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Chemistry > Chemical Reactions and Equations > Balanced Chemical Equations
Chemical Reactions and Equations

Balanced Chemical Equations

In the reaction Mg + O2 → MgO, the number of atoms of each element on either side of the arrow is not equal. Therefore, this is an unbalanced chemical equation. Then what is a balanced equation? Let’s find out below and learn how to balance a chemical equation.

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Balanced Equations

As you have already learned earlier, mass can neither be created nor destroyed. This law applies to a chemical reaction too. This means that the total mass of elements in the products of a chemical reaction has to be equal to the total mass of the elements in the reactants. The number of atoms of each element remains the same, before and after a chemical reaction. For example, let’s look at the following chemical equation –

Zinc + Sulphuric acid → Zinc sulfate + Hydrogen

This is further simplified as –

Zn + H2SO4 → ZnSO4 + H2

Now, let us look at the number of atoms of each element on LHS and RHS –

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

As you can see, the number of atoms of each element on LHS and RHS are the same. Therefore, this chemical equation is a ‘balanced equation’. Now, look at the following equation –

Fe +H2O → Fe3O4 +H2

If you notice, the number of atoms of each element are not the same on both sides of the arrow. Therefore, this is an ‘unbalanced’ equation. We can balance this equation using the following steps –

Step I

First, draw boxes around each formula without changing anything inside the boxes.

balanced equation

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Step II

List the number of atoms of each element on either side of the arrow in the equation –

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

Step III

To balance the above equation, let’s start with the compound with the maximum number of atoms i.e. Fe3O4. Now, in this compound select the element with the maximum number of atoms i.e. oxygen. As you can see, there are 4 oxygen atoms on the RHS, whereas there is only one oxygen atom on the LHS. Let’s try to balance the oxygen atoms –

Atoms of oxygen In reactants In products
(i) Initial 1 (in H2O) 4 (in Fe3O4)
(ii) To balance 1 x 4 4

Remember – To equalize the number of atoms in a reaction, you cannot change the formulae of the compounds or the elements in the reaction. For example, to balance the number of oxygen atoms, we can add 4 as a coefficient to get 4H2O and not H2O4 or (H2O)4. Now, the partly balanced equation looks like this –

balanced equation

Step IV

Now, let’s balance the hydrogen atoms in the equation –

Atoms of hydrogen In reactants In products
(i) Initial 8 (in 4H2O) 2
(ii) To balance 8 2 x 4

Therefore, we add 4 as a coefficient to H2 on the RHS. The equation will now be –

balanced equation

Step V

Now, you will find that only one element is left unbalanced i.e. Fe. Let’s try to balance that –

Atoms of iron In reactants In products
(i) Initial 1 (in Fe) 3 (in Fe3O4)
(ii) To balance 1 x 3 3

Therefore, we add 3 as a coefficient to Fe on the LHS –

balanced equation

Step VI

Now, we count the atoms of each element on both sides of the equation to the check the correctness of the equation

3Fe + 4H2O → Fe3O4 + 4H2

The number of atoms of each element on both sides is now equal, therefore this equation is now a balanced equation. As we make trials to balance the equation using smallest whole numbers as coefficients, this method is called the ‘hit and trial method’ of balancing equations.

Step VII

The last step is to write the physical states of the reactants and products. This makes the balanced equation more informative. The phases of the reactants and products are mentioned using notations such as (g) for gaseous, (l) for liquid, (s) for solid and (aq) for aqueous. If the reactant or product exists as a solution in water, it is written as aqueous (aq).

The balanced equation above now becomes –

3Fe(s) + 4H2O(g) → Fe3O4 (s) + 4H2(g)

Note – For H2O, the symbol (g) is used to indicate that here, water has been used in the form of steam. Physical states of the reactants and products are usually not specified in the equation unless necessary. A lot of times, the reaction conditions such as the temperature, pressure, catalyst etc are indicated above and/or below the arrow in the equation. For example –

balanced equation

Solved Examples for You

Question: Write the balanced equations for the following chemical reactions –

  1. HNO3 +Ca(OH)2 → Ca(NO3)2 + H2O
  2. BaCl+ Al2(SO4 )3 → BaSO4 + AlCl3
  3. Zinc + Silver nitrate → Zinc nitrate + Silver
  4. Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Solutions:

  1. 2HNO3 +Ca(OH)2 → Ca(NO3)2 + 2H2O
  2. 3BaCl+ Al2(SO4 )3 → 3BaSO4 + 2AlCl3
  3. Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
  4. Ca(OH)2 + CO→ CaCO3 + H2O
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