ACT SSC Chemistry — Unit 4
Redox Reactions — ACT SSC Chemistry — Unit 4 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: Define a redox reaction.
A redox (reduction-oxidation) reaction involves the transfer of electrons between chemical species. One substance loses electrons (oxidation) while another substance gains electrons (reduction) simultaneously. These processes always occur together.
Q2: Explain the role of an oxidising agent (oxidant) in a redox reaction.
An oxidising agent, or oxidant, is the chemical species that causes another substance to be oxidised by accepting electrons from it. In doing so, the oxidising agent itself gets reduced.
Q3: Explain the role of a reducing agent (reductant) in a redox reaction.
A reducing agent, or reductant, is the chemical species that causes another substance to be reduced by donating electrons to it. In this process, the reducing agent itself gets oxidised.
Q4: How can changes in oxidation states identify whether a reaction is redox?
If the oxidation state of at least two elements changes during a reaction, it is a redox reaction. One element's oxidation state must increase (oxidation), and another's must decrease (reduction).
Q5: What is a displacement reaction of metals, and how is it a redox reaction?
A displacement reaction involves a more reactive metal displacing a less reactive metal from its compound in solution. The more reactive metal is oxidised, and the less reactive metal ion is reduced, involving electron transfer.
Sample Quiz Questions
Q1: What is the defining characteristic of oxidation in terms of electron transfer?
Answer: Loss of electrons
Oxidation is defined as the process where a chemical species loses electrons, resulting in an increase in its oxidation state.
Q2: Which species acts as an oxidising agent (oxidant) in a redox reaction?
Answer: The species that accepts electrons.
An oxidising agent accepts electrons from another species, causing that species to be oxidised, while the oxidising agent itself is reduced.
Q3: For a polyatomic ion, the sum of the oxidation states of all atoms must equal:
Answer: The charge of the ion.
The sum of the oxidation states of all atoms within a polyatomic ion must equal the overall charge of that ion.
Revision Tip
Half-equation balancing is procedural — drill a Revizi deck with 10+ half-equations covering acidic and basic media until balancing is automatic.
Related Concepts
Last updated: 3 September 2026 · 20 sample flashcards · 20 sample quiz questions