How to Balance Chemical Equations

How to Balance Chemical Equations

A chemical equation is a symbolic representation of chemical reaction in the form of symbols and formulas, wherein the reactants are given on the left-hand side and the products on the right-hand side. A balanced equation is an equation for a chemical reaction in which the number of atoms for each element in the reaction and the total charge are the same for both the reactants and the products. The numbers placed in before the formulas to balance equations are called coefficients.
There are two methods for balancing a chemical equation-
1. Oxidation Number Charge Method
2. Half Reaction Method

Oxidation Number Charge Method

This method is based on the fact that in a balanced redox reaction, the total increase in oxidation number is equal to the total decrease in oxidation number. The following rules are use to balance the chemical equation by oxidation number charge method-
1. Assign oxidation numbers to elements that undergo changes in oxidation numbers.
2. First, balance only those atoms of elements that undergo oxidation-number changes and write the oxidation number for all the atoms balanced.
3. Connect the atoms of the element undergoing oxidation by drawing a bracket and do the same for the reduction process. Show increase in oxidation number for oxidation process and decrease in oxidation number for reduction process.
4. Insert coefficients into the equation to make the total increase and decrease in oxidation numbers equal.
5. Now balance the atoms of other elements except H and O by inspection. H and O atoms in aqueous solutions are balanced in the following way-
a. Acidic Medium: For one excess oxygen atom, add one H₂O on the other side and two H on the same side.
b. Alkaline Medium: For one excess oxygen atom, add one H₂O on the same side and two OH on the other side.
In case H-atoms are still unbalanced, for one excess H-atom, add one OH on the same side and one H₂O, on the other side.
c. In some cases both H and O atoms are balanced together by adding H₂O only.
Example-

Balance the given equation in acidic medium-
Fe⁺² + MnO₄⁻ → Fe⁺³ + Mn⁺²

Permanganate ion reacts with bromide ion in basic medium to give manganese dioxide and bromate ion. Write the balanced ionic equation for the reaction by oxidation number charge method.

Half Reaction Method

In this method, the two half equations are balanced separately and then added together to give balanced equation. 1. Seperate the two half reactions i.e. oxidation half reaction and reduction half reaction
2. Balance the atoms in the half-reactions other than the hydrogen and oxygen
3. a. Acidic Medium: For one excess oxygen atom, add one H₂O on the other side and two H on the same side.
3. b. Alkaline Medium: For one excess oxygen atom, add one H₂O on the same side and two H on the other side.
4. Balance the charges by adding electrons to each half-reaction
5. Equalize the electrons by multiplying everything in one or both equations by a coefficient
6. Add the two half-reactions together now.
Example-

Balance the given equation in acidic medium-
Fe⁺² + Cr₂O₇⁻² → Fe⁺³ + Cr⁺³

Separate the equation into half reactions-
Oxidation Half Reaction: Fe⁺²(aq) → Fe⁺³(aq)
Reduction Half Reaction: Cr₂O₇^–2(aq) → Cr⁺³(aq)

Permanganate(VII) ion, MnO₄^– in basic solution oxidises iodide ion, I^– to produce molecular iodine(I₂) and manganese(IV) oxide (MnO₂). Write a balanced ionic equation to represent this redox reaction by half reaction method.