation.

Example: A + B → C + D

In this equation, A and B are called reactants and C and D are called the
products. The arrow shows the direction of the chemical reaction.
Condition, if Xany, is written generally above the arrow.
When hydrogen reacts with oxygen, it gives water. This reaction can be
represented by the following chemical equation:

Hydrogen + Oxygen → Water

H2 + O2 → H2O

In the first equation, words are used and in second, symbols of substances
are used to write the chemical equation. For convenience, the symbol of
substance is used to represent chemical equations.

A chemical equation is a way to represent the chemical reaction in a

concise and informative way.

A chemical equ ation can be divided into two types: Balanced Chemical
Equation and Unbalanced Chemical Equation.

a) Balanced Chemical Equation: A balanced chemical equation has the

same number of atoms of each element on both sides.

Example: Zn + H2SO4 → ZnSO4 + H2

In this equation, numbers of zinc, hydrogen and sulphate are equal on both
sides, so it is a Balanced Chemical Equation.

According to the Law of Conservation of Mass, mass can neither be created

nor destroyed in a chemical reaction. To obey this law, the total mass of
elements present in reactants must be equal to the total mass of elements
present in products.

(b) Unbalanced Chemical Equation: If the number of atoms of each

element in reactants is not equal to the number of atoms of each element
present in the product, then the2 chemical equation is called Unbalanced
Chemical Equation.

Example: Fe + H2O → Fe3O4 + H2

In this example, the number of atoms of elements are not equal on two
sides of the reaction. For example, on the left-hand side only one iron atom
is present, while three iron atoms are present on the right-hand side.
Therefore, it is an unbalanced chemical equation.

To Make Equations More Informative:

Writing the symbols of physical states of substances in a chemical equation:

By writing the physical states of substances, a chemical equation becomes

more informative.

⦁ Gaseous state is represented by symbol (g).

⦁ Liquid state is represented by symbol (l).

⦁ Solid state is written by symbol (s).

⦁ Aqueous solution is written by symbol (aq).

⦁ Writing the condition in which reaction takes place: The condition is

generally written above and/or below the arrow of a chemical equation.

Thus, by writing the symbols of the physical state of substances and

condition under which reaction takes place, a chemical equation can be
made more informative.

What are the types of a chemical reaction?

Chemical reactions can be classified in following types:

(i) Combination Reaction: Reactions in which two or more reactants

combine to form one product are called Combination Reactions. It is also
known as synthesis reaction.

A general combination reaction can be represented by the chemical

equation given here:

A + B → AB

Examples:
When magnesium is burnt in the air (oxygen), magnesium oxide is formed.
In this reaction, magnesium is combined with oxygen.

Mg(s) + O2(g) → 2MgO(s)

Magnesium + Oxygen → Magnesium Oxide

When carbon is burnt in oxygen (air), carbon dioxide is formed. In this

reaction, carbon is combined with oxygen.

C (s) + O2(g) → CO2(g)

Carbon + Oxygen → Carbon dioxide

(ii) Decomposition Reaction: Reactions in which one compound

decomposes in two or more compounds or elements are known as
Decomposition Reaction. The reaction is just the opposite of combination
reaction.

A general decomposition reaction can be represented as follows:

AB → A + B

Examples:

When calcium carbonate is heated, it decomposes into calcium oxide and

carbon dioxide.

CaCO3(s) heat−→− CaO(s) + CO2(g)

Calcium carbonate → Calcium oxide + Carbon dioxide

Thermal Decomposition: The decomposition of a substance on heating is

known as Thermal Decomposition.

Example: 2Pb(NO3)2(s) heat−→− 2PbO(s) + 4NO2(g) + O2(g)

2FeSO4 -----------> Fe2O3 + SO2 + SO3

Electrolytic Decomposition: Reactions in which compounds decompose into
simpler compounds because of passing of electricity, are known as Electrolytic
Decomposition. This is also known as Electrolysis.
Example: When electricity is passed in water, it decomposes into hydrogen and
oxygen.
2H2O(l) -------> 2H2(g) + O2(g)

Photolysis or Photo Decomposition Reaction: Reactions in which a

compound decomposes because of sunlight are known as Photolysis or Photo
Decomposition Reaction.
Example: When silver chloride is put in sunlight, it decomposes into silver
metal and chlorine gas.
2AgCl(s) (white) Sunlight−→−−−−− 2Ag(s) (grey) + Cl2(g)

Photographic paper has a coat of silver chloride, which turns into grey when
exposed to sunlight. It happens because silver chloride is colourless while silver
is a grey metal. Silver chloride and silver bromide are used in black and white
photography.

