Topics

What a redox account has to contain

The six things this covers

Oxidation–reduction at this level is six things, and the list is closed. Nothing outside it comes up, so you can prepare against it exactly.

The six things the report has to do

What	What you must be able to do
Oxidation numbers	Assign an oxidation number to any atom in a formula, and spot which ones change
Electron transfer	Say which species lost electrons and which gained them, and how many
Oxidants and reductants	Name the oxidant (the species reduced) and the reductant (the species oxidised)
Observations	Describe what you would see — colours, precipitates, gases, a metal coating
Half equations	Write a balanced half equation for the oxidation and one for the reduction
Overall equations	Combine the two halves into a balanced overall equation with no electrons left in it

The three depths of an answer

Identify the species and say whether each is oxidised or reduced, with a reason.
- The reason is either electrons lost/gained or a change in oxidation number. Without a reason, nothing has been explained.
Explain: write balanced half equations, and link them to the observations.
- "The purple MnO₄⁻ was reduced to colourless Mn²⁺, shown by the half equation …" is the shape of that sentence.
Justify: bring the half equations, the overall equation, the observations and the electron transfer together into one account.
- This is not "more facts". It is the same facts, connected.

What is NOT in this topic

Do not spend time on these — they come later:

Standard reduction potentials ( $E°$ values) and cell potentials.
Predicting whether a reaction is spontaneous from $E°$ .
Electrochemical and electrolytic cells, salt bridges, electroplating.

Also left for later: redox titration calculations, and balancing half equations in basic solution. Everything here is balanced in acidic solution using H⁺ and H₂O.

The shape of a redox question

You are given a set of reactions, often with observations recorded from a practical you have done.
You are usually given a table of substances: name, formula and appearance — but nothing that pairs an oxidant with a reductant. Working out which species takes the electrons and which gives them is the whole task.
You then explain each reaction: what was oxidised, what was reduced, the half equations, the overall equation, and how the observations show it.

The species you should know on sight

These are the species that turn up again and again. Know them cold:

Oxidants — O₂, I₂, Br₂, Cl₂, OCl⁻, H⁺, Fe³⁺, Cu²⁺, H₂O₂, MnO₄⁻ in acid, Cr₂O₇²⁻ in acid, concentrated HNO₃, IO₃⁻.
Reductants — metals, C, H₂, Fe²⁺, Br⁻, I⁻, H₂S, SO₂, SO₃²⁻, HSO₃⁻, H₂O₂.
Note that H₂O₂ appears on both lists. Hydrogen peroxide can act as either, depending on what it meets — one of the sharpest things in the topic, and it has its own page later.

What a redox account has to contain

Covers AS91167

The six things this covers

Oxidation–reduction at this level is six things, and the list is closed. Nothing outside it comes up, so you can prepare against it exactly.

The six things the report has to do

What	What you must be able to do
Oxidation numbers	Assign an oxidation number to any atom in a formula, and spot which ones change
Electron transfer	Say which species lost electrons and which gained them, and how many
Oxidants and reductants	Name the oxidant (the species reduced) and the reductant (the species oxidised)
Observations	Describe what you would see — colours, precipitates, gases, a metal coating
Half equations	Write a balanced half equation for the oxidation and one for the reduction
Overall equations	Combine the two halves into a balanced overall equation with no electrons left in it

The three depths of an answer

Identify the species and say whether each is oxidised or reduced, with a reason.
- The reason is either electrons lost/gained or a change in oxidation number. Without a reason, nothing has been explained.
Explain: write balanced half equations, and link them to the observations.
- "The purple MnO₄⁻ was reduced to colourless Mn²⁺, shown by the half equation …" is the shape of that sentence.
Justify: bring the half equations, the overall equation, the observations and the electron transfer together into one account.
- This is not "more facts". It is the same facts, connected.

What is NOT in this topic

Do not spend time on these — they come later:

Standard reduction potentials ( $E°$ values) and cell potentials.
Predicting whether a reaction is spontaneous from $E°$ .
Electrochemical and electrolytic cells, salt bridges, electroplating.

Also left for later: redox titration calculations, and balancing half equations in basic solution. Everything here is balanced in acidic solution using H⁺ and H₂O.

The shape of a redox question

You are given a set of reactions, often with observations recorded from a practical you have done.
You are usually given a table of substances: name, formula and appearance — but nothing that pairs an oxidant with a reductant. Working out which species takes the electrons and which gives them is the whole task.
You then explain each reaction: what was oxidised, what was reduced, the half equations, the overall equation, and how the observations show it.

The species you should know on sight

These are the species that turn up again and again. Know them cold:

Oxidants — O₂, I₂, Br₂, Cl₂, OCl⁻, H⁺, Fe³⁺, Cu²⁺, H₂O₂, MnO₄⁻ in acid, Cr₂O₇²⁻ in acid, concentrated HNO₃, IO₃⁻.
Reductants — metals, C, H₂, Fe²⁺, Br⁻, I⁻, H₂S, SO₂, SO₃²⁻, HSO₃⁻, H₂O₂.
Note that H₂O₂ appears on both lists. Hydrogen peroxide can act as either, depending on what it meets — one of the sharpest things in the topic, and it has its own page later.