ACT SSC Chemistry — Unit 3
Redox Reactions — ACT SSC Chemistry — Unit 3 Flashcards & Quiz
Redox reactions are part of Unit 3 Equilibrium and Redox Reactions in the BSSS Chemistry T course (from 2023), under 'Oxidation and reduction'. The course models displacement reactions of metals, combustion, corrosion and electrochemical processes as electron transfer, represented with half-equations. You need to assign oxidation numbers, balance half-equations, compare oxidising and reducing agents using standard electrode potentials, calculate cell potentials, and contrast galvanic cells (including fuel cells) with electrolytic cells driven by an external power supply.
Key Points
- Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG). The two always occur together.
- Oxidation number rules: free element = 0; simple ion = its charge; O is usually −2; H is usually +1; the numbers sum to the overall charge.
- Half-equations in acidic solution: balance other atoms, then O with H₂O, then H with H⁺, then charge with electrons.
- Standard electrode potentials rank oxidising agents: the more positive the E°, the stronger the oxidising agent.
- Galvanic cells (including fuel cells) run a spontaneous reaction: anode (oxidation) is negative, cathode (reduction) is positive, electrons flow anode → cathode through the external circuit.
- Electrolytic cells use an external power supply to drive a non-spontaneous reaction: anode (oxidation) is positive, cathode (reduction) is negative.
- E°cell = E°(cathode) − E°(anode), using reduction potentials; a positive value means the reaction is spontaneous under standard conditions.
Common Mistakes to Avoid
- Confusing oxidation (loss of electrons) with reduction (gain of electrons). Remember OIL RIG.
- Forgetting that electrode polarity is reversed between galvanic and electrolytic cells, even though oxidation is always at the anode.
- Multiplying E° values by the coefficients used to balance electrons. E° does not depend on the amount of substance.
- Mixing methods: if you use E°cell = E°(cathode) − E°(anode), use both tabulated reduction potentials as written and do not also reverse a sign.
- Leaving the salt bridge, electron flow direction or electrode signs off a galvanic cell diagram.
Exam Strategy
ACT SSC Chemistry is assessed through tasks set and marked by your college, but redox questions commonly ask you to balance equations, calculate a cell potential or predict whether a reaction occurs. Method: (1) assign oxidation numbers, (2) identify what is oxidised and what is reduced, (3) balance the half-equations and combine them so electrons cancel, (4) calculate E°cell from the table of standard electrode potentials, (5) interpret the sign, and say whether the set-up is galvanic or electrolytic.
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, so drill a Revizi deck of 10 or more half-equations in acidic solution until balancing is automatic.
Related Concepts
Last updated: 9 October 2026 · 20 sample flashcards · 20 sample quiz questions