Q1.
A green solution, X, is thought to contain [Fe(H2O)6]2+ ions.
(a)     The presence of these ions can be confirmed by reacting separate samples of solution X with aqueous ammonia and with aqueous sodium carbonate.
Write equations for each of these reactions and describe what you would observe.
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(4)
(b)     A 50.0 cm3 sample of solution X was added to 50 cm3 of dilute sulfuric acid and made up to 250 cm3 of solution in a volumetric flask.
A 25.0 cm3 sample of this solution from the volumetric flask was titrated with a 0.0205 mol dm−3 solution of KMnO4
At the end point of the reaction, the volume of KMnO4 solution added was 18.70 cm3.
(i)      State the colour change that occurs at the end point of this titration and give a reason for the colour change.
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(2)
(ii)     Write an equation for the reaction between iron(II) ions and manganate(VII) ions.
Use this equation and the information given to calculate the concentration of iron(II) ions in the original solution X.
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(5)
(Total 11 marks)
Q2.
(a)     In the Haber Process for the manufacture of ammonia, the following equilibrium is established in the presence of a heterogeneous catalyst.
N2(g) + 3H2(g)  2NH3(g)
Identify the heterogeneous catalyst used in this process and state what is meant by the term heterogeneous.
A heterogeneous catalyst can become poisoned by impurities in the reactants. 
Give one substance which poisons the heterogeneous catalyst used in the Haber Process and explain how this substance poisons the catalyst.
(5)
(b)     State what is observed when an excess of aqueous ammonia reacts with an aqueous iron(II) salt. Write an equation for this reaction.
(4)
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(Total 9 marks)
Q3.
A representation of a hydrogen–oxygen fuel cell that operates in alkaline conditions is
Pt|H2|H2O||O2|OH–|Pt
(a)     (i)      Write a half-equation for the reaction that occurs at each electrode.
Use the half-equations to deduce an overall equation for the cell.
Half-equation at positive electrode ______________________________
Half-equation at negative electrode ______________________________
Overall equation _____________________________________________
(3)
(ii)     State and explain the effect, if any, of increasing the pressure of oxygen on the e.m.f of this cell.
Effect on e.m.f. _________________________________________________
Explanation ____________________________________________________
______________________________________________________________
(2)
(b)     Complete the diagram to show how the e.m.f. of a hydrogen–oxygen fuel cell changes with time.
 
(1)
(c)     (i)      Suggest the effect, if any, on the e.m.f. of this cell if the surface area of each platinum electrode is increased.
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(1)
(ii)     State the main environmental advantage of using a hydrogen–oxygen fuel cell to power a car.
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(1)
(d)     Suggest why the use of a hydrogen–oxygen fuel cell might not be carbon-neutral.
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(1)
(Total 9 marks)
Q4.
Hydrogen–oxygen fuel cells are used to provide electrical energy for electric motors in vehicles.
(a)     In a hydrogen–oxygen fuel cell, a current is generated that can be used to drive an electric motor.
(i)      Deduce half-equations for the electrode reactions in a hydrogen–oxygen fuel cell.
Half-equation 1 _________________________________________________
Half-equation 2 _________________________________________________
(2)
(ii)     Use these half-equations to explain how an electric current can be generated.
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(2)
(b)     Explain why a fuel cell does not need to be recharged.
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(1)
(c)     To provide energy for a vehicle, hydrogen can be used either in a fuel cell or in an internal combustion engine.
Suggest the main advantage of using hydrogen in a fuel cell rather than in an internal combustion engine.
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(1)
(d)     Identify one major hazard associated with the use of a hydrogen–oxygen fuel cell in a vehicle.
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(1)
(Total 7 marks)
Q5.
Use the standard electrode potential data in the table below to answer the questions which follow.
                                                                                                        E/ V
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                       Ce4+(aq) + e–          Ce3+(aq)                              +1.70
   MnO–(aq) + 8H+(aq)+ 5e–          Mn2+(aq) + 4H2O(l)               +1.51
                         Cl2(g) + 2e–          2Cl–(aq)                               +1.36
       VO2+(aq) +2H+(aq) + e–          VO2+(aq) + H2O(l)               +1.00
                       Fe3+(aq) + e–          Fe2+(aq)                               +0.77
   SO42–(aq) + 4H+(aq) + 2e–          H2SO3(aq) + H2O(l)            +0.17
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(a)     Name the standard reference electrode against which all other electrode potentials are measured.
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(1)
(b)     When the standard electrode potential for Fe3+(aq) / Fe2+(aq) is measured, a platinum electrode is required.
(i)      What is the function of the platinum electrode?
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(ii)     What are the standard conditions which apply to Fe3+(aq)/Fe2+(aq) when measuring this potential?
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(3)
(c)     The cell represented below was set up under standard conditions.
Pt|H2SO3(aq), SO42–(aq)||MnO4–(aq), Mn2+(aq)|Pt
Calculate the e.m.f. of this cell and write an equation for the spontaneous cell reaction.
Cell e.m.f. __________________________________________________________
Equation  __________________________________________________________
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(3)
(d)     (i)      Which one of the species given in the table is the strongest oxidising agent?
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(ii)     Which of the species in the table could convert Fe2+(aq) into Fe3+(aq) but could  not convert Mn2+(aq) into MnO4–(aq)?
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(3)
(e)     Use data from the table of standard electrode potentials to deduce the cell which would have a standard e.m.f. of 0.93 V. Represent this cell using the convention shown in part (c).
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(2)
(Total 12 marks)
Q6.
The table below shows some standard electrode potential data.
 
