Consider the following equations
Zn (s) + H2SO4 → Zn SO4 ↓+ H2 (g) (1)
Fe + H2O→ Fe3O4 + H2 (2)
KClO3 →KCl + O2 (3)
According to the law of conservation of mass,
Mass can neither be created nor destroyed in a chemical reaction.
It means the mass of the elements or number of atoms of various elements present in reactants and products should be equal in a chemical reaction.
Let us examine the number of atoms of different elements of reactants and products in chemical equation – 1.
The number of atoms of each element present in reactants and products are same.
Such an equation which has an equal number of atoms or masses of various elements on both sides of the arrow (reactants and products) is called balanced chemical equation .
Let us examine chemical equation – 2
and chemical equation – 3
Converse to equation – 1, in equation 2 and 3, number of atoms of various elements present in reactants and products are not same.
Such an equation which has an unequal number of atoms of one or more elements in reactants and products is called unbalanced chemical equation.
Unbalanced chemical equation has to be made balanced to justify the law of conservation of mass.
Balancing of an unbalanced chemical equation
1.Which of the following reaction is not correctly balanced ?
(a) Zn (s) + H2SO4 → Zn SO4 ↓+ H2 (g)
(b) C (s) + O2 (g) → CO2 (g) + heat
(c) AgCl → Ag + Cl2
(d) 2Cu + O2 → 2CuO