O Level Notes : Chemistry - Redox Reactions

Redox reactions are reactions that involve oxidation and reduction. There are several definitions of oxidation and reduction, but we will go through a general idea of them.

 

Oxidation:

Oxidation is a gain in oxygen, or a loss of hydrogen. Any combustion process can be classified as oxidation, as it involves a gain in oxygen. Oxidation can also be defined as a loss of electrons, or an increase in the oxidation state. Substances which help oxidation to take place are called oxidising agents. Oxygen itself is the obvious oxidising agent, but substances with lots of oxygen in their molecules can also act as oxidizing agents. An oxidising agent itself, however, becomes reduced.

 

Reduction:

Reduction is a loss of oxygen, or a gain in hydrogen. It is also a gain in electrons, or a decrease in the oxidation state. Substances which help reduction to take place are called reducing agents. These agents themselves become oxidized to reduce.

 

 

For example, let us consider this ionic equation:

Cu2+ (s) + H2 (g)    Cu (s) + 2H+ (aq)

 

This is a redox reaction. In it, Copper’s oxidation state changes from +2 to 0. As it is a decrease in oxidation state, it means that in this reaction copper is being reduced.

On the other hand, Hydrogen’s oxidation state changes from 0 to +1. As it is an increase in oxidation state, it means that hydrogen is being oxidised.

 

Oxidising agents:

These are the electron acceptors. Some examples are:

  • Non-metals (chlorine, bromine, etc). This is because non-metals are good at accepting electrons.
  • Acidified Potassium Manganate (VII)
  • Acidified Potassium Dichromate (VI)

 

Reducing Agents:

These are the electron donors. Some examples are:

  • Metals (potassium, sodium, etc). This is because they are good at donating electrons.
  • Potassium Iodide
  • Carbon
  • Hydrogen
  • Carbon monoxide

 

Color Changes

It is often helpful to identify redox reactions by the color changes involved. Some of the color changes that you should remember are:

  • Acidified Potassium Manganate (VII): this is a powerful oxidising agent. It works by reducing its Manganate (VII) ion to Manganate (II) ion. The colour changes from purple to colourless.
  • Acidified Potassium Dichromate (VI): this is also a powerful oxidising agent. It works by reducing its dichromate (VI) ion to chromium (III) ion. The colour changes from orange to green.
  • Aqueous Potassium Iodide: this is a powerful reducing agent. Its iodide ion is oxidised to iodine. The colour changes from colourless to brown.

 

 

These colour changes can also be used as tests. Test for oxidising agent by adding potassium iodide which turns from colourless to brown, if present. Test foe reducing agent by adding acidified potassium dichromate (VI) which turns from orange to green, if present.

 

 

Let us do a couple of questions together now.

 

Q1. Which underlined agent is ‘not’ an oxidising agent?

  1. Cl2 (g) + 2Br- (aq)  ⇒   2Cl- (aq) + Br2 (l)
  1. PbS (s) + 4H2O2 (aq)   ⇒  PbSO4 (s) + 4H2O (l)
  1. SO2 (g) + 2H2S(g)   ⇒   2H2O (l) + 3S (s)
  1. Mg (s) + Cl2 (g)   ⇒  MgCl2 (s)

 

For answer, let us consider all options one by one.

 

In option A, Cl2 is underlined. Its oxidation state changes from 0 to -1, which means it is being reduced. We learnt that oxidising agents themselves get reduced. However, an alternate and a more accurate method would be to see weather or not Br is getting oxidised. Its oxidation state changes from -1 to 0, which means that it is getting oxidised. Therefore Cl2 here is acting as an oxidising agent, so it is eliminated as the answer.

In option B, H2O2 is underlined. So let’s take a look at PbS. There can be two ways. Either check the oxidation state, which changes from 0 to +8 (increases), or you can see that it is gaining Oxygen. It both ways, you can see that PbS is getting oxidised. Therefore, H2O2 is acting as an oxidising agent, so we eliminate option B as well.

In option C, SO2 is underlined. So let’s look at H2S. The hydrogen gains oxygen, while the Sulphur’s oxidation state changes from -2 to 0 (increases). Therefore SO2 is an oxidising agent. So we eliminate option C as well.

In option D, Mg is underlined. So we take a look at Cl2. its oxidation state changes from 0 to -2, which is a decrease. Therefore, we can say that Cl2 is getting reduced, so Mg is acting as a reducing agent.

So, option D is the right answer.

 

 

 

Q2. For the following equations, state which substances have been oxidised, and which have been reduced, giving your reasons.

a). C (s) + H2O (g)  ⇒    CO (g) + H2 (g)

 

Ans. Oxidation: Carbon is being oxidised in this reaction. Its oxidation state increases from 0 to +2. It also gains oxygen, changing from C to CO.

Reduction: Hydrogen is being reduced. Its oxidation state decreases from +2 to 0. It also loses oxygen.

b). Fe2O3 (s) + 3CO (g)  ⇒    2Fe (s) + 2CO2 (g)

 

Ans. Oxidation: CO is getting oxidised in this reaction. Its oxidation state increases from 0 to +2, and it also gains oxygen.

Reduction: Fe becomes reduced. It loses oxygen. Its oxidation state decreases from +3 to 0.