In this post, let’s go through the key concepts in redox reactions as tested in O Level Chemistry.
What is a redox reaction
A redox reaction is one that involves oxidation and reduction.
So what’s an oxidation reaction and a reduction reaction then?
An oxidation reaction is one that involves a gain in oxygen, gain in oxidation state, loss of hydrogen and/ or loss of electrons.
A reduction reaction is one that involves a loss in oxygen, decrease in oxidation state, gain in hydrogen and/ or gain in electrons.
Since all redox reactions involve oxidation and reduction, there will be an oxidizing and reducing agent.
- An oxidizing agent is one that oxidizes another substance (and itself is being reduced).
- A reducing agent is one that reduces another substance (and itself is being oxidized).
Oxidation number
We can find the oxidation number (also known as oxidation state) using this method:
- oxidation number of elements is 0.
- When it exists as a compound…
- oxygen has an oxidation number of -2 (except for hydrogen peroxide, where the oxidation state is -1)
- hydrogen has an oxidation number of +1 (except when it exists as a hydride e.g. NaH, where its oxidation state is -1)
- for most compounds, the oxidation state of each element is usually the same as the charge of the ions they form.
Examples of Oxidising Agents
Potassium manganate (VII)
Acidified potassium manganate (VII) is an oxidising agent. When it oxidises another substance, it changes from purple to colourless.
Example:
When hydrogen peroxide is added to acidified potassium manganate solution, purple potassium manganate (KMnO4) acts as a oxidising agent, and itself is being reduced to Mn2+, which is colourless. Hydrogen peroxide (H2O2) is being oxidized to form oxygen (that is why you notice a lot of bubbles being produced). Hydrogen peroxide is a reducing agent in this example, as it reduces potassium manganate to manganese(II).
Potassium dichromate (VI)
Acidified potassium dichromate (VI) is an oxidising agent. When it oxidises another substance, it changes from orange to green.
When iron (II) sulfate solution is added to acidified potassium dichromate (VI) solution, the orange acidified potassium dichromate (VI) (K2Cr2O7) acts as a oxidising agent, and itself is being reduced to Cr3+, which is green. Iron (II) sulfate, which is pale green, is being oxidised to iron (III) sulfate which is yellow or brown. You won’t see any yellow or brown colour at the end of the reaction, as the colour intensity of Cr3+ “mask” away the yellow or brown colour of iron (III) sulfate.
Examples of Reducing Agents
Potassium iodide
Potassium iodide is a reducing agent. When it reduces another substance, it changes from colourless to brown, as iodide is converted to iodine.
When potassium iodide is added to hydrogen peroxide, a brown solution is formed, due to the formation of iodine.
Hydrogen peroxide (H2O2) acts as an oxidising agent, oxidising potassium iodide (KI) to iodine (I2). Hydrogen peroxide itself is reduced to water. As such, potassium iodide is the reducing agent here.
Hydrogen peroxide – Reducing and Oxidising Agent
Hydrogen peroxide can act as an reducing or oxidizing agent, depending on what is added to it.
Hydrogen peroxide as a reducing agent
For example, when hydrogen peroxide is added to acidified potassium manganate solution, hydrogen peroxide (H2O2) is being oxidized to form oxygen. Hydrogen peroxide is a reducing agent in this example, as it reduces potassium manganate to manganese(II).
Hydrogen peroxide as an oxidizing agent
When hydrogen peroxide and potassium iodide are added together, hydrogen peroxide (H2O2) acts as an oxidizing agent, oxidising potassium iodide (KI) to iodine (I2). Hydrogen peroxide itself is reduced to water.
Disproportionation of hydrogen peroxide
Hydrogen peroxide can also undergo disproportionation. A disproportionation reaction is one in which a substance is being oxidized and reduced simultaneously. In the presence of manganese(IV) oxide (or MnO2), hydrogen peroxide is oxidized to oxygen and reduced to water simultaneously.
In the disproportionation of hydrogen peroxide, water and oxygen are formed:
hydrogen peroxide —> water + oxygen
2H2O2 —> 2H2O + O2