 	E / V	
ZnO(s)  +  H2O(I)  +  2e–   Zn(s)  +  2OH–(aq)	–1.25	
Fe2+(aq)  +  2e–   Fe(s)                     	–0.44	
O2(g)  +  2H2O(I)   +  4e–    4OH–(aq)               	+0.40	
2HOCl(aq)  +  2H+(aq)  +  2e–   Cl2(g)  +  2H2O(I)	+1.64	
(a)     Give the conventional representation of the cell that is used to measure the standard electrode potential of iron as shown in the table.
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(2)

(b)     With reference to electrons, give the meaning of the term reducing agent.
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(1)
(c)     Identify the weakest reducing agent from the species in the table.
Explain how you deduced your answer.
Species ____________________________________________________________
Explanation _________________________________________________________
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(2)
(d)     When HOCl acts as an oxidising agent, one of the atoms in the molecule is reduced.
(i)      Place a tick (✓) next to the atom that is reduced.
 
Atom that is
reduced	Tick (✓)	
 H	 	
 O	 	
 Cl	 	
(1)
(ii)     Explain your answer to part (i) in terms of the change in the oxidation state of this atom.
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(1)
(e)     Using the information given in the table, deduce an equation for the redox reaction that would occur when hydroxide ions are added to HOCl
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(2)
(f)      The table is repeated to help you answer this question.
 
 	E / V	
ZnO(s)  +  H2O(I)  +  2e–   Zn(s)  +  2OH–(aq)	–1.25	
Fe2+(aq)  +  2e–   Fe(s)                     	–0.44	
O2(g)     +  2H2O(I)   +  4e–    4OH–(aq)               	+0.40	
2HOCl(aq)  +  2H+(aq)  +  2e–   Cl2(g)  +  2H2O(I)	+1.64	
The half-equations from the table that involve zinc and oxygen are simplified versions of those that occur in hearing aid cells.
A simplified diagram of a hearing aid cell is shown in the following figure.
 
(i)      Use data from the table to calculate the e.m.f. of this cell.
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Answer = ________________
(1)
(ii)     Use half-equations from the table to construct an overall equation for the cell reaction.
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(1)
(iii)    Identify which of A or B, in the figure, is the positive electrode. Give a reason for your answer.
Positive electrode _______________________________________________
Reason _______________________________________________________
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(2)
(iv)    Suggest one reason, other than cost, why this type of cell is not recharged.
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(1)
(Total 14 marks)
Q7.
Hydrogen–oxygen fuel cells can operate in acidic or in alkaline conditions but commercial cells use porous platinum electrodes in contact with concentrated aqueous potassium hydroxide. The table below shows some standard electrode potentials measured in acidic and in alkaline conditions.
 
Half-equation	Eο /V	
  O2(g) + 4H+(aq) + 4e–  →      2H2O(l)	+1.23	
  O2(g) + 2H2O(l) + 4e–   →      4OH–(aq)	+0.40	
2H+(aq) + 2e–   →      H2(g)	0.00	
2H2O(l) + 2e–   →      2OH–(aq) + H2(g)	– 0.83	
(a)     State why the electrode potential for the standard hydrogen electrode is equal to 0.00V.
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(1)
(b)     Use data from the table to calculate the e.m.f. of a hydrogen–oxygen fuel cell operating in alkaline conditions.
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(1)
(c)     Write the conventional representation for an alkaline hydrogen–oxygen fuel cell.
 
