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 MnO₄^- and hydrogen peroxide H₂O₂ as an example.

Since H₂O₂ can be both oxidised and reduced, we will determine what happens to MnO₄^- first.

Manganate is a strong oxidising agent and will be reduced.

Therefore we will expect to find MnO₄^- on the left hand side of the half equation in the data booklet, since all half equations are written in reduction form.

Since MnO₄^- is reduced, then hydrogen peroxide must be oxidised.

So we need to choose the half equation where H₂O₂ appears on the right hand side of the half equation.

1. Choosing the relevant half equations

For reduction of MnO₄^-, it has to be on the left hand side of the half equation.

In acidic medium, MnO₄^- will be reduced to Mn²⁺.

In neutral or alkaline medium, MnO₄^- will be reduced to MnO₂.

So the half equation that we will use is the one where MnO₄^- is reduced to Mn²⁺ and E value is +1.52V.

For oxidation of H₂O₂, 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 H₂O₂ is on the right hand side, and O₂ is on the left hand side and E value is +0.68V.

Notice there is another half equation where H₂O₂ is on the left hand side (E = +1.77V).

But we will reject that since H₂O₂ and MnO₄^- cannot be both reduced in the same reaction.

2. Determine Redox Reaction

With the 2 half equations chosen we now know that MnO₄^- is reduced while H₂O₂ 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

Back to other previous Physical Chemistry Video Lessons.

Found this A Level Chemistry video useful?

This free chemistry video lesson is brought to you by Chemistry Guru, Singapore's renowned A Level Chemistry tuition centre since 2010.

Please like this video and share it with your friends!

Join my 18,000 subscribers on my YouTube Channel for new A Level Chemistry video lessons every week.

Check out other A Level Chemistry Video Lessons here!

Need an experienced tutor to make Chemistry simpler for you?

Do consider signing up for my A Level H2 Chemistry Tuition classes at Bishan or online tuition classes!