(iii) Displacement Reaction: The chemical reactions in which a more reactive

element displaces a less reactive element from a compound is known as
Displacement Reactions. Displacement reactions are also known as Substitution
Reaction or Single Displacement/ replacement reactions.
A general displacement reaction can be represented by using a chemical
equation as follows :
A + BC → AC + B
Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’
is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and reaction
will not be taking place.
Examples:
When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc
chloride.
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

Fe + CUSO4(aq) ---------> FeSO4 + Cu

Cu + AgNO3(aq) ----------> Cu(NO3)2 + Ag

iv) Double Displacement Reaction: Reactions in which ions are exchanged
between two reactants forming new compounds are called Double Displacement
Reactions.
AB + CD → AC + BD
Examples:
When the solution of barium chloride reacts with the solution of sodium
sulphate, white precipitate of barium sulphate is formed along with sodium
chloride.
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) (white precipitate) + 2NaCl(aq)

When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride
and water are formed.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Note: Double Displacement Reaction, in which precipitate is formed, is also

known as precipitation reaction. Neutralisation reactions are also examples of
double displacement reaction.

Precipitation Reaction: The reaction in which precipitate is formed by the

mixing of the aqueous solution of two salts is called Precipitation Reaction.
Example:

Neutralization Reaction: The reaction in which an acid reacts with a base to

form salt and water by an exchange of ions is called Neutralization Reaction.
Example:

(v) Oxidation and Reduction Reactions:

Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen
or metallic element from a compound is known as Oxidation.
Elements or compounds in which oxygen or non-metallic element is added or
hydrogen or metallic element is removed are called to be Oxidized.
Reduction: Addition of hydrogen or metallic element or removal of oxygen or
non-metallic element from a compound is called Reduction.
The compound or element which goes under reduction in called to be Reduced.
Oxidation and Reduction take place together.
Oxidizing agent:

 The substance which gives oxygen for oxidation is called an Oxidizing

agent.
 The substance which removes hydrogen is also called an Oxidizing agent.
 Oxidising agent is the substance which oxidises the other but self gets
reduced.

Reducing agent:

 The substance which gives hydrogen for reduction is called a Reducing

agent.
 The substance which removes oxygen is also called a Reducing agent.
 Reducing agent is the substance which reduces the other but self gets
oxidised.

The reaction in which oxidation and reduction both take place simultaneously is
called Redox reaction.
When copper oxide is heated with hydrogen, then copper metal and water are
formed.
CuO + H2 → Cu + H2O
(i) In this reaction, CuO is changing into Cu. Oxygen is being removed from
copper oxide. Removal of oxygen from a substance is called Reduction, so
copper oxide is being reduced to copper.

(ii) In this reaction, H2 is changing to H2O. Oxygen is being added to hydrogen.

Addition of oxygen to a substance is called Oxidation, so hydrogen is being
oxidised to water.

Exothermic and Endothermic Reactions:

Exothermic Reaction: Reaction which produces energy is called Exothermic
Reaction. Most of the decomposition reactions are exothermic.
Example:
Respiration is a decomposition reaction in which energy is released.
When quick lime (CaO) is added to water, it releases energy.

Endothermic Reaction: A chemical reaction in which heat energy is absorbed

is called Endothermic Reaction.
Example: Decomposition of calcium carbonate.

Effects of Oxidation Reactions in Everyday life: Corrosion and Rancidity.

Corrosion: The process of slow conversion of metals into their undesirable
compounds due to their reaction with oxygen, water, acids, gases etc. present in
the atmosphere is called Corrosion.
Example: Rusting of iron.

Rusting: Iron when reacts with oxygen and moisture forms red substance which
is called Rust.

The rusting of iron is a redox reaction.

Corrosion (rusting) weakens the iron and steel objects and structures such as
railings, car bodies, bridges and ships etc. and cuts short their life.
Methods to Prevent Rusting

 By painting.
 By greasing and oiling.
 By galvanisation.

Corrosion of Copper: Copper objects lose their lustre and shine after some
time because the surface of these objects acquires a green coating of basic
copper carbonate, CuCO3.Cu(OH)2 when exposed to air.
Corrosion of Silver Metal: The surface of silver metal gets tarnished (becomes
dull) on exposure to air, due to the formation of a coating of black silver
sulphide (Ag2S) on its surface by the action of H2S gas present in the air.

Rancidity: The taste and odour of food materials containing fat and oil changes
when they are left exposed to air for a long time. This is called Rancidity. It is
caused due to the oxidation of fat and oil present in food materials.

Methods to prevent rancidity:

 By adding anti-oxidant.
 Vacuum packing.
 Replacing air by nitrogen.
 Refrigeration of foodstuff.