How to Balance a Chemical Equations?

In our last articles, we learnt about chemical reactions and different ways to represent it. Now we will learn about how to balance the chemical equations representing a chemical reaction.

Table of Contents

1. Introduction
2. Points to be remembered while balancing a chemical equation
3. Steps to balance a chemical equation
4. Example
5. Practice Questions

Introduction

Let us recall the law of conservation of mass, which states that –

          “Mass can neither be created nor destroyed in a chemical reaction.”

This law is also emphasized in a postulate of Dalton atomic theory, which states that –

          “Atoms can neither be created nor destroyed; only the rearrangement of atoms takes place in a chemical reaction.”

Postulate of Dalton atomic theory is the manifestation of law of conservation of mass. It is important to understand and apply this law while balancing a chemical equation. The important points to be remembered while balancing a chemical equation are discussed below.

Points to be remembered while balancing a chemical equation

From the above discussion and applying the law of conservation of mass, following points must be considered while balancing a chemical reaction –

1. The total mass of the elements present in the reactants must be equal to the total mass of the elements present in the products of a chemical reaction.

2. In order to make sure this, the number of atoms of each element present in the reactants must be equal to the number of atoms of each element present in the products of a chemical reaction.

« A chemical reaction is said to be balanced when the number of atoms of each element is equal on the both sides (reactants and products) of the reaction.
                              Reactants → Products

Steps to balance a chemical equation

An unbalanced chemical equation can be balanced by following the principles of Hit-and-Trial method. Some of the steps to be followed while balancing a chemical equation are as follows:-
Step - 1 :
     Identify the reactants and products involved in the chemical reaction and write their respective chemical formulas.

Example –

When a magnesium ribbon is burnt in oxygen, magnesium oxide is formed. This reaction can be represented or written as :

Step - 2 :
     Write the skeletal chemical equation by writing the formula of the each reactant on the left – hand side and each product on the right – hand side of the arrow (→).
Example –

Skeletal equation is written by writing the chemical formula of elements / compounds in reactant and product –

Step - 3 :

• Make a list of all elements present in the reactants and products.
• Count and write the number of atoms of each element present in  the reactants and the products separately.
• Examine whether the number of atoms of each element in the reactants and products are equal or not.

Example –

Write all the elements with their number of atoms in a list and examine whether the atoms of each element is same or not  –

Element	Number of atoms in reactants (LHS)	Number of atoms in products (RHS)	Remarks
Mg	1	1	Equal
O	2	1	Unequal

Here, the atoms of magnesium are equal (balanced) but the atoms of oxygen are not equal (unbalanced). Hence, the equation is unbalanced and we need to balance it.

At this stage, if the number of atoms of each element in the reactants and products are equal, then the equation is already balanced and there is NO need to move further.

Step - 4 :
     If the equation is unbalanced, draw boxes around each chemical formula of the reactants and products. The formula written inside the box is not changed while balancing the equation.
Example –

Draw the boxes around the chemical formula of each element / compound.

Step - 5 :

     Change the coefficients of the atoms / molecules outside the box UNTIL the number of atoms of each element on both the sides of the equation is balanced. 

• It is often convenient to start balancing with the compound that contains the maximum number of atoms. 
• In that compound, select the element which has the maximum number of atoms.

Example – Using the above criteria, 

1. First, we select MgO and the element oxygen in it. There are two oxygen atoms (O₂) on the reactant side and only one oxygen atom (O) on the product side. Add a coefficient of “2” to the MgO on product side to equalize the number of oxygen atoms.

• The coefficient increases the number of molecules / atoms of that compound / element proportionally. 
• Here, 2MgO represents 2 molecules of Magnesium oxide (MgO). MgO molecule has one atom of magnesium and one atom of oxygen, whereas 2MgO has two atoms of magnesium and two atoms of oxygen.
• When no coefficient is written before the chemical formula of a compound / element in an equation. It represents “1 molecule / atom” of that compound / element.


2. Multiply the number of atoms previously present in the element / compound with the added coefficient and examine again whether the equation is balanced or not.

Element	Number of atoms in reactants (LHS)	Number of atoms in products (RHS)	Remarks
Mg	1	1 x 2 = 2	Unequal
O	2	1 x 2 = 2	Equal




Now, the oxygen atoms become equal (balanced). But, the magnesium atoms become unequal (unbalanced). Equation is still unbalanced.








In the second attempt, we select Mg atom on the reactant side. There is one magnesium (Mg) atom on the reactant side and two magnesium (Mg) atoms in two molecules of MgO on the product side. Add a coefficient of “2” to the Mg on reactant side to equalize the number of magnesium atoms.

Step - 6 :

Finally, examine the number of atoms of each element on both sides of the balanced chemical equation to check its correctness

Example –

Count atoms of each element on both sides of the balanced chemical equation.

Element	Number of atoms in reactants (LHS)	Number of atoms in products (RHS)	Remarks
Mg	1 x 2 = 2	1 x 2 = 2	Equal
O	2	1 x 2 = 2	Equal

Now, both the magnesium and oxygen atoms become equal (balanced). Hence, the equation is balanced.

Step - 7 :

     ( optional ) Additional information like physical states of the compounds, specific conditions for the reaction to occur can also be mentioned in the balanced chemical equation.
Example –

Here Mg and MgO are in solid (s) state while O₂ is a gas (g). This information can be represented in the equation as –

Hence, the balanced chemical equation of the chemical reaction can be written as: -

Another Example

When the natural gas (methane) is burnt in air, it gives carbon dioxide and water.
This reaction can be represented or written as :

1. Count the number of atoms of each elements on both sides (reactants and products) of the arrow :

Element	Number of atoms in reactants (LHS)	Number of atoms in products (RHS)	Remarks
C	1	1	Equal
H	4	2	Unequal
O	2	2 + 1 = 3	Unequal




Balance the Hydrogen (H) atoms –
There are four hydrogen atoms (H) on the reactant side and two hydrogen atom (H) on the product side.
Add a coefficient of “2” to the H₂O on the product side to equalize the number of hydrogen atoms.

Again examine the number of each atoms in the reaction –

Element	Number of atoms in reactants (LHS)	Number of atoms in products (RHS)	Remarks
C	1	1	Equal
H	4	2 x 2 = 4	Equal
O	2	2 + (1 x 2) = 4	Unequal



Balance the Oxygen (O) atoms –
There are two oxygen atoms (O) on the reactant side and four oxygen atom (O) on the product side.
Add a coefficient of “2” to the O₂ on the reactant side to equalize the number of oxygen atoms.

Finally, examine the number of each atoms in the reaction –

Element	Number of atoms in reactants (LHS)	Number of atoms in products (RHS)	Remarks
C	1	1	Equal
H	4	2 x 2 = 4	Equal
O	2 x 2 = 4	2 + (1 x 2) = 4	Equal

Now, the chemical equation is balanced.

The balanced chemical equation for the chemical reaction can be written as :

Practice Questions

Balance the following chemical equations :-

1. H₂ + O₂ → H₂O
2. AgBr → Ag + Br₂
3. Fe + H₂O → Fe₃O₄ + H₂
4. NaCl + AgNO₃ → AgCl + NaNO₃
5. Pb(NO₃)₂ → PbO + NO₂ + O₂

 Related Articles 

• Learning with fun Activity : Balancing a Chemical Equation
• Types of Chemical Reactions
• Chemical Reactions - Definition and Concept

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