SACE Chemistry — Stage 2
Redox Reactions — Flashcards & Quiz
Redox (reduction-oxidation) reactions involve the transfer of electrons between species, and SACE Chemistry Stage 2 expects you to assign oxidation states, balance half-equations, and apply the electrochemical series to predict spontaneity and calculate cell EMF. Galvanic cells convert chemical energy to electrical energy through redox and are the practical application you should be able to analyse.
Key Points
- Oxidation is loss of electrons (OIL); reduction is gain of electrons (RIG). Mnemonic: OIL RIG.
- Oxidation state rules: free elements = 0; ions = their charge; O is usually –2 (except peroxides); H is usually +1 (except metal hydrides).
- Half-equations show one species: balance atoms first, then charges with electrons, then add H⁺ and H₂O if in acidic solution.
- Galvanic cell: cathode = reduction (positive terminal), anode = oxidation (negative terminal). Electrons flow anode → cathode externally.
- Cell EMF = E°(cathode) – E°(anode). Positive EMF means the reaction is spontaneous under standard conditions.
- Salt bridge maintains charge neutrality — cations move toward cathode, anions toward anode.
Common Mistakes to Avoid
- Confusing oxidation (loss of e⁻) with reduction (gain of e⁻) — use OIL RIG.
- Forgetting to flip the sign of E° when reversing a half-equation from reduction to oxidation.
- Multiplying E° by stoichiometric coefficients — reduction potentials are intensive and don't scale.
- Confusing cathode and anode polarity between galvanic (cathode positive) and electrolytic (cathode negative) cells.
- Leaving the salt bridge out of diagrams — without it, the circuit stops.
Exam Strategy
SACE Stage 2 redox questions ask you to (1) assign oxidation states, (2) balance half-equations, (3) calculate cell EMF, or (4) predict spontaneity. Method: identify the species being oxidised and reduced, write the two half-equations, balance atoms then charges with electrons, combine for the net ionic equation. For cell EMF, use E°ₐₐll = E°ᶜₐₜₕₒdₑ – E°ₐₙₒdₑ. Always show working step by step.
Revision Tip
Redox half-equation balancing is procedural — drill a Revizi deck of increasingly tricky half-cell combinations including acidic and basic media.
Last updated: March 2026