ACT SSC Chemistry — Unit 4
Redox Reactions — Flashcards & Quiz
Redox reactions involve the transfer of electrons between species, and ACT SSC Chemistry Year 12 Unit 4 expects you to assign oxidation states, balance half-equations, predict spontaneity from E° values, and compare galvanic (spontaneous) with electrolytic (non-spontaneous) cells. The electrochemical series is your essential lookup table.
Key Points
- Oxidation = loss of electrons (OIL); Reduction = gain of electrons (RIG). Mnemonic: OIL RIG.
- Oxidation state rules: free element = 0; simple ion = its charge; O usually –2; H usually +1; sum = overall charge.
- Half-equations: balance atoms first, then charges with electrons, then H⁺ and H₂O for acidic solution.
- Galvanic cells (spontaneous): cathode is POSITIVE, anode is NEGATIVE. Electrons flow anode → cathode externally. E°ₐₐll > 0.
- Electrolytic cells (non-spontaneous): cathode is NEGATIVE, anode is POSITIVE. External power source drives the reaction.
- Cell EMF: E°ₐₐll = E°(cathode) – E°(anode). Positive = spontaneous.
Common Mistakes to Avoid
- Confusing oxidation (loss of e⁻) and reduction (gain of e⁻) — remember OIL RIG.
- Forgetting that anode polarity is opposite in galvanic vs electrolytic cells.
- Multiplying E° by stoichiometric coefficients — it's intensive.
- Forgetting to flip the sign of E° when reversing a half-equation.
- Ignoring the salt bridge in galvanic cell diagrams.
Exam Strategy
BSSS Unit 4 redox questions ask you to balance equations, calculate cell EMF, or predict spontaneity. Method: (1) assign oxidation states, (2) identify what is oxidised and reduced, (3) balance the half-equations, (4) calculate E°ₐₐll, (5) interpret the sign. Distinguish galvanic from electrolytic where relevant.
Sample Flashcards
Q1: Outline the steps for balancing a redox equation using half-equations.
1) Write separate oxidation and reduction half-equations. 2) Balance atoms other than O and H. 3) Balance O by adding H₂O. 4) Balance H by adding H⁺. 5) Balance charge by adding electrons. 6) Multiply half-equations so electrons are equal. 7) Add and simplify.
Revision Tip
Half-equation balancing is procedural — drill a Revizi deck with 10+ half-equations covering acidic and basic media until balancing is automatic.
Last updated: March 2026 · 1 flashcards