# What is the oxidation number method?

Apr 28, 2015

The oxidation number method is a way of keeping track of electrons when balancing redox equations.

The general idea is that electrons are transferred between charged atoms.

Here's how the oxidation number method works for a very simple equation that you could probably balance in your head.

${\text{Zn" + "HCl" → "ZnCl"_2 + "H}}_{2}$

Step 1. Identify the atoms that change oxidation number

Left hand side: $\text{Zn}$ = 0; $\text{H}$ = +1; $\text{Cl}$ = -1
Right hand side: $\text{Zn}$ = +2; $\text{Cl}$ = -1; $\text{H}$ = +1

The changes in oxidation number are:
$\text{Zn}$: 0 → +2; Change = +2
$\text{H}$: +1 → 0; Change = -1

Step 2. Equalize the changes in oxidation number

Each $\text{Zn}$ atom has lost two electrons, and each $\text{H}$ atom has gained one electron.

You need 2 atoms of $\text{H}$ for every 1 atom of $\text{Zn}$. This gives us total changes of +2 and -2.

Step 3. Insert coefficients to get these numbers

$\textcolor{red}{1} {\text{Zn" + color(red)(2)"HCl" → color(red)(1)"ZnCl"_2 + color(red)(1)"H}}_{2}$

The balanced equation is

$\textcolor{red}{{\text{Zn" + 2"HCl" → "ZnCl"_2 + "H}}_{2}}$