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Question 19N.3.hl.TZ0.20

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Date November 2019 Marks available [Maximum mark: 9] Reference code 19N.3.hl.TZ0.20
Level hl Paper 3 Time zone TZ0
Command term Calculate, Deduce, Formulate, Outline, Suggest Question number 20 Adapted from N/A
20.
[Maximum mark: 9]
19N.3.hl.TZ0.20

A proton-exchange membrane (PEM) fuel cell uses pure hydrogen gas as the fuel and a proton exchange membrane as the electrolyte.

 

(a)

Deduce the half-equations for the reactions occurring at the electrodes.


Anode (negative electrode):

Cathode (positive electrode):

[2]

Markscheme

Anode (negative electrode):
H2 (g) → 2H+ (aq) + 2e− ✔

Cathode (positive electrode):
O2 (g) + 4H+ (aq) + 4e− → 2H2O (l) ✔

NOTE: Accept any correct integer or fractional coefficients. Award [1 max] for M1 and M2 if correct half-equations are given at the wrong electrodes OR if incorrect reversed half-equations are given at the correct electrodes.

(b(i))

Calculate the cell potential, Eθ, in V, using section 24 of the data booklet.

[1]

Markscheme

(+)1.23 «V» ✔

NOTE: Do not accept “-1.23 «V»”.

(b(ii))

Suggest how PEM fuel cells can be used to produce a larger voltage than that calculated in (b)(i).

[1]

Markscheme

connect several fuel cells in series
OR
increase pressure/concentration of reactant/hydrogen/oxygen ✔

NOTE: Do not accept changes in [H+]/pH as they do not affect cell potential in this case.
Do not accept reference to quantity for “concentration”.

(c)

Suggest an advantage of the PEM fuel cell over the lead-acid battery for use in cars.

[1]

Markscheme

liquid in cell is less/not corrosive
OR
does not contain lead/toxic chemicals
OR
larger energy density/charge capacity/current per unit mass
OR
does not have to be charged prior to use / is always ready for use «as long as fuel is available» ✔

A dye-sensitized solar cell (DSSC) uses light energy to produce electricity.

(d(i))

Outline the functions of the dye, TiO2 and the electrolyte in the operation of the DSSC.

Dye: 

TiO2:

Electrolyte:

[3]

Markscheme

Dye:
absorbs photons/light
OR
releases electrons ✔

TiO2:
conducts current/electricity
OR
semiconductor ✔

Electrolyte:
reduces/regenerates «the oxidized» dye ✔

(d(ii))

Suggest an advantage of the DSSC over silicon-based photovoltaic cells.

[1]

Markscheme

Any one of:
cheaper/ease of manufacture
OR
plentiful and renewable resources «to construct DSSC cells» ✔

use light of lower energy/lower frequency/longer wavelength
OR
use of nanoparticles provides large surface area for exposure to sunlight/sun/light
OR
can absorb better under cloudy conditions ✔

operate at lower «internal» temperatures
OR
better at radiating heat away «since constructed with thin front layer of conductive plastic compared to glass box in photovoltaic cells» ✔

better conductivity ✔

more flexible/durable ✔

NOTE: Accept “lower mass/lighter «so greater flexibility to integrate into windows etc.»” OR “greater power-conversion efficiency «with latest DSSC models»”.

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