# How to balance a reaction by Hit and trial method and by Partial equation method. Explain with some examples.

How to balance a reaction by Hit and trial method and by Partial equation method. Explain with some examples.

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Method 1 :

Hit and Trial Method:

The simplest method to balance a chemical equation is by the hit and trial method. This method is also called trial and error method, or inspection method. In this method, coefficient before the formulae or symbols of the reactants and products are adjusted in such a way that the total number of atoms of each element only both the sides become equal. This is called material balance, or mass balance. In this method first of all, atoms of the element which appears least in the chemical equation should be balanced. Then, the next one, and so only.

The method involves in the following steps :

1. Write the correct formula of the reactants and products with plus sign in between with an arrow pointing from the reactants to the products.

2.Balance the atom of that element which occur at minimum number of places on both side of the chemical equation first. Atoms which occur at a maximum number of places are balanced last of all.

3. When an elementary gas ( diatomic) such as hydrogen, oxygen, nitrogen appears as a reactant or product, the equation is balanced more easily keeping the elementary gas in the atomic state. The balanced atomic equation is then made molecular by multiplying the whole equation by two.

4. Verify that the number of atoms of each element is balanced in the final equation.

5.The chemical equation can be made more informative by mentioning the physical states of the reactants and products. Thus, the gaseous, liquid, aqueous and solid states of the reactants and products are represented by the notations (g), (l), (aq) and (s).

Or,

# Balancing of chemical equation by hit and trial method is illustrated through the reaction which involves the burning of methane gas in the excess of air (or oxygen) to form carbon dioxide gas and water. The reactions is,

Methane + Oxygen (from air) → Carbon dioxide + Water

(i) The skeleton equation for this reaction is,

CH4 (g) + O2 (g) → CO2 (g) + H2O (l)

(ii) The number of atoms of each element on both the sides of the arrow. As the number of atoms of His and one on both the sides are not equal, the skeleton equation is not a balanced chemical equation.

(iii) Inspection of the skeleton equation shows that both carbon(C) and hydrogen(H) occur twice, and oxygen(O) appears thrice. So, a start is made by balancing carbon or hydrogen atoms. Carbon is already balanced. There are 4 hydrogen atoms only the left side, and two hydrogen atom only the right side. So, hydrogen can be balanced by placing a coefficient of 2 before H2O on the right hand side.

(iv) Now, there is one carbon and four hydrogen atoms only either side of the equation. Thus, carbon and hydrogen are balanced. In this partly balanced chemical equation, there are four oxygen atoms only the right-hand side, while there are two only the left-hand sides, by 2.

(v) The resulting equation is

CH4 (g) + 2O2 (g) → CO2 (g) + H2O (l)

The number of atoms of each element, only both the sides of the equation

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) are equal. Therefore, the above chemical equation is a balanced chemical equation.

Method 2:

Partial Equation Method :

For a reaction in which the same elements is repeated in several compounds, the partial equation method is more helpful.

The method involves following steps:

1.The chemical reaction represented by the equation is supposed to proceed in two or more steps.

2. The skeleton equations representing each steps are written and then balance by hit and trial method. These equations are known as partial equation method.

3. If necessary, partial equations are multiplied by suitable integers so as to cancel those intermediate products which don't occur in the final equation.

4. The partial equations are added to get the final balanced equation.

Example :

So, first step: decide on the partial equations.

P4 + HNO3 ---------> H3PO4 + NO +H2O

-> P4 + H2O ---------------> H3PO4

Then, using hit and trial method,

P4 + H2O ------------> 4H3PO4  + 10[O]

(addition of 10 atoms of oxygen i.e. nascent oxygen to make the equation balanced)

-> HNO3 -> NO2 + H2O

Again using hit and trial method,

2HNO3 +[O] ->  2NO2 + H2O

(addition of 1 atom of nascent oxygen to make the equation balanced)

NOTE : THE SAME EXTRA ATOM HAS TO BE MULTIPLIED ON BOTH THE SIDES AS PER THE SITUATION AND MUST BE CUT AT LAST

Now adding both the equations to make a single balanced equation,

P4 + H2O ------------> 4H3PO4  + 10[O]

+ 2HNO3 +[O] -------------->  2NO2 + H2O } ×10

————————————————————

P4 + 20HNO3 ---------------> 4H3PO4 + 2NO + 9H2O

AT last the oxygens and 1 water molecule get cancelled and the above answer remains.