SACE Chemistry — Stage 2
Redox Reactions — SACE Chemistry — Stage 2 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.
Sample Flashcards
Q1: What is a fuel cell, and what happens at each electrode of a hydrogen fuel cell?
A fuel cell is a galvanic cell in which the reactants are supplied continuously, so it produces electricity directly from the oxidation of a fuel for as long as fuel is supplied. In a hydrogen fuel cell with an acidic electrolyte: anode (negative, oxidation): H₂(g) → 2H⁺(aq) + 2e⁻; cathode (positive, reduction): O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l). Electrons flow through the external circuit from anode to cathode; H⁺ ions move through the electrolyte towards the cathode. Flow cells store their electrolytes in external tanks that are pumped through the cell, so capacity can be increased simply by using larger tanks.
Q2: How does an electrolytic cell differ from a galvanic cell?
An electrolytic cell uses electrical energy to drive a non-spontaneous reaction. It requires an external power supply. The anode is positive (connected to + terminal) and the cathode is negative (connected to - terminal) — opposite polarity to galvanic cells. Both still follow: oxidation at anode, reduction at cathode.
Sample Quiz Questions
Q1: A hydrogen fuel cell produces water as its only product.
Answer: TRUE
The overall reaction is 2H₂ + O₂ -> 2H₂O. The only product is water, making hydrogen fuel cells zero-emission at point of use.
Q2: In a hydrogen fuel cell, the anode is the positive electrode.
Answer: FALSE
A fuel cell is a galvanic cell: oxidation of hydrogen at the anode releases electrons, so the anode is NEGATIVE.
Q3: In an electrolytic cell, the anode is connected to the positive terminal of the power supply.
Answer: TRUE
In electrolytic cells, the anode is positive (connected to + terminal) and the cathode is negative (connected to - terminal). This is opposite to galvanic cells.
Revision Tip
Redox half-equation balancing is procedural — drill a Revizi deck of increasingly tricky half-cell combinations including acidic and basic media.
Related Concepts
Last updated: 9 October 2026 · 2 sample flashcards · 3 sample quiz questions