A2 · Practice questions · Standard cell potentials and feasibility

Subtract, then read the sign.

Six original Cambridge-style questions on cell potentials and feasibility.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Positive minus negative, then check the sign.

E_cell = E(positive) − E(negative) = E_right − E_left.
Electrons flow in the wire from negative to positive.
Feasible if E_cell > 0, but rate and conditions can intervene.
ΔG = −nFE, then ÷ 1000 for kJ.

Calculation

[3 marks]

A cell is made from a Zn²⁺/Zn half-cell and an Ag⁺/Ag half-cell under standard conditions. Calculate E^⊖_cell, write the overall equation and state the direction of electron flow.

E^⊖_cell = +0.80 − (−0.76) = +1.56 V. ✓
Zn + 2Ag⁺ → Zn²⁺ + 2Ag. ✓
Electrons flow through the external wire from the zinc to the silver electrode. ✓

Calculation

[3 marks]

Calculate ΔG^⊖ for the reaction Zn + Cu²⁺ → Zn²⁺ + Cu, given E^⊖_cell = +1.10 V.

n = 2 mol of electrons transferred per mole of reaction. ✓
ΔG^⊖ = −nFE = −2 × 96 500 × 1.10 = −212 300 J mol⁻¹. ✓
ΔG^⊖ = −212 kJ mol⁻¹; negative, so feasible. ✓

Analysis

[3 marks]

Use E^⊖ values to predict whether acidified dichromate(VI) will oxidise (a) iron(II) ions and (b) chloride ions under standard conditions.

(a) E^⊖_cell = 1.33 − 0.77 = +0.56 V, positive, so Fe²⁺ is oxidised to Fe³⁺. ✓
(b) E^⊖_cell = 1.33 − 1.36 = −0.03 V, negative, so Cl⁻ is not oxidised under standard conditions. ✓
So dichromate titrations of Fe²⁺ can be done in HCl, unlike manganate(VII). ✓

Explain

[2 marks]

Give two reasons why a reaction predicted to be feasible from E^⊖ values may not be observed.

The activation energy may be high, so the rate is too slow to observe (E^⊖ gives no information about kinetics). ✓
Conditions may not be standard: different concentrations or temperature change the electrode potentials, especially if E^⊖_cell is small. ✓

Analysis

[3 marks]

Will bromine oxidise iron(II) ions? Use E^⊖(Br₂/Br⁻) = +1.07 V and E^⊖(Fe³⁺/Fe²⁺) = +0.77 V, and calculate ΔG^⊖ for the reaction as written with 2 mol of Fe²⁺.

Br₂ + 2Fe²⁺ → 2Br⁻ + 2Fe³⁺. ✓
E^⊖_cell = 1.07 − 0.77 = +0.30 V, positive, so feasible. ✓
ΔG^⊖ = −2 × 96 500 × 0.30 = −57.9 kJ mol⁻¹. ✓

Explain

[2 marks]

Explain the role of the salt bridge in an electrochemical cell, and what carries the charge through it.

It completes the circuit and keeps each solution electrically neutral. ✓
Ions (e.g. K⁺ and NO₃⁻) move through it; electrons only travel through the external wire. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.