5.3 questions ms
 
1.	(a)	gains electrons  (1)	1
 [1]
 
2.	(a)	(i)	Fe2+	1
(ii)	F2O	1
(iii)	Fe2+	1
Cl–	1
Use list principle if more than two answers
(b)	(i)	e.m.f. = E(rhs) – E(lhs)	1
= 1.52 – 0.77 = 0.75
(0.75 scores first mark also)	1
(ii)	Fe2+ ® Fe3+ + e–	1 
(iii)	Decrease	1
(Increase is CE, no further marks)
	Equilibrium (or reaction) shifts to R	1
(or L if refers to half equation in table)
(or in favour of more Fe3+)
(or more Fe3+ formed)
(or more electrons formed)
	Electrode potential (for Fe3+/Fe2+) less positive (or decreases)	1
[10]

3.	(a)	Fe2+ or Fe(II)	1
(b)	(i)	6 or (VI)	1
(ii)	3 or (III)	1
(c)	(i)	0.5	1
(ii)	2Mn2+ + 8H2O + 5S2O ® 10SO + 2MnO + 16H+
Both SO and MnO on right	1
Balanced	1
[13]

4.	(a)	(i)	0.60 V	1
(ii)		1

(b)	(i)		Species	1
Balanced	1
(ii)	The concentration of the ions change or are no longer standard or
the e.m.f is determined when no current flows	1
(iii)	Unchanged	1
(iv)	Increased	1
Equilibrium  displaced to the right	1
	Electrons more readily accepted or more reduction occurs
or electrode becomes more positive (Q o L)	1
(c)		1
5 or V	1
	1
[12]

5.	(a)	Oxidising agents take/remove/accept/gain electrons  (1)
Not ‘electron pair’	1
(b)	(i)	Cl2(g) at 100 kPa / 1 bar / 1atm  (1)
[Cl–] = 1 molar / 1M  (1)
Allow 1M HCl
Temperature = 298K / 25°C  (1)
Do not use list principle for other incorrect species
(ii)	Lower [Cl–] or reduce temperature
Increase pressure or concentration of Cl2  (1)
CE if change incorrect
Equilibrium displace to right  (1)
or if reduced temperature given, reaction exothermic
Allow a correct explanation when no change given	5
(c)	(i)	Cl–  (1)
(ii)	Fe3+ ;  (1)  NO3–  (1)
Penalise by list principle
Note: H+ is incorrect
(iii)	V2+,  (1)  Fe2+  (1)
Penalise by list principle	5
[11]

6.	(a)	(Standard) hydrogen (electrode) (1)	1

(b)	(i)	To allow transfer of electrons / provide a reaction surface (1)
(ii)	298 K (1)
Both F3+ (aq) and Fe2+ (aq) have a concentration of 1 mol dm–3 (1) (QoL)
OR [H+] = 1 mol dm–3 
NOT zero current or 100 kPa
		3
(c)	+1.34 V (1)
2 MnO4– + 5 H2SO3 ® 2 Mn2+ + 5 SO42– + 3 H2O +4 H+
Correct species / order (1)
Balanced and cancelled (1)
Allow one for 2 MnO4– + 5 H2SO3 ® 2 Mn2+ + 5 SO42–
		3
(d)	(i)	Ce4+ (aq) (1)
(ii)	VO2+ (aq) (1); Cl2 (1)
Penalise additional answers to zero
		3 
(e)	Pt | Fe2+ (aq), Fe3+ (aq) || Ce4+(aq), Ce3+ (aq) | Pt
Correct species (1)
Correct order (1)
Deduct one mark for each error
		2
[12]
 
7.	(a)	Cell e.m.f.: 1.93 (v) CE if negative value given (1)
Half equation: Mg ® Mg2+ + 2 e– (1)
or 
Ignore state symbols
Mark on after an AE
		2 
(b)	Change in e.m.f.: increases (1)
Mark on even if incorrect
	Explanation: Equilibrium displaced to Mg2+ or to the left (1)
cell reaction or overall reaction goes to the right
Electrode is more negative or E decreases
or gives more electron
or forms more Mg2+ ions
Mark separately
		3 

(c)	Cell e.m.f. : –0.84 (V) (1)
	Explanation: Fe is giving electrons or forming Fe2+
or reaction goes in the reverse direction (1)
Mark on after AE
		2
N.B. In (a) and (c) mark on if no value given,
but CE in both (a) and (c) if e.m.f. = 0
[7]

8.	(a)	(i)	(standard) hydrogen (electrode) / hydrogen half cell not
hydrogen cell (1)
reference electrode / electrode to which others are compared (1)	2
(ii)	0.00(V) / 0 / zero (1)	1
 
(b)	(i)	emf = –0. 14 –(–0.25)
	= + 0.11 V / allow 0.11V not – 0.11V (1)	1
(ii)	electrode D / Sn2+ /Sn /tin / right hand electrode  (1)	1
(iii)	Ni + Sn2+ ® Ni2+ + Sn (ignore state symbols)  (1)	1

(c)	(i)	e.m.f = –0.44 – (–0.14) = –0.30 (V) / emf for cell is – ve
comparison of standard electrode potentials  (1)
	+ve e.m.f for feasible reaction / tin is a weaker reducing agent
\ would not occur  (1)
if correct DG argument used, allow both marks	2
 
