Chemical Reactions and Equations

 Definition of Chemical Equation

A chemical equation is defined as a shorthand notation of an actual chemical reaction in terms of the
symbols and formulae along with the number of atoms and molecules of its reactants and products.

 Physical States of Reactants and Products

As per IUPAC recommendation, the physical state is indicated by its symbol in a small bracket in
front of the substance in a chemical equation. For example :
The gaseous state is indicated by (g) as CO2 (g)
The liquid state is indicated by (l) as H2O (l)
The solid state is indicated by (s) as Fe (s)
The solution of substance in water is indicated by (aq) as NaOH (aq)

 Information Conveyed by Chemical Equation

We get the following information from a balanced chemical equation :
1. The formula, symbol, names and physical states of the reactants and products.
2. The relative number of atoms and molecules of the reactants that take part in the reaction.
3. The relative number of atoms and molecules of the products formed in the reaction.
4. The ratio of the moles of the reactants and products.
5. The ratio of the masses of the reactants and products.
6. The ratio of the volumes of the gaseous reactants and products.

 Advantages of The Uses of A chemical Equation

(a)The representation of actual chemical reaction becomes easy by using equation. It saves time and
space in writing.
(b) To prepare a known amount of the product we can calculate the amount of the reactant to be
taken.
(c)The effect of temperature, pressure and concentration on the rate of a reaction can be investigated
with the help of a perfectly balanced chemical equation.
(d) The chemical equation can be understood by the chemists belonging to any country of the world
irrespective of the language they speak. For example, the chemical equation N2 + 3H2  2NH3,can
be understood by chemist as : nitrogen and hydrogen combine in 1 : 3 mole ratio to give 2 mole ratio
of ammonia.
 Limitations of a Chemical Equation
(a) A chemical equation does not tell us about the rate of the reaction and the time taken for the
completion of the reaction.
(b) Some reactions may be even explosive. This is not revealed by a chemical equation.
(c)As such chemical equation does not tell about the actual quantity of the reactants consumed or
products formed. It gives only ratio. Nevertheless, chemical equation is very useful to calculate the
amount of all other substances consumed or formed if the amount of any one is known.

 Characteristics of A Chemical Reaction

 Evolution of Gas
some chemical reactions take place with the evolution of a gas :
some reactions in which a gas is evolved are described below :
Heating a mixture of potassium chlorate (KClO3 and magnese dioxide (MnO2) gives oxygen gas.

MnO2 is used as a catalyst in this reaction.

 Change of Colour
There are some reactions in which there is a colour change.
For example :
When red lead oxide is heated, yellow lead monoxide is formed.

 Formation of precipitate
When silver nitrate solution is mixed with a solution of sodium chloride, white precipitate of silver
chloride is formed.

When potassium iodide solution is added to solution of lead nitrate, yellow precipitate of lead
iodide is formed.
 Energy Changes
When coal (or carbon) is bumt, heat and light are produced.

When a small quantity of water is added to quicklime, a large amount of heat is evolved.

This reaction takes place when lime is added to water for preparting the lime suspension which is
used for whitewashing.

Limestone (CaCO3) is burnt to obtain lime (or quicklime). In this reaction, heat is absorbed.

 Change of physical state

In certain reactions, the physical state of products is different from that of the reactants. That is there
is a change of state during a chemical reaction.
When a mixture of hydrogen and oxygen is ignited with an electric spark at room temperature, liquid
water is formed.

 Types of Chemical Reactions

 Combinations Reactions
 (a) Combination reactions involving two elements

(i)
 (ii)

In all the above reactions, two elements combine to form a single new compound and are, therefore
combinations reactions. These reactions are also called synthesis reactions.
 (b) Combination reactions involving an element and a compound
(i) Carbon monoxide burns in oxygen to form carbon dioxide

(ii) Nitric oxide combines with oxygen at room temperature to form brown fumes of nitrogen
dioxide (NO2)

 (c) Combination reaction involving two compound

(i) Quick lime (CaO) reacts with water to form calcium hydroxide (slaked lime)

