Online Learning | CBSE, ICSE Study Materials - Alyss Edusolutions Pvt. Ltd.
X
Please select who you are ?

## Chapter : Chemical Reactions and Equations

Chemistry on the whole is an ocean of chemical reactions. What is a chemical reaction?

A chemical reaction is a symbolic representation of the reactants and the products using their chemical formulae. Reactants are the basically termed as the substances which are given as inputs and they undergo a change in the reaction. A product is a substance formed after a reaction.

E.g. Hydrogen plus Chlorine gives hydrochloric acid. In this reaction, hydrogen and chlorine are called as the reactants and hydrochloric acid is the product.

The reactants are usually written on the left hand side (LHS) and the product is mentioned on the right hand side (RHS).

Let us consider an example for a chemical reaction. i.e. burning of coal. Coal burns in the presence of oxygen and converts into ash. Chemically, coal is carbon compound. Hence when coal is burnt it produces carbon dioxide (CO2).

## Writing a Chemical Equation

Chemical equations can be made briefer when they are written as chemical formula instead of words. We have learnt that a chemical equation contains-

1 - Reactants

2 - Products

3 - An arrow separates  the reactants and products

Let us consider the above burning of coal as an example. Coal is a carbon hence it is represented by symbol C. O2 stands for oxygen. Hence the reaction is framed as follows:

C + O2 → CO2

Reactants              Products

CO2 stands for carbon dioxide.

Representation of physical states of reactants and products:

i - For solids it is "(s)".

ii - For liquids it is "(l)".

iii - For gases it is "(g)".

iv - For aqueous solutions it is “(aq)".

v - For gas produced in the reaction it is "(↑)".

vi - For precipitate formed in the reaction it is “(↓)".

vii - Direction of reaction is indicated by "(→)".

A chemical equation is helpful to understand a chemical reaction in an easy way.

In a chemical equation the masses of reactants and products may or may not be equal. But according to law of conservation of mass "the total mass of the reactants and the products should be equal". So in order maintain the law true it is necessary to balance a chemical equation.

Steps involved in balancing of chemical equation:

i -  Determining the reactants and products in a reaction.

ii - Counting the number of atoms of each element on both sides of the equation.

iii - Selecting the elements that occur for the least number of times in the equation.

iv - Balance atoms of each element on both sides of the reaction.

v - Always leave hydrogen and oxygen for last to balance.

vi - Balance the hydrogen atoms lastly followed by balancing of oxygen atoms.

Example:

Formation of Ammonia

N2 + H2 → NH3

Step1: 2-Nitrogen atoms, 2-Hydrogen atoms → 1 –Nitrogen atom, 3-Hydrogen atoms

Step2: In the above equation number of Nitrogen atoms on both sides are not equal, multiply with suitable integer to balance the Nitrogen atoms on both sides.

So, multiply with "2" on the product side.

N2 + H2 → 2NH3

In the above equation the number of Nitrogen atoms were balanced.

Step3: Hydrogen atoms on both sides were not balanced. So, multiply with suitable integer.

Multiply with "3" to Hydrogen on reactant side.

N2 + 3H2 → 2NH3

Now the above equation is balanced.

## Types of Chemical Reactions:

### 1 . Combination Reaction

It’s a reaction in which two reactions are combined into one product.

Calcium oxide reacts strongly with water to produce slaked lime which is chemically called as calcium hydroxide and releases a large amount of heat.

In the above reaction, calcium oxide and water combine to form a single product called calcium hydroxide. This type of reaction in which a single product is formed from two or more reactants is known as a combination reaction.

### 2 . Decomposition Reaction

It is a reaction in which single reactant breaks down to give simpler products.

Calcium carbonate decomposes to calcium oxide and carbon dioxide when heated. This is called as decomposition reaction. This decomposition of calcium carbonate is a very important reaction in various industries. Calcium oxide is called lime or quick lime and is used in the manufacture of cement. If the decomposition reaction is carried out in presence of heat it is called as thermal decomposition.

### 3 . Displacement Reaction

Displacement reaction is a reaction in which an element is displaced or removed by another element.

The following chemical reaction takes place in this Activity

In this reaction, iron has displaced or removed another element, copper, from copper sulphate solution. This reaction is known as displacement reaction.

### 4 . Double Displacement Reaction

It is a reaction in which there is an exchange of ions between the reactants.

### 5 . Oxidation and Reduction

If a substance gains oxygen during a reaction, it is said to be oxidized.

The surface of copper powder becomes coated with black copper(II) oxide. Why has this black substance formed? This is because oxygen is added to copper and copper oxide is formed.

If a substance loses oxygen during a reaction, it is said to be reduced.

The hydrogen is gaining oxygen and is being oxidised. In other words, one reactant gets oxidised while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions.