Mini-Lesson

Electron transfer — redox

This mini-lesson covers Reactivity 3.2: oxidation and reduction, oxidation states, oxidising and reducing agents, and voltaic and electrolytic cells. This HL lesson also builds in the Additional Higher Level (AHL) material.

Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.

Reactivity 3.2

Oxidation and reduction

Redox is electron transfer. Remember OIL RIG:

Oxidation Is Loss of electrons (oxidation number rises).
Reduction Is Gain of electrons (oxidation number falls).

Oxidation states: free elements = 0; simple ions = their charge; O usually −2, H usually +1; the sum equals the overall charge.

Calculate

Oxidation state (1)

1What is the oxidation state of sulfur in the sulfate ion, SO₄²⁻?

Let S = x. x + 4(−2) = −2, so x = +6.

Calculate

Oxidation state (2)

2What is the oxidation state of manganese in the permanganate ion, MnO₄⁻?

x + 4(−2) = −1, so x = +7.

Calculate

Oxidation state (3)

3What is the oxidation state of chromium in the dichromate ion, Cr₂O₇²⁻?

2x + 7(−2) = −2, so 2x = +12, x = +6.

Reactivity 3.2

Agents and half-equations

An oxidising agent takes electrons (and is itself reduced); a reducing agent gives electrons (and is oxidised). Redox equations split into two half-equations that must balance for electrons.

A more reactive metal displaces a less reactive one from solution — the basis of the reactivity series.

Quick check

Define oxidation

?In terms of electrons, oxidation is:

Reactivity 3.2

Voltaic (galvanic) cells

A voltaic cell uses a spontaneous redox reaction to make electricity. Oxidation happens at the anode (negative), reduction at the cathode (positive); a salt bridge completes the circuit.

The cell voltage is E°cell = E°(cathode) − E°(anode).

Calculate

Cell potential

4A cell uses Cu²⁺/Cu (E° = +0.34 V) as cathode and Zn²⁺/Zn (E° = −0.76 V) as anode. Calculate E°cell.

V

E°cell = E°(cathode) − E°(anode) = 0.34 − (−0.76).

Reactivity 3.2

Electrolytic cells

An electrolytic cell does the opposite: it uses electrical energy to drive a non-spontaneous redox reaction. Positive ions (cations) move to the cathode to be reduced; negative ions (anions) go to the anode to be oxidised. Used for extracting reactive metals and electroplating.

Quick check

Electrolysis electrodes

?During electrolysis, positive ions (cations) are attracted to and reduced at the:

AHL — Reactivity 3.2

Electrode potentials & electrolysis

AHL uses standard electrode potentials to predict feasibility (spontaneous when E°cell > 0) and links them to Gibbs energy through ΔG° = −nFE°cell. Quantitative electrolysis uses Q = It and n(e⁻) = Q ÷ F (Faraday's constant, 96 500 C mol⁻¹).

AHL calculate

Gibbs energy of a cell

HFor a cell with n = 2 and E°cell = 1.10 V, calculate ΔG° in kJ (F = 96 500 C mol⁻¹).

kJ

ΔG° = −nFE° = −(2 × 96 500 × 1.10) J, then ÷ 1000 for kJ.

Sort it

Oxidation, reduction or electrolysis?

Tap a phrase, then the box it belongs in.

🔺 Oxidation

🔻 Reduction

🔌 Electrolysis

Match it

Match term to meaning

Tap an item on the left, then its match on the right.

Term

Meaning

Recap

The big ideas to know

Redox: OIL RIG — oxidation is loss, reduction is gain of electrons

Oxidation states: elements 0; O = −2, H = +1; sum = overall charge

Voltaic cell: spontaneous, makes electricity; E°cell = E°cathode − E°anode

Electrolytic cell: electrical energy drives a non-spontaneous reaction

You've covered Electron transfer — redox for IB Diploma Chemistry HL. Press Finish to see your score.

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