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(2)
(d)     Use the appropriate half-equations to construct an overall equation for the reaction that occurs when an alkaline hydrogen–oxygen fuel cell operates. Show your working.
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(2)
(e)     Give one reason, other than cost, why the platinum electrodes are made by coating a porous ceramic material with platinum rather than by using platinum rods.
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(1)
(f)      Suggest why the e.m.f. of a hydrogen–oxygen fuel cell, operating in acidic conditions, is exactly the same as that of an alkaline fuel cell.
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(1)
(g)     Other than its lack of pollution, state briefly the main advantage of a fuel cell over a re-chargeable cell such as the nickel–cadmium cell when used to provide power for an electric motor that propels a vehicle.
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(1)
(h)     Hydrogen–oxygen fuel cells are sometimes regarded as a source of energy that is carbon neutral. Give one reason why this may not be true.
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(1)
(Total 10 marks)
Q8.
An experiment was carried out to measure the e.m.f. of this cell.
Al(s)   Al3+(aq)   H+(aq)   H2(g)   Pt(s)
(a)     The aluminium used as the electrode is rubbed with sandpaper prior to use.
Suggest the reason for this.
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(1)

(b)     Draw a labelled diagram of a suitable apparatus for the right-hand electrode in this cell. You do not need to include the salt bridge or the external electrical circuit.

 

 

(2)
(c)     A simple salt bridge can be prepared by dipping a piece of filter paper into potassium carbonate solution. Explain why such a salt bridge would not be suitable for use in this cell.
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(2)
(Total 5 marks)
Q9.
The electrons transferred in redox reactions can be used by electrochemical cells to provide energy.
Some electrode half-equations and their standard electrode potentials are shown in the table below.
 
Half-equation	Eο/V	
Cr2O72–(aq) + 14H+(aq) + 6e– → 2Cr3+(aq) + 7H2O(l)	+1.33	
Fe3+(aq) + e– → Fe2+(aq)	+0.77	
2H+(aq) + 2e– → H2(g)	0.00	
Fe2+(aq) + 2e– → Fe(s)	–0.44	
Li+(aq) + e– → Li(s)	–3.04	
(a)     Describe a standard hydrogen electrode.
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(4)
(b)     A conventional representation of a lithium cell is given below.
This cell has an e.m.f. of +2.91 V
Li(s) | Li+(aq) || Li+(aq) | MnO2(s) , LiMnO2(s) | Pt(s)
Write a half-equation for the reaction that occurs at the positive electrode of this cell.
Calculate the standard electrode potential of this positive electrode.
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(2)
(c)     Suggest what reactions occur, if any, when hydrogen gas is bubbled into a solution containing a mixture of iron(II) and iron(III) ions. Explain your answer.
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(2)
(d)     A solution of iron(II) sulfate was prepared by dissolving 10.00 g of FeSO4.7H2O
(Mr = 277.9) in water and making up to 250 cm3 of solution. The solution was left to stand, exposed to air, and some of the iron(II) ions became oxidised to iron(III) ions. A 25.0 cm3 sample of the partially oxidised solution required 23.70 cm3 of 0.0100 mol dm–3 potassium dichromate(VI) solution for complete reaction in the presence of an excess of dilute sulfuric acid.
Calculate the percentage of iron(II) ions that had been oxidised by the air.
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(6)
(Total 14 marks)

