Class 10 Chemical Reactions

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Balancing a Chemical Equation

If you will carefully observe the above chemical equation, you will notice that the number of magnesium atoms is the same on both sides of the arrow. However, the number of oxygen atoms is not equal on both sides of the arrow. Hence, this is an unbalanced chemical equation.

The Law of Conservation of Mass says that mass cannot be created nor can be destroyed. So, a chemical reaction must obey the law of conservation of mass. Due to this, it is necessary to have a balanced chemical equation.

Balanced Chemical Equation:

A chemical equation in which a number of atoms of each element are equal on both sides of the arrow is called a balanced chemical equation.

Let us take the example of a reaction between zinc and sulphuric acid, which is given by the following equation:

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The following table shows the number of atoms of various elements on both either side of this equation.

Element Number of atoms of reactants (LHS) Number of atoms of products (RHS)
Zn 1 1
H 2 2
S 1 1
O 4 4

This table shows that the number of atoms of each element is the same on both sides of this equation. Hence, this is a balanced chemical equation.

 Chemical Equation Balancing:

While writing a chemical equation, you will often observe that the equation is not a balanced equation. In such a case, you need to balance the chemical equation; by writing suitable numbers before particular reactants/products. Let us take the following equation to understand how to balance a chemical equation.

You need to follow these steps in order to balance a chemical equation:

(1) Make a table in write the number of atoms of elements present in reactants and in products.

Element Number of atoms in reactants (LHS) Number of atoms in products (RHS)
Fe 1 3
H 2 2
O 1 4

(2) Select the atom which has a maximum number of atoms. Balance its number on either side of the equation. In this case, the number of oxygen atoms is maximum; and it is 4 on the RHS.

For balancing the number of oxygen atoms on both sides, you need to multiply the oxygen on the LHS by 4. By doing so, the number of oxygen atoms would become equal on both sides of the equation.

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(3) After the above step, the number of hydrogen atoms becomes 8 on the LHS. But the number of hydrogen atoms on the RHS is still 2; which needs to be balanced. For this, you need to multiply the number of hydrogen atoms on RHS by 4.

(4) After the above step, the number of oxygen and hydrogen atoms became equal on both sides of the equation. Now, let us count the number of iron atoms on both sides. A number of iron atoms are 3 on RHS but it is 1 on LHS. In order to balance the number of iron atoms, you need to multiply the number of iron atoms on LHS by 3.

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(5) After the above step, count the number of atoms of each element on both sides. This can be shown by the following table; which shows that a number of atoms of various elements are the same on both sides of the equation. Thus, it is now a balanced chemical equation.

Element Number of atoms in reactants (LHS) Number of atoms in products (RHS)
Fe 3 3
H 8 8
O 4 4

This equation can now be written as follows:

Example: Balance the following chemical equation:

Step 1: Make a table to show a number of each atom on LHS and RHS of the equation.

Element Number of atoms on LHS Number of atoms on RHS
H 3 2
N 1 2
O 5 7
Ca 1 1

Step 2: Select the atom with a maximum number of atoms; which is oxygen in this reaction. The number of oxygen atoms on LHS is 5 and that on RHS is 7. But oxygen is being repeated twice on LHS and RHS hence it can be confusing to balance the oxygen atoms. Instead of working on oxygen atoms; let us take nitrate ion for convenience. A number of nitrate ions are 1 on LHS and 2 on RHS. To balance the number of nitrate ions, let us multiply nitrate ions on LHS by 2.

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Now, the number of hydrogen atoms is 4 on LHS and it is 2 on RHS. To balance this, let us multiply hydrogen atoms by 2 on  RHS.

Now, a number of different atoms on each side are as follows:

Element Number of atoms on LHS Number of atoms on RHS
H 4 4
N 2 2
O 8 8
Ca 1 1

So, this is a balanced equation; which can be written as follows:

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Important Note: If a polyatomic ion (such as nitrate or sulphate) is present in an equation, you should balance that ion before any other atom.

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