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Date	November 2016	Marks available	2	Reference code	16N.3.hl.TZ0.21
Level	HL	Paper	3	Time zone	TZ0
Command term	State	Question number	21	Adapted from	N/A

Question

A fuel cell is an energy conversion device that generates electricity from a spontaneous redox reaction.

The Geobacter species of bacteria can be used in microbial fuel cells to oxidise aqueous ethanoate ions,
CH₃COO⁻(aq), to carbon dioxide gas.

State the half-equations for the reactions at both electrodes.

[2]

A concentration cell is an example of an electrochemical cell.

(i) State the difference between a concentration cell and a standard voltaic cell.

(ii) The overall redox equation and the standard cell potential for a voltaic cell are:

Zn (s) + Cu²⁺ (aq) → Zn²⁺ (aq) + Cu (s) E^θ_cell = +1.10 V

Determine the cell potential E at 298 K to three significant figures given the following concentrations in mol dm⁻³:

[Zn²⁺] = 1.00 × 10⁻⁴ [Cu²⁺] = 1.00 × 10⁻¹

Use sections 1 and 2 of the data booklet.

(iii) Deduce, giving your reason, whether the reaction in (b) (ii) is more or less spontaneous than in the standard cell.

[3]

Dye-sensitized solar cells (DSSC) convert solar energy into electrical energy.

(i) Describe how a DSSC converts sunlight into electrical energy.

(ii) Explain the role of the electrolyte solution containing iodide ions, I⁻, and triiodide ions, I₃⁻, in the DSSC.

[4]

Markscheme

Negative electrode (anode): CH₃COO⁻ (aq) + 2H₂O (l) → 2CO₂ (g) + 7H⁺ (aq) + 8e⁻

Positive electrode (cathode): O₂(g) + 4H⁺ (aq) + 4e⁻ → 2H₂O (l)

Accept equilibrium signs in equations.
Award [1 max] if correct equations are given at wrong electrodes.

i
concentration cell has different concentrations of electrolyte «solutions» «but same electrodes and electrolytes»
OR
standard voltaic cell has different electrodes/electrolytes «but same concentration of electrolytes»
Accept “both half-cells in concentration cell made from same materials”.

ii
« $E = 1.10 - \left( {\frac{{RT}}{{nF}}} \right)\ln \frac{{\left[ {{\text{Z}}{{\text{n}}^{2 + }}} \right]}}{{\left[ {{\text{C}}{{\text{u}}^{2 + }}} \right]}} = 1.10 - \left( {\frac{{8.31 \times 298}}{{2 \times 96500}}} \right)\ln \frac{{{{10}^{ - 4}}}}{{{{10}^{ - 1}}}} = 1.10 + 0.0886 =$ »

(+) 1.19 «V»
3 significant figures needed for mark.

iii
more spontaneous because E > E^θ_cell

photon/«sun»light absorbed by the dye/photosensitizer/«transition» metal complex
OR
dye/photosensitizer/«transition» metal complex excited by photon/«sun»light

electron«s» move«s» to conduction band
OR
electron«s» transferred to semiconductor/TiO₂

I₃⁻ + 2e⁻ → 3I⁻ «at cathode»
OR
triiodide ions/I₃⁻ reduced into/produce iodide ions/I⁻ «at cathode»

iodide ions/I⁻ reduce dye/act as reducing agent AND oxidized into/produce triiodide ions/I₃⁻OR
dye⁺ + e⁻ → dye AND 3I^- → I₃⁻ + 2e⁻

Examiners report

[N/A]

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Options » C: Energy » C.6 Electrochemistry, rechargeable batteries and fuel cells (HL only)

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