Mark schemes
Q1.
(a)     [Fe(H2O)6]2+ + 2NH3 → Fe(H2O)4(OH)2 + 2NH4+
Allow equation with OH− provided equation showing formation of OH− from NH3 given
1
Green precipitate
1
[Fe(H2O)6]2+ + CO32− → FeCO3 + 6H2O
1
Green precipitate
effervescence incorrect so loses M4
1
(b)     (i)      Colourless / (pale) green changes to pink / purple (solution)
Do not allow pale pink to purple
1
Just after the end−point MnO4− is in excess / present
1
(ii)     MnO4− + 8H+ + 5Fe2+ → Mn2+ + 4H2O + 5Fe3+
1
Moles KMnO4 = 18.7 × 0.0205 / 1000 = (3.8335 × 10−4)
Process mark
1
Moles Fe2+ = 5 × 3.8335 × 10−4 = 1.91675 × 10−3
Mark for M2 × 5
1
Moles Fe2+ in 250 cm3 = 10 × 1.91675 × 10−3 = 0.0191675 moles in 50 cm3
Process mark for moles of iron in titration (M3) × 10
1
Original conc Fe2+ = 0.0191675 × 1000 / 50 = 0.383 mol dm−3
Answer for moles of iron (M4) × 1000 / 50
Answer must be to at least 2 sig. figs. (0.38)
1
[11]
Q2.
(a)     Iron
1
Heterogeneous; catalyst in a different phase from
that of the reactants
1
          Poison; a sulphur compound (allow sulphur)
1
Poison strongly adsorbed onto active sites/ blocked
1
Poison not desorbed or reactants not adsorbed or
catalyst surface area reduced
1
(b)     Pale green solution                                                                                          1
Green precipitate formed
1
Insoluble in excess ammonia
1
Equation:
 Species
1
Balance
1
NB     Allow equations with H2O and OH – if reaction of H2O with NH3 also given
Max 4
[9]
Q3.
(a)     (i)      M1 Positive electrode O2 + 2H2O +4e(-) → 4OH-
M2 Negative electrode H2 + 2OH- → 2H2O + 2e(-)
Allow multiples, ignore state symbols
If equations both correct but at the wrong electrodes allow 1 mark
1
1
2H2 + O2 → 2H2O
Mark independently
Must be this way round
1
(ii)     Increase (emf )
If decrease/no change then CE=0/2; if blank then mark on
1
2H2 + O2 → 2H2O will move to the right
Or overall equation moves to the right
Allow O2 + 2H2O +4e- → 4OH- will move to the right / oxygen half equation moves to the RHS / EΘ O2|OH- half cell moves to the right
1
(b)      
Must start at y-axis
1
(c)     (i)      Unchanged
1
(ii)     Water is the only product / fuel cell does not give out pollutants such as NOx or CO2 or SO2 or C or CO or CxHy or unburnt hydrocarbons
Not fuel cell does not give out pollutants unless pollutant stated
1
(d)     CO2 is released because fossil fuels are burned to produce electricity to generate hydrogen
OR
CO2 is released when methane reacts with steam to produce hydrogen
Allow CO2 is released to produce the hydrogen
1
[9]
Q4.
(a)    (i)      H2 + 2OH- → 2H2O + 2e- / H2 → 2H+ + 2e-
Any order
1
O2 + 4e- + 2H2O → 4OH- / O2 + 4H+ + 4e- → 2H2O
1
(ii)     Hydrogen (electrode) produces electrons
Ignore reference to salt bridge
Do not allow at negative / positive electrode – must identify hydrogen and oxygen
1
Oxygen (electrode) accepts electrons
Allow electrons flow to the oxygen electrode
1
(b)    Hydrogen / the fuel / reactants supplied continuously / fed in
Do not accept oxygen supplied as the only statement
1
(c)    In the fuel cell, a greater proportion of the energy available from the hydrogen–oxygen reaction is converted into useful energy
Allow less energy wasted / more efficient
Do not allow reference to safety
1
(d)    Hydrogen is flammable / H+ corrosive / OH– corrosive / hydrogen explosive
1
[7]
Q5.
(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]
Q6.
(a)    Pt|H2|H+||Fe2+|Fe
Allow 1 for correct order of symbols but lose second mark for a wrong phase boundary(s) / Pt missing / extra Pt on RHS, additional phase boundary
Note, allow one mark only for correct symbol in reverse:
Fe|Fe2+||H+|H2|Pt
Allow dashed lines for salt bridge
Ignore state symbols
Ignore 2 if used before H+
2
(b)    Electron donor
Allow (species that) loses electrons
Do not allow reference to electron pairs
1
(c)    Cl2 / chlorine
If M1 blank or incorrect cannot score M2
1
(Species on RHS / electron donor) has most positive / largest EΘ / 
