Chemical Reactions and Equations (Class 10 Science) Part 2

 

Balanced chemical equation

            A balanced chemical equation is that in which the total number of atoms of each element are equal on both sides of the equation. The balancing of a chemical equation is based on law of conservation of mass. According to this – “mass can neither be created nor be destroyed during a chemical reaction.”

 

            The method used for balancing chemical reaction is called hit and trial method as we make trials to balance the equation by using the smallest whole number coefficient. In this method the number of atoms of each element remains the same, before and after a chemical reaction.

 

BALANCING A CHEMICAL EQUATION

            Several steps are involved in balancing a chemical equation. These steps are as follows :-

Step (a) Writing unbalanced equation and enclosing the formulae in brackets.

(Na) + (H₂O) → (NaOH) + (H₂)

 

Step (b) Making list of number of atoms of different atoms of different elements as present in unbalanced equation.

Element	Number of Atoms in Reactants (LHS)	Number of Atoms in Products (RHS)
Na	1	1
H	2	3
O	1	1

 

Step (c) Balancing first element

            From the table shown above, it is clear that it is only the hydrogen atoms, which are unbalanced. So , firstly we try to balance it.

Atoms of H	In reactants	In products
Initialy	2 (in H2O)	3 (1 in NaOH and 2 in H2)
To Balance	2x2	2x1 in NaOH 2 in H2 = 4H atoms

Step (d) Balancing second element

            We examine the obtained equation and select another element which is still unbalanced. In the above equation, Na is still unbalanced. To balance the number of Na- atoms,

Atoms of Na	In reactants	In products
Initialy	1 in (Na)	2 (in NaOH)
To Balance	2x1	2

 

Thus after balancing Na and removing the brackets, we get the equation:

2Na + 2H₂O → 2NaOH + H₂

 

Step (e) Balancing other element

            If we further examine the reaction, no element is found to be unbalanced. This method of balancing chemical equation is called hit and trial method.

 

Step (f) Checking the correction of equation

            To check the correctness of the equation, we further tabulate the number of atoms of each element separately.

 

Element	Number of Atoms in Reactants (LHS)	Number of Atoms in Products (RHS)
Na	2	2
H	4	4
O	2	2

 

 Making a chemical equation more informative

(i)                The physical states of the reactants and products can be represented by using symbols, (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution.

(ii)             The specific conditions of the reaction like temperature, pressure, catalyst etc., are written above or below the arrow in the chemical equation.

(iii)           Evolution of heat or absorption of heat can be indicated by writing [+Heat] on the right hand side or left hand side of the equation.

Example –

C_(s) + O_2(g) → CO_2(g) + Heat

Mg_(s) + H₂SO_4(aq) → MgSO_4(aq) + H₂↑

part 1       Part 3