(ii) Decomposition Reactions

The conversion of a single compound into two or more simple substances is called decompo-
sition. For exmaple : AB  A + B

 Types of Decomposition Reaction

(a) Thermal Decomposition

(b) Electrolytic Decomposition Reaction

(i)

(ii)
(c) Photodecompostion Reactions
Displacement Reactions
The chemical reactions in which an atom or a group of atoms
in the molecule is replaced by another atom or a group of
atoms are called displacement reactions.
Let AL be any molecule
and X be the displacing group, then the displacement reaction
may be written as :
X +A- L X - L+A
Here A is the leaving group.
These reactions are generally found to occur in the solution.
The elements involved may be metals or non-metals, i.e.
a more active metal may displace a less active metal or
a more active nonmetal may displace a less active non-metal from its compound.
Before we take up examples of displacement reactions, it is important to understand the relative
reactivities of metals.

Reaction 1
When an iron nail is dipped in a copper sulphate solution, it gets coated with copper.

 Displacement of a less active non metal by a more active non metal

Reaction 1

Reaction 2

 Double Displacement Reactions

Ex. 1

Ex. 2

In this duble displacement reaction, Cl- ions and OH- ions have displaced each other to form insoluble
aluminium hydroxide and ammonium chloride solution.
 Exothermic and Endothermic Reactions
(a) Exothemic Reaction
The chemical reactions in which formation of products is accompanied by evolution of heat are
known as exothermic reaction.
In such cases, the sign “ + Heat” is written alongwith the products i.e.
Rection - Products + Heat
Examples :-

(b) Endothermic Reaction

The chemical reactions in which formation of products is accompanied by the absorption of heat are
known as endothermic reactions.
In such cases, the sign “ + Heat” is written alongwith the reactants or - ‘Heat’ with the products
reactants  Products - Heat
Example :-

 Oxidation and Reduction Reactions

Oxidation
(a) There is addition of oxygen to a substance.
(i) The chemical reactions in which oxygen is added to a substance.
 (b) There is removal of hydrogen from a substance
(ii) The chemical reaction is which hydeogen is lost from a substance.
(1)

(2)

(3)

(4)

(5)

 Reduction
(a) There is addition of hydrogen to a substance.
Example
(1) (2)
(3) (4)
(b) there is removal of oxygen from a substance

(1) (2)

(3) (4)

(5)
 Redox Reactions
Generally, oxidation and reduction reaction takes palce simultaneously because in a chemical reac-
tion, one atom, ion or molecule of a substance loses electron while another atom, ion or molecule
gains electron. In this way oxidation of one substance while reduction of another substance takes
place. Thus such reactions are known as Redox reactions.
Example

(1) (2)

 Corrosion
The process of slowly eating up of the metals due to attack of atompheric gases such as oxygen,
carbon dioxide, hydrogen sulphide, water vapour etc. on the surface of the metals so as to convert
the metal into oxide, carbonate, sulphide etc. is known as corrosion.
 Rusting of Iron

The surface of silver metal gets tarnished (i.e. loses lustre and becomes dull) on exposure to air. This is due to
the formation of a coating of black silver sulphide (Ag2S) on its surface by the action of H2S present in the air.

 Method to prevent rusting :

Rusting can be prevented if iron objects are not allowed to come in contact with the damp air. some
common method generally used are given below.
(i) By painting the iron articles such as window grills, iron gates, steel fumiture, railway coaches
bodies of cars, buses etc.
(ii) By greasing and oiling the iron articles such as mechanical tools, machine parts etc.
(iii) By galvanisation, i.e. coating the surface of iron objects with a thin layer of zinc.

 Rencidity
Example :-
1. Rancidity can be prevented by adding anti-oxidants to foods containing fats and oils.
Ex. BHA (Butylated Hydroxy-Anisole) and BHT (Butylated Hydroxy-Toluene).
2. Rancidity can be prevented by packaging fat and oile containing foods in nitorgen gas.
3. Rencidity can be retarded by keeping food in refrigerator.
4. Rancidity can be retarded by storing food in air-tight containers.
5. Rancidity can be retarded by storing foods away from light.