What is the oxidation number method?

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Answer 1 · 2015-04-28T17:06:31

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.

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

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

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

The balanced equation is

$Zn + 2 HCl \to {ZnCl}_{2} + H_{2}$ $color(red)("Zn" + 2"HCl" → "ZnCl"_2 + "H"_2)$