has highest potential
Do not allow reference to e.m.f. or E(cell)
1
(d)    (i)      Cl / chlorine
1
(ii)     Chlorine +1 to chlorine 0
CE if chlorine not identified in part (i)
Allow chlorine +1 to chlorine –1 (in Cl–)
Allow oxidation state decreases by one OR two
Allow oxidation state changes by –1 OR –2
1
(e)    4HOCl + 4H+ + 4OH-→ 2Cl2 + O2 + 6H2O
OR
4HOCl → 2Cl2 + O2 + 2H2O
Allow one mark for any incorrect equation that shows
HOCl → Cl2 + O2
Allow multiples
Ignore state symbols
Penalise one mark for uncancelled or uncombined species (eg H2O + H2O instead of 2H2O)
2
(f)    (i)      e.m.f. = 0.40 – (–1.25) = 1.65 (V) / +1.65 (V)
Allow –1.65 (V)
1
(ii)     2Zn + O2 → 2ZnO
Allow multiples
Ignore state symbols
Do not allow uncancelled species
If more than one equation given, choose the best
1
(iii)    A / stainless lid
If M1 incorrect or blank CE=0
1
O2 (electrode) has a more positive EΘ / oxygen (electrode) requires / gains electrons from external circuit
Or reference to the overall equation and a link to electrons going into A
Allow oxygen is reduced and reduction occurs at the positive electrode
OR Zinc (electrode) has more negative EΘ
Do not allow reference to e.m.f. or E(cell)
1
(iv)    (Cell) reaction(s) cannot be reversed / zinc oxide cannot be reduced to zinc by passing a current through it / zinc cannot be regenerated
Allow danger from production of gas / oxygen produced / hydrogen produced
1
[14]
Q7.
(a)     By definition
allow ‘set to this value’
1
(b)     1.23 V
Allow + or –
1
(c)     Pt|H2(g)|OH–(aq),H2O(l)||O2(g)|H2O(l),OH–(aq)|Pt
H2O not essential, allow reverse order
Correct but with Pt missing
1
Includes Pt with correct representation
1
(d)     Uses O2 + 2H2O + 4e– → 4OH–
And (2×) 2OH– + H2 → 2H2O + 2e–
1
2H2 + O2 → 2H2O
1
(e)     Increases the surface area (so reaction faster)
1
(f)      Overall reaction is the same (2H2 + O2 → 2H2O)
Or shows e.m.f. is the same
1
(g)     Hydrogen and oxygen supplied continuously
OR
Can be operated without stopping to recharge
Or can be refuelled quickly
Allow any one mark
1
(h)     Hydrogen may need to be made using an energy source that is
not ‘carbon neutral’
1
[10]
Q8.
(a)     To remove the oxide layer on the aluminium
Do not allow ‘cleaning’ or ‘removal of grease’.
Do not allow ‘removal of impurities’ without qualification.
1
(b)     An appropriate method for delivering H2 gas over a Pt electrode
Need H2 gas and Pt electrode labelled (allow gas delivered directly below the electrode).
1
The Pt electrode must clearly be in contact with a solution of a named acid.
Ignore any concentration or pressure values.
Ignore absence of bubbles.
Allow if electrode is below outer acid level.
1
(c)     The carbonate ion reacts with the acid (in the SHE) / reaction between carbonate and Al3+
Lose this mark if aluminium carbonate formed but mark on.
1
Reaction given (either equation or products specified)
OR H+ / Al3+ concentrations change / cell e.m.f. altered
1
[5]
Q9.
(a)     Hydrogen/H2 gas/bubbles
1
1.0 mol dm–3 HCl/H+
1
At 298K and 100kPa
Allow 1 bar instead of 100 kPa
Do not allow 1 atm
1
Pt (electrode)
1
(b)     Li+ + MnO2 + e– → LiMnO2
Ignore state symbols
1
–0.13(V)
1
(c)     Fe3+ ions reduced to Fe2+
Can score from equation/scheme
1
Because E(Fe3+(/Fe2+)) > E(H+/H2)/E(hydrogen)
Allow emf/Ecell +ve/0.77V
Allow Fe3+ better oxidising agent than H+
Allow H2 better reducing agent than Fe2+
Only award this explanation mark if previous mark given
1
(d)     Moles Cr2O72– =23.7 × 0.01/1000 = 2.37 × 10–4
1
1 mol Cr2O72– reacts with 6 mol Fe2+ so moles
Fe2+ in 25 cm3 = 6 × 2.37 × 10–4 = 1.422 × 10–3
1
M1 × 6
Moles Fe2+ in 250 cm3 = 1.422 × 10–2
M2 × 10 or M4/10
1
Original moles Fe2+ = 10.00/277.9 = 0.0360
Independent mark
1
Moles Fe2+ oxidised = 0.0360 – 0.0142 = 0.0218
M4 – M3
1
% oxidised = (0.0218 × 100)/0.0360 = 60.5%
(M5 × 100)/M4
Allow 60 to 61
Note Max 3 if mol ratio for M2 wrong
eg 1:5 gives 67.1%
1:1 gives 93.4%
Note also, 39.5% (39-40) scores M1, M2, M3 and M4 (4 marks)
1
[14]