What is the oxidation number method?

1 Answer
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.

"Zn" + "HCl" → "ZnCl"_2 + "H"_2Zn+HClZnCl2+H2

Step 1. Identify the atoms that change oxidation number

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

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

Step 2. Equalize the changes in oxidation number

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

You need 2 atoms of "H"H for every 1 atom of "Zn"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"_21Zn+2HCl1ZnCl2+1H2

The balanced equation is

color(red)("Zn" + 2"HCl" → "ZnCl"_2 + "H"_2)Zn+2HClZnCl2+H2