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.

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.

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.

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

i

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     

ii

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⁻