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How to balance a chemical equation

Balancing of a Chemical Equation

 Balancing of a chemical equation means making the number of atoms of each element equal on both sides of the equation. The importance of a balanced chemical equation lies in the fact that it satisfies the law of conservation of mass, i.e., in a chemical reaction, total mass of all the products is equal to the total mass of all the reactants. Check out Chemistry Formulas and NCERT Solutions for class 11 Chemistry prepared by Physics Wallah. 

Steps Involved In The Balancing Of A Chemical Equation:

Step 1. To write the word equation. Write down the equation in the word form by writing the names of the reactants on the left side and those of products on the right side. This step is not required if the equation is  given in terms of symbols and formulas.

Step 2. To write the skeletal chemical equation. Write down the symbols and formulae of the various reactants and products. This gives us the skeletal chemical equation. 

Step 3. To enclose the formulae in boxes. Enclose the formula of each reactant and product in a box. This is done to remember that during balancing of the chemical equation, the formula of any reactant or product cannot be changes.

Step 4. To list the number of atoms of different elements on LHS (Reactants) and RHS (Products).

Step 5. To start balancing of different elements. Having selected the compound with the biggest formula as above, first balance the element of this compound which has the highest number of atoms. Then balance other elements one by one. To balance the atoms of an element, put a small whole number coefficient before the formula of the compound (or symbol of the element for elementary substance). 

If selection of the biggest formula appears inconvenient, balance the atoms of that element which occurs at minimum number of places on both sides of the equation. Atoms which occur at maximum places are balances last of all.

Step 6. To check the correctness of the balanced equation. Finally check the correctness of the balanced equation by counting the number of atoms of each element on both sides of the equation.

The above method of balancing of chemical equations is called Hit and Trial method as we keep on trying to balance the equation using smallest whole number coefficients. 

  • To make the number of atoms of any element equal on both sides of the equation, we cannot change the subscripts of the formula. We can only place a suitable whole number coefficient before the formula. For example, to have four O-atoms, we write 4 H2O and not H2O4 or (H2O)4 . 
  • The coefficient placed before the formula multiples every atom of that formula by that number. For example, 2 NH3 means 2 N atoms and 6 H atoms. Similarly, 3 H2O means 6 H atoms and 3 O-atoms. 

Examples of Balancing of Equations

Example 1. Write the balanced chemical equation for the following reaction: Steam is passed over heated iron to form magnetic oxide of iron (Fe3O4) and hydrogen*.

Solution.

Step I Writing the equation in the word form.

Iron + Steam How to balance a chemical equation Magnetic oxide of iron + Hydrogen

Step II. Writing the skeletal chemical equation and enclosing the formulae in boxes. 

  Fe + H2O How to balance a chemical equation Fe3O4 + H2

Remember that there are three oxides of iron. These are:

  • FeO called ferrous oxide or iron (II) oxide as valency of iron in this oxide is II.
  • Fe2O3 called ferric oxide or iron (III) oxide because valency of iron in this oxide is III.
  • Fe3O4 called magnetic oxide of iron. It is a mixture of the above two oxides (FeO + Fe2O3). Thus, it is a double oxide and is called iron (II, III) oxide.

  Also, remember that steam is water vapours. Hence, its formula is same as that of water, viz., H2O.

Step III. Listing number of atoms of different elements.

Element           No.of atoms on LHS      No. of atoms on RHS

Fe                            1                                           3

H                             2                                           2

O                            1                                           4

Step IV. Selecting the biggest formula (i.e., Fe3O4) and balancing the element with highest number of atoms, i.e., oxygen. There are 4 O-atoms on RHS and 1 O-atom on LHS. To balance O-atoms, multiply H2O on LHS by 4. We get 

How to balance a chemical equation

Step V. To balance Fe atoms on both sides.
There are 3 Fe atoms on RHS and 1 Fe atom on LHS. To balance Fe atoms, multiply Fe on LHS by 3. We get 

How to balance a chemical equation

Step VI. To balance H-atoms

There are 8 H atoms on LHS and 2 H atoms on LHS. To balance H-atoms, multiply H2 by 4. We get 

How to balance a chemical equation

Step VII. Checking the correctness of the balanced equation.
                     LHS        RHS

Fe atoms      3             3

H atoms       8              8

O atoms      4               4

Hence, the equation is balanced, i.e., the final balanced equation may be written as 

How to balance a chemical equation

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