# Question #3a2a3

Jan 20, 2017

In balancing equations, addition of ${H}_{2} O$ is used to balance either the oxygen or hydrogen atoms.

#### Explanation:

In using the oxidation number method for balancing equations, we go through a series of steps in which we balance, in order, (for an acidic reaction)

1. The change in oxidation number for the oxidizing agent and the reducing agent.

2. The number of oxygen atoms in the equation or half-reaction

3. The charge on each side of the equation.

It is valid to add water molecules at step 2, for two reasons. First, these reactions often are occurring in aqueous solution and water is an available reactant or an acceptable product. Second, since the oxidation number of oxygen and hydrogen in ${H}_{2} O$ are $- 2$ and $+ 1$ respectively, adding ${H}_{2} O$ does not upset the oxidation number balance we achieved in the first step!

(In balancing a basic reaction, the steps are a bit different, but the idea is still the same.)
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