Balance Redox Reaction via Half Eqn Method

Balancing Redox Equations via the Half-Equation Method can be done via the following systematic steps.

1. Balance Redox Equation in Acidic Medium
Let's have this reaction as an example:

balance redox reaction using half equation method example

We can determine which species is oxidised and reduced by comparing the oxidation number:

- Oxidation state of manganese decreases from +7 to +4 hence MnO4- is reduced

- Oxidation state of iodine increases from -1 to 0 hence I- is oxidised

We can then write down the half equations for oxidation and reduction:

balance redox reaction half equations

Next we need to balance each half equation.

For half equation in acidic medium, the steps are:

1. Balance elements oxidised or reduced

2. Balance oxygen using water

3. Balance hydrogen using H+

4. Balance charge using electron

So after applying these 4 steps for each half equation, we'll end up with these two balanced half equations:

balance redox reaction half equation balanced in acidic medium

Next we need to modify each half equation so that the number of electrons for both half equations are the same (lowest common multiple).

In this case the lowest common multiple is 6, so we need to multiply the reduction half equation by 2, and the oxidation half equation by 3.

We can then add the oxidation and reduction half equations together to get an overall redox equation in acidic medium.

balance redox reaction overall balanced in acidic medium

2. Balance Redox Equation in Alkaline Medium
If we want to balance the redox reaction in alkaline medium, an additional step is required which is to add OH- to neutralise the H+.

From the balanced redox equation in acidic medium that we have done previously, there are 8H+ on the left hand side of the equation.

Therefore we need to add  H- on the left side to neutralise the 8H+, forming 8H2O.

We also need to add  H- on the right side to keep the equation balanced.

Usually there will be H2O on both sides of the equation to cancel out.

This will give us the overall balanced redox equation in alkaline medium:

balance redox reaction balanced in alkaline medium

Watch this video tutorial to learn how to balance a redox equation step-by-step!

Topic: Redox Titrations, Physical Chemistry, JC, H2, A Level Chemistry, Singapore

You can also view this video lesson with screenshots and detailed explanation.

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