Electrode Potentials and Fuel Cells Test Ms
1.	(i)	6HNO3 + S → H2SO4 + 6NO2 + 2H2O/
4HNO3 + S → H2SO4 + 4NO2 + H2/
2HNO3 + S → H2SO4 + NO2 + N2	1
(ii)	from (+)5 to (+)4 	1
[2]
 
2.	4NO2 + O2 + 2H2O → 4HNO3 (1)
	N from +4 to +5
O from 0 to –2 (1) Could be below equation	2
[2]
 
3.	(a)	A = Platinum(electrode)
B = H+(aq) / HCl(aq) / other suitable acid
C = Voltmeter / galvanometer
D = Cl2(g)
State symbols needed for B and D
All correct = 2, 3 correct = 1	2
(b)	(i)	Arrow marked on or close to wire via voltmeter pointing from
hydrogen half cell to chlorine half cell	1
Electrons flow to half cell with more +ve standard electrode
potential	1
(ii)	Pressure = 1 Atm / 100 kPa
Temp = 298 K / 25°C
Concentration = 1 mol dm–3
All 3 correct = 2 marks 2 correct = 1 mark	2
 (c)	The standard electrode potential for ClO3– / ½Cl2 is more positive
than that of ½ Cl2 / Cl–	1
ClO3– has a greater tendency to gain electrons than Cl2 / ClO3– is a
better oxidising agent than Cl2	1
Alternative:
Because Eο is positive, the reaction will go from left to right
therefore ClO3– is reduced so it must be a better oxidising
agent than chlorine.
[8]

4.	(a)	Emf of a cell / voltage / potential difference / cell potential	1
Comprising half cell combined with standard hydrogen electrode	1
Conc = 1 mol.dm–3; Pressure (of H2) = 1 atm; Temp = 298K	1
(all of above = 1 mark)
(b)	+0.16 V (unit required)	1
[4]

5.	(a)	Cr2O72– + 14H+ + 6Fe2+ → 2Cr3+ + 6Fe3+ + 7H2O	1
		Cr2O72– / Cr3+ has more positive electrode potential	1
		Therefore Cr2O72– is the stronger oxidising agent which
oxidises Fe2+ to Fe3+ (ora)	1
 
(b)	Emf = (+) 0.56 V	1
[4]

6.	
(a)	(i)	Diagram shows:
Voltmeter + salt bridge + complete circuit	1
Solution labelled Cu2+ and electrode labelled Ag	1

(ii)	Direction from Cu(s) to Ag(s) (must be in / close to wire)	1
(iii)	0.80 – 0.34 = 0.46 V	1
(iv)	Cu + 2Ag+ → Cu2+ + 2Ag	1
 
(b)	Standard Electrode Potential for chlorine is more positive than
Fe3+ therefore it is a better oxidising agent than Fe3+ (do not
accept Eο is larger or smaller)	1
Standard Electrode Potential for iodine is less positive than
Fe3+ therefore it is a poorer oxidising agent than Fe3+	1
(Accept release of electrons/equilibrium arguments)
[7]

7.	(a)	(i)	2MnO4– + 10Cl– + 16H+ → 2Mn2+ + 5Cl2 + 8H2O	1
	correct species on both sides of equation equation balanced	1
(ignore electrons for first mark, penalise for balance)
(ii)	Chlorine –1 → 0	1
Manganese +7 → +2	1
Link to (i) and allow ecf
(iii)	Chloride ion oxidised (not chlorine)	1
Manganate(VII) ion reduced (not manganese)	1
 (b)	0.16 V too small/rate too slow/insufficient activation
energy/not standard conditions	1
[7]

 8.	(i)	oxidation: Fe → Fe2+ + 2e– (1)
reduction: V3+ + e– → V2+ (1)	2
(ii)	Ecell = 0.18 V (1)	1
[3]

 9.	(i)	system III × 2 and reversed + system IV (1)
2H2 + O2 → 2H2O/
H2 + ½O2 → H2O (1)	2
(ii)	advantages:
only H2O formed/ non-polluting
greater efficiency (1)
	disadvantages:
H2 difficult to store (1)
H2 difficult to manufactured initially /
limited life cycle of H2 adsorber/absorber (1)	4
[6]