Electrode Potential and Fuel Cells Hw Ms
1.	(i)	SO42– → H2S: S from +6 to –2 (1)
I– → I2: I from –1 to 0 (1)	2
(ii)	10H+ + SO42– + 8I– → 4I2 + H2S + 4H2O (1)	1
[3]

2.	Oxidation state of nitrogen goes from +5 to +4 (1)
	Oxidation state of oxygen goes from –2 to 0 (1);
	Correct linking of changes of oxidation state with
reduction and with oxidation (1)	3
If oxidation state of barium given is incorrect max 1 for the oxidation numbers.
Allow ecf from wrong oxidation states for the correct linking mark
Both oxidation and reduction needed
[3]
 
3.	(a)	Emf/voltage/potential difference (of electrochemical cell)	1
comprising a (Cu/Cu2+) half cell combined with a standard
hydrogen electrode	1
1 atm, 1 mol.dm–3, 298K (all 3 needed but can transfer mark if stated in (b))	1

(b)	Salt bridge and voltmeter	1
Platinum electrode dipping into 1 mol dm–3 H+	1
Hydrogen gas feed	1
(Accept a suitable alternative standard electrode)
[6]

4.	(a)	VO2+	1
(b)	(i)	B and D	1
(ii)

	Allow ecf from (b) (i)
Solutions can be reversed.	4
(iii)	298 K / 25 °C temperature
all solutions 1 mol dm–3
Both needed for 1 mark. Ignore any reference to pressure	1
[7]

5.	(a)	Hydrogen ion concentration: 1.00 mol dm–3 (1)
Hydrogen gas pressure: 100 kPa (1)	2 
(b)	Explanation of change: Equilibrium displaced to left (1)
	to reduce constraint (1)
Change in electrode potential: Becomes negative or decreases (1)	3
allow more negative
(c)	(i)	0.43V (1)
(ii)	Half-equation: 2Br– ® Br2 + 2e– (1)
Overall equation: 2BrO3– + 10Br– + 12H+ ® 6Br2 + 6H2O (2)	4
	 or BeO3– + 5Br– + 6H+ ® 3Br2 + 3H2O
species (1)
balanced (1)
[9]

6.	(a)	Platinum electrode  (1)
Hydrogen gas  (1)
H+(aq)  or  HCl  or  H2SO4  (1)
	298 k  (1)
100 kPa  or  1 atm  or 1 bar  (1)
1 M  [H+]  (1)	6
(b)	(i)	1.61 V  (1)
(ii)	Positive  (1)	2

(c)	(i)	1.36 V  (1)
Left to right  (1)
Allow on diagram	2
(d)	Ce4+ + e–  ®  Ce3+  (1)
V3+ + H2O  ®  VO2+ + 2H+ + e–
or reversed
or	V3+ + 2H2O  ®  VO2++ 4H+ + 2e–  (1)
or reversed
Ce4+ + V3+ + H2O  ®  VO2+ + 2H+ + Ce3+
or	2Ce4+ + V3+ + 2H2O  ® VO2+ + 4H+ + 2Ce3+  (1)	3
(e)	VO2+  (1)      VO2+  (1)	2
[15]

7.	(a)	most powerful reducing agent:  Zn;	1 
(b)	(i)	reducing species:	Fe2+	1
(ii)	oxidising species:	Cl2;	1
(c)	(i)	standard electrode potential	1.25 V;	1
(ii)	equation: Tl3+ + 2 Fe2+ ® 2Fe3+ + T1+	balanced;	1
	correct direction;	1
 
 [6]