# Determine Feasibility of Redox Reaction using Standard Electrode Potentials

In Electrochemistry we need to know how to use standard cell potential (E_{cell}) to predict thermodynamic feasibility of a redox reaction.

Let's use a mixture of acidified manganate MnO_{4}^{-} and hydrogen peroxide H_{2}O_{2} as an example.

Since H_{2}O_{2} can be both oxidised and reduced, we will determine what happens to MnO_{4}^{-} first.

Manganate is a strong oxidising agent and will be reduced.

Therefore we will expect to find MnO_{4}^{-} on the left hand side of the half equation in the data booklet, since all half equations are written in reduction form.

Since MnO_{4}^{-} is reduced, then hydrogen peroxide must be oxidised.

So we need to choose the half equation where H_{2}O_{2} appears on the right hand side of the half equation.

## 1. Choosing the relevant half equations

**For reduction of MnO _{4}^{-}, it has to be on the left hand side of the half equation.**

In acidic medium, MnO_{4}^{-} will be reduced to Mn^{2+}.

In neutral or alkaline medium, MnO_{4}^{-} will be reduced to MnO_{2}.

So the half equation that we will use is the one where MnO_{4}^{-} is reduced to Mn^{2+} and E value is +1.52V.

**For oxidation of H _{2}O_{2}, it has to be on the right hand side of the half equation.**

So the half equation that we will use is the one where H_{2}O_{2} is on the right hand side, and O_{2} is on the left hand side and E value is +0.68V.

Notice there is another half equation where H_{2}O_{2} is on the left hand side (E = +1.77V).

But we will reject that since H_{2}O_{2} and MnO_{4}^{-} cannot be both reduced in the same reaction.

## 2. Determine Redox Reaction

With the 2 half equations chosen we now know that MnO_{4}^{-} is reduced while H_{2}O_{2} is oxidised.

## 3. Calculate Feasibility

We can calculate **standard cell potential** or E_{cell} of this reaction using formula **E _{cell} = E_{R} - E_{O}**.

**Since E _{cell} is a positive value, the reaction is thermodynamically feasible.**

## 4. Balance Redox Reaction

If the half equations can be found in the data booklet, this is very simple as there is no need to use the half equation method to balance the redox equation.

Topic: Electrochemistry, Physical Chemistry, A Level Chemistry, Singapore

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