A2 LEVEL CHEMISTRY

ELECTRODE POTENTIALS AND FUEL CELLS

ASSESSED HOMEWORK

Answer all questions
Max 49 marks

	
Name		…………………………………………………………..

Mark		……../49	……....%	Grade ………

1.	In order to obtain full marks in this question, you must show all your working clearly.
	In its reactions, sulphuric acid, H2SO4, can behave as an acid, an oxidising agent and as a dehydrating agent.
	The displayed formula of pure sulphuric acid is shown below.
		
	Concentrated sulphuric acid will readily oxidise halide ions to the halogen.
	The equation below represents the unbalanced equation for the oxidation of iodide ions by sulphuric acid.
					
		H+	+	SO42–	+	I–	→	I2	+	H2S	+	H2O
(i)	Write the oxidation numbers of sulphur and iodine in the boxes above the equation.
[2]
(ii)	Balance the equation above.

[1]
[Total 3 marks]

2.	The carbonates and nitrates of Group 2 elements decompose when heated.
Barium nitrate decomposes when heated to make barium oxide, nitrogen dioxide and oxygen.
		2Ba(NO3)2(s) → 2BaO(s) + 4NO2(g) + O2(g)
Use oxidation states to explain why this decomposition reaction involves both oxidation and reduction.
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[3]

3.	The standard electrode potential of Cu2+(aq) + 2e–  Cu(s) is +0.34 V.
(a)	Define the term standard electrode potential.
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[3]
(b)	Complete the diagram to show how the standard electrode potential of
Cu2+(aq) + 2e–  Cu(s) could be measured.

[3]
[Total 6 marks]

4.	The standard electrode potentials for some redox systems involving vanadium are shown below.
These are labelled A, B, C and D.
		Eο/ V
	A	VO2+ + 2H+ + e–	 VO2+ + H2O	+1.00
	B	V3+ + e–	 V2+	–0.26
	C	V2+ + 2e–	 V	–1.20
	D	VO2+ + 2H+ + e–	 V3+ + H2O	+0.34
(a)	Which of the vanadium species shown in A, B, C and D is the most powerful oxidising agent?
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[1]

(b)	A student wishes to set up a cell with a standard cell potential of 0.60V.
(i)	Which two of the redox systems, A, B, C or D, should he choose?
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[1]
(ii)	Complete the labelling of the following diagram which shows the cell with a standard cell potential of 0.60V.

[4]
(iii)	The emf of this cell is only 0.60 V under standard conditions. What do you understand by the expression standard conditions?
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[1]
[Total 7 marks]

5.	Use the data below, where appropriate, to answer the questions which follow.
Standard electrode potentials	E/V	
       2H+(aq)  +  2e–  ®  H2(g)	0.00	
            Br2(aq)  +  2e–  ®  2Br–(aq)	+1.09	
(aq)  +  12H+(aq)  +  10e–  ®  Br2(aq)  +  6H2O(l)	+1.52	
	Each of the above can be reversed under suitable conditions.
(a)	State the hydrogen ion concentration and the hydrogen gas pressure when, at 298 K, the potential of the hydrogen electrode is 0.00 V.
Hydrogen ion concentration ......................................................................................
Hydrogen gas pressure ..............................................................................................
(2)
(b)	The electrode potential of a hydrogen electrode changes when the hydrogen ion concentration is reduced. Explain, using Le Chatelier’s principle, why this change occurs and state how the electrode potential of the hydrogen electrode changes.
Explanation of change ...............................................................................................
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Change in electrode potential ....................................................................................
(3)

(c)	A diagram of a cell using platinum electrodes X and Y is shown below.

(i)	Use the data above to calculate the e.m.f. of the above cell under standard conditions.
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(ii)	Write a half-equation for the reaction occurring at electrode X and an overall equation for the cell reaction which occurs when electrodes X and Y are connected.
Half-equation ...................................................................................................
Overall equation ..............................................................................................
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(4)
(Total 9 marks)

6.	Use the data given below, where appropriate, to answer the questions which follow.
Standard electrode potentials in acid solution	E/V	
Mg2+ (aq) + 2e– ® Mg(s)	–2.37
Zn2+ (aq) + 2e– ® Zn(s)	–0.76
Sn4+(aq) + 2e–  ® Sn2+(aq)	+0.15
VO2+(aq) + 2H+(aq)   + e–   ® V3+(aq) + H2O(l)	+0.34
 (aq) + 2H+(aq)   + e–  ® VO2+(aq) + H2O(l)	+1.02
	Ce4+(aq)  + e–  ® Ce3+(aq)	+1.70	
(a)	Give the components of the standard reference electrode used in determining the standard electrode potentials above.  State the conditions under which this standard reference electrode has a potential of 0.00 V.
Components.................................................................................................................
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Conditions...................................................................................................................
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(6)

(b)	A diagram of a cell is shown below.

(i)	Calculate the overall standard potential of this cell.
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(ii)	State the polarity of the zinc electrode.
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(2) 
(c)	A diagram of a cell is shown below.

(i)	Calculate the overall standard potential of the cell.
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(ii)	Deduce the direction of electron flow in the external circuit when the inert electrodes are connected together.
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(2)

(d)	Using data from the table on page 6, derive an equation for the overall redox process which occurs when a solution containing Ce4+,(aq) is added to a solution containing
V3+ (aq).
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(3)
(e)	Which, if any, of the four vanadium-containing species, V2+ (aq), V3+ (aq), VO2+ (aq) and VO2+ (aq), will convert Sn2+(aq) into Sn4+ (aq) in acid solution?
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(2)
(Total 15 marks)

7.	Where appropriate, use the standard electrode potential data in the table below to answer the questions which follow.
							Eο/V	
Zn2+(aq)	+	2e–	→	Zn(s)			–0.76 	
V3+(aq)	+	e–	→	V2+(aq)			–0.26	
+ 2H+(aq)	+	2e-	→		+	H2O(l) 	+0.17 	
VO2+(aq) +2H+(aq)	+	e–	→	V3+(aq)	+	H2O(l)	+0.34	
Fe3+(aq)	+	e–	→	Fe2+(aq)			+0.77	
+ 2H+(aq)	+	e–	→	VO2+(aq)	+	H2O(l)	+1.00	
Cl2(aq)	+	2e–	→	2Cl–(aq)			+1.36 	
 
(a)	From the table above select the species which is the most powerful reducing agent.
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(1)

(b)	From the table above select
(i)	a species which, in acidic solution, will reduce  to VO2+(aq) but will not reduce VO2+(aq) to V3+(aq),
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(ii)	a species which, in acidic solution, will oxidise VO2+(aq) to .
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(2) 
(c)	The cell represented below was set up under standard conditions.
Pt|Fe2+(aq), Fe3+(aq)||Tl3+(aq),Tl+(aq)|Pt	Cell e.m.f. = + 0.48 V
(i)	Deduce the standard electrode potential for the following half-reaction.
Tl3+(aq) + 2e– → Tl+(aq)
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(ii)	Write an equation for the spontaneous cell reaction.
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(3)
 (Total 6 marks)