(ii)	manganese will decrease in size / disappear / eaten away / dissolves /
solution turns (pale) pink (1)
effervescence / bubbles (of colourless gas) / fizzing not gas
given off (1)
	reaction likely to occur is Mn + (2)H+ ® Mn2+ + H2  (1)
or the same ideas expressed in words
	+ve e.m.f. / +1.18 V / Mn is strong reducing agent / has
large – ve E()  (1)
(not just Mn is more reactive)	4
[12]

9.	(a)	oxidising agent accepts electrons  (1)	1

(b)	Stronger oxidising agent  	H+  (1)
Equation 	 (1)
e.m.f. 		0.000 – (–0.004) = + 0.004 V  (1)	3
 
(c)	Equation 	AgF(s) + Cl–(aq) ® AgCl(s) + F–(aq)  (1)
e.m.f. 	= + 0.78 – 0.22 = +0.56 V  (1)	2
(d)	(i)	Silver lies above hydrogen in the electrochemical series
Or Ag+(aq) is a stronger oxidising agent than H+(aq)  (1)
(ii)	Hydrogen halide 	Hl   (1)
	Equation 	H+(aq) + 1–(aq) + Ag(s) ® Agl(s) + H2(g)  (1)
e.m.f. 	40.00 – (–0.15) = + 0.15 V  (1)	4
[10]

10.	(a)	Reducing agent	Br2 (1)
Half equation	Br2 + 6H2O ® 2BrO + 12H+ + 10e– (1)	2
(b)	(i)	Temperature 	298 K or 25°C (1)
Concentration	1.00 M or 1.00 mol dm–3 (1)
(ii)	Secondary standard electrode 	calomel 
	or Ag/AgCl (1)
	Reason 	Easier to use 
	or hydrogen electrode (1)
	difficult to use	4 
(c)	(i)	[Co(H2O)6]3+ (1)
(ii)	[Co(H2O)6]2+ (1)
(iii)	E for [Co(CN)6]3–/[Co(CN)6]4– is negative with respect to H+/H2 (1)
Electrons flow to H+/H2 and H+ reduced (1)	4
[10]

11.	(a)	Fe + Cu2+ ® Cu + Fe2+  (1)
	allow use of soluble Cu salt eg CuSO4
ignore state symbols	1

(b)	(i)	
	junctions correct  (1)
ignore state symbols
allow alternative symbols for salt bridge
allow if junctions are correct but order is wrong providing
metals on each side of salt bridge are the same ie Fe | Fe2+ not Fe | Cu2+
	order of species correct  (1)
do not give this mark if cell reversed	2 
(ii)	e.m.f. = + 0.34 – (–0.44)
	= + 0.78 V  (1)
must have + sign
allow – 0.78 (V) if reverse cell given in (i)	1
(c)	(i)	e.m.f for cell must be positive for reaction to occur / be feasible /  (1)
spontaneous or DG must be negative)
	Cu(s) + 2H+ ® products
e.m.f = – 0.34\won’t happen /
sensible comparison of the magnitude of E for the electrodes
eg ‘Cu electrode more positive than hydrogen electrode\won’t work’  (1) 
Cu(s) + NO + 4H+ ® products
e.m.f = + 0.96 – 0.34 = +0.62\can occur /
similar sensible comparison  (1)	3 
(ii)	3Cu + 2 NO + 8H+ ® 3Cu2+ + 2NO +4H2O
	species  (1)
balanced – this mark dependent on first mark  (1)	2
(d)	(i)	2Fe + O2 + 2H2O ® 2Fe2+ + 4OH– or 2Fe(OH)2  (1)
ignore state symbols	1
(ii)	anode (only give this mark if explanation attempted)  (1)
	Fe loses e– (\ negative pole) / oxidation occurs  (1)
this mark dependent on anode for first mark	2
(iii)	e.m.f. = + 0.06 V or reference to E for electrodes  (1)
	reasoned argument  (1)
eg positive\should occur / difference so small that reaction unlikely	2
[14]

12.	(a)	Name 	hydrogen electrode (1)
	Conditions 	1 M H+(aq) or 1 M HCl(aq) or 0.5 M H2SO4(aq) (1)
	298 K (1)
	Hydrogen gas at 1 bar or 100 kPa (1)	4
(b)	(i)       E value  1.21 v (1)
           Equation   S2O + 2Ag ® 2SO + 2Ag+ [2] 
(ii)	Change, if any, in electrode potential 	Less positive or decrease (1)
	Explanation 		Equilibrium displaced to the left (1)
		More electrons released (1)	6
[10]

13.	 	(i) Oxidising agents

		Cu2+(aq) (1), H2O2(aq) (1), C12(aq) (1) (3)
if Cr2O(aq) is included, deduct one mark 
(ii)	2I– ® I2    I(–1) ® I(0) (1)
Mn2+ ® MnO   Mn(II) ® Mn(VII) (1)
Cr2O ® 2CrO no change (penalised above)
Cl2 ® ClO–    Cl(0) ® Cl(1) (1)
(iii)	Cu2(aq) + I–(aq) + e– ® Cu+(s) (1)
I–(aq) ® ½ I2(aq) + e– (not reverse)  (1)
H2O2(aq) + 2H+(aq) + 2e– ® 2H2O (1) (1)
Mn2+(aq) + 4H2O (1) ® MnO(aq) + 8H+(aq) + 5e– (not reverse) (1)
Cl2(aq) + e– ® Cl– (aq) (1)
½ Cl2(aq) + 2OH–(aq) ® ClO–(aq) + H2O(l) + e– (not reverse) (1)
Penalise missing or incorrect state symbols once only
(12 MAX)
		11
[11]