Balancing of Chemical Equation

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A chemical equation should be balanced so as to satisfy the requirements of the law of conservation of mass, as no matter is destroyed or created during a chemical reaction.

The two popular methods of balancing are the hit and trial method and partial equation method.

Hit and Trial Method or Inspection Method

• The symbols and formulae of the reactant and products are written as a skeletal equation.

• Any elementary gas (O₂, H₂, N₂ etc.) appearing on either side of the skeletal equation, is written in the atomic state.

• The formula containing the maximum number of atoms is selected to begin the process of balancing. If this method is not convenient, then balancing of the atoms begin with atoms, which appear minimum number of times.

• Atoms of elementary gases are balanced at the last.

• When the balancing is complete, the equation is converted to the molecular form.

Problem

12. Balance the chemical equation,

By hit and trial method.

Solution

(i) Skeletal equation

(ii) Elementary gas in atomic form

(iii) Starting with KMnO₄: K and Mn are balanced

(iv) Balancing O atoms

(v) Balancing H atoms

(vi) Balancing Cl atoms

The equation is now balanced in the atomic form.

(vii) To make it molecular

So the balanced molecular equation is

Partial Equation Method

When equations contain many reactants and products they cannot be balanced by the hit and trial method. They are then balanced by the partial equation method. In this method the overall reaction is assumed to take place through two or more simpler reactions, which can be represented by partial equations. The steps involved are:

Steps and problem

• The given chemical equation is split into two or more partial equations.

• Each partial equation is separately balanced by the hit and trial method.

• These balanced partial equations are multiplied with suitable coefficients in order to exactly cancel out those common substances which do not appear in the overall chemical equations.

• The balanced partial equations so obtained, are added to arrive at the balanced chemical equation.

Problem

13. Balance the equation,

by partial equation method.

Solution

The given skeleton equation can be split into two partial equations.

Partial eq.1

Partial eq.2

The two partial equations are balanced by hit and trial method.

Balanced partial eq.1

Balanced partial eq.2

NaClO does not appear in the overall equation and so to cancel it, the partial balanced equation 1 is multiplied by 3 and the two equations are added to get the overall balanced equation.

Limiting Reagent

In a desired chemical reaction some reactants may be present in lesser or greater proportions than the stoichiometry as indicated by the balanced chemical equation. The reactant, which is completely used up first as per the stoichiometry, limits the amount of product that can be formed and does not allow the reaction to go further. This is the limiting reagent. The excess reactants are left behind as unconsumed reagents, being limited by the limiting reagent. In such cases the desired reaction does not go to 100% completion.