Question

What is the oxidation number method?

Answer 1

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.

`"Zn" + "HCl" → "ZnCl"_2 + "H"_2`

Step 1. Identify the atoms that change oxidation number

Left hand side: `"Zn"` = 0; `"H"` = +1; `"Cl"` = -1
Right hand side: `"Zn"` = +2; `"Cl"` = -1; `"H"` = +1

The changes in oxidation number are:
`"Zn"`: 0 → +2; Change = +2
`"H"`: +1 → 0; Change = -1

Step 2. Equalize the changes in oxidation number

Each `"Zn"` atom has lost two electrons, and each `"H"` atom has gained one electron.

You need 2 atoms of `"H"` for every 1 atom of `"Zn"`. This gives us total changes of +2 and -2.

Step 3. Insert coefficients to get these numbers

`color(red)(1)"Zn" + color(red)(2)"HCl" → color(red)(1)"ZnCl"_2 + color(red)(1)"H"_2`

The balanced equation is

`color(red)("Zn" + 2"HCl" → "ZnCl"_2 + "H"_2)`