Oxidation Numbers and Redox Reactions: Quick Revision

A brief note on Oxidation Numbers and Redox Reactions. Read it before a test. Try each question at the end before you open its answer.

Redox ideas

• Oxidation is loss of electrons, and a rise in oxidation number. Reduction is gain of electrons, and a fall in oxidation number.
• They always happen together. The electrons lost equal the electrons gained.
• The reducing agent gives electrons and is oxidised. The oxidising agent takes electrons and is reduced.
• Classical ideas: oxidation adds oxygen or removes hydrogen. Reduction removes oxygen or adds hydrogen.

Oxidation number rules

• Free element: 0. Simple ion: its charge. Fluorine: $-1$.
• Hydrogen: $+1$, but $-1$ in metal hydrides. Oxygen: $-2$, but $-1$ in peroxides, $-\tfrac{1}{2}$ in superoxides and $+2$ in $\text{OF}_2$.
• Alkali metals $+1$. Alkaline earth metals $+2$. Aluminium $+3$.
• All the numbers in a species add up to its charge. A fraction is an average.
• Stock notation: $\text{FeCl}_3$ is iron(III) chloride.

Which agents go one way

• Highest state (for example Mn at $+7$, N at $+5$): only an oxidising agent.
• Lowest state (for example S at $-2$, N at $-3$): only a reducing agent.
• In between (for example S at $+4$): either.

The five kinds

• Combination: two substances join, with a free element.
• Decomposition: one compound breaks up. Redox only if an oxidation number changes.
• Displacement: an element takes the place of another. Metals, hydrogen and halogens.
• Disproportionation: one element goes both up and down.
• Comproportionation: two states of one element become one.

A worked example

For $2\text{Al} + 3\text{Cl}_2 \rightarrow 2\text{AlCl}_3$, find what is oxidised and what is reduced, and count the electrons.

1. Oxidation numbers. Aluminium goes $0 \rightarrow +3$. Chlorine goes $0 \rightarrow -1$.
2. Who is who. Aluminium is oxidised, so it is the reducing agent. Chlorine is reduced, so it is the oxidising agent.
3. Electrons lost. Each aluminium atom loses 3, and there are 2 atoms, so 6 electrons.
4. Electrons gained. Each chlorine atom gains 1, and there are 6 atoms, so 6 electrons.
5. Check. The electrons lost equal the electrons gained.

Practice with new numbers

A program to try

The program solves for an oxidation number from the total charge.

Mistakes to avoid

• Mixing up the agent and the change. The reducing agent is the one oxidised.
• Forgetting the subscript when solving for one element.
• Using $-2$ for oxygen in a peroxide.
• Calling every decomposition a redox reaction.
• Confusing disproportionation with comproportionation.

Check yourself

1. What is the oxidation number of sulfur in $\text{SO}_3$?

2. In $\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$, which substance is the reducing agent?

3. Can fluorine be oxidised in a compound?

4. What kind of reaction is $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$?