Oxidation–reduction (internal)
Internal 3 credits Redox Half-equations Oxidation numbers Reactivity series Electrochemical cellsAS 91167, 3 credits. The standard's ladder: Achieved describes and identifies redox reactions; Merit makes and explains links between reactions, OBSERVATIONS and EQUATIONS; Excellence justifies, evaluates, compares/contrasts, or analyses those same links. That three-way link — what you saw, what it means, what the equation says — is the whole standard.
1 Reading the notation
A closed scope (oxidation numbers, electron transfer, half-equations) — no cell potentials or EMF calculations required.📖 Terms the schedule grades you on
4 terms- Demonstrate understanding (Achieved)
- Describe, identify, name, give an account of redox reactions.
- In-depth (Merit)
- Make and explain links between reactions, observations, and equations.
- Comprehensive (Excellence)
- Justify, evaluate, compare/contrast, or analyse those links, with consistent conventions.
- Scope (note 4)
- Oxidation numbers, electron transfer, oxidants/reductants, observations, half-equations, overall equations.
2 Oxidation numbers and the electron picture
OIL RIG, and how to prove a reaction is redox even with no obvious electron transfer visible.Proving a reaction is redox
- Assign oxidation numbers to every element on both sides.
- If ANY element's oxidation number changes, the reaction is redox.
- The element whose number DECREASES is reduced; the one whose number INCREASES is oxidised.
3 Balancing half-equations
Atoms, then oxygen, then hydrogen, then charge — always in that order. Internal-style Copper wire placed in silver nitrate solution Link the observation to the chemistry. OBSERVATION: grey crystals form on the wire; the solution turns pale blue. INFERENCE: silver metal is depositing and Cu²⁺ is entering solution. HALF-EQUATIONS: Cu(s) → Cu²⁺(aq) + 2e⁻ (oxidation); Ag⁺(aq) + e⁻ → Ag(s) (reduction). BALANCED OVERALL: Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + 2Ag(s). EXPLANATION: copper is above silver in the reactivity series, so it loses electrons more readily — copper is oxidised and is the reducing agent. ✓ Observation → inference → half-equations → overall equation → reactivity explanation. That is the three-way link the Merit criterion asks for.4 Reading observations as evidence
Colour changes, coatings, and effervescence — each one earns a place in the link if tied to an equation. ✕Describing colour changes without linking them
Listing several accurate colour observations without connecting each one to its half-equation and the electron transfer it represents stays at Achieved. Merit requires the explicit three-way link — reaction, observation, AND equation — every time.Practice
Recall it first, then use it — testing yourself builds the memory, where rereading the notes above only feels easier.
Recall and application shuffled together — you have to work out what each item wants before you can answer it, which is the part the exam actually tests. Both kinds feed the same spacing schedule, so anything you miss comes back sooner.
Say the answer out loud before flipping — that attempt is where the learning happens, even when you get it wrong.
Exam-style only. Instant feedback, shuffled each attempt, best score saved on this device.