Redox and Electrochemistry - Worksheets, Questions and Revision

12 original exam-style questions - 5 pages of questions with a full mark scheme - free printable PDF.

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A-Level · Chemistry

AC5 Redox and Electrochemistry

AQA 7402/7405/7408 · Calculator allowed · about 125 minutes
Total Marks
Name: _______________________________    Date: ____ / ____ / ______
Answer ALL questions. Show all your working.
1
Potassium manganate(VII), KMnO4, is a strong oxidising agent that is widely used in redox titrations. In acidified solution it reacts with iron(II) ions according to the equation:
MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 5Fe3+ + 4H2O
(a)State the oxidation state of manganese in the MnO4- ion.(1)
(b)State the oxidation state of chromium in the dichromate(VI) ion, Cr2O72-.(1)
(c)The half-equation for the reduction of MnO4- is:
MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O
Use this half-equation and the half-equation for the oxidation of Fe2+ to Fe3+ to show that the overall balanced ionic equation is:
MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 5Fe3+ + 4H2O(3)
(d)Identify the oxidising agent in this reaction and explain your answer in terms of electron transfer.(2)
(Total for Question 1 is 7 marks)
2
When chlorine gas is bubbled into cold, dilute sodium hydroxide solution, a disproportionation reaction occurs.
(a)State the oxidation state of chlorine in Cl2, and the oxidation states of chlorine in the two chlorine-containing products of this reaction, Cl- and ClO-.(2)
(b)Write ionic half-equations for the reduction and the oxidation processes that occur when chlorine reacts with cold, dilute hydroxide ions, OH-.(2)
(c)Combine your two half-equations from part (b) to give the overall ionic equation for the reaction of chlorine with cold, dilute sodium hydroxide solution.(2)
(d)State the term used to describe a reaction, such as this one, in which the same element is simultaneously oxidised and reduced.(1)
(Total for Question 2 is 7 marks)
3
A student sets up a Daniell cell using a zinc half-cell (Zn(s) in 1.00 mol dm-3 ZnSO4(aq)) and a copper half-cell (Cu(s) in 1.00 mol dm-3 CuSO4(aq)) at 298 K, connected by a salt bridge.
Standard electrode potentials: E(Zn2+/Zn) = -0.76 V; E(Cu2+/Cu) = +0.34 V
V high-resistance voltmeter Zn Cu salt bridge 1.00 mol dm⁻³ ZnSO₄(aq) 1.00 mol dm⁻³ CuSO₄(aq) Zn half-cell Cu half-cell
(a)Define the term 'standard electrode potential' of a half-cell.(2)
(b)Write the conventional cell diagram (cell notation) for this Daniell cell under standard conditions.(2)
(c)Calculate the standard emf of this cell.(2)
(d)State which electrode is the negative terminal of the cell and explain your answer.(2)
(Total for Question 3 is 8 marks)
4
This question is based on the required practical for measuring the emf of an electrochemical cell. A student builds the Daniell cell described in Question 3, connecting the two half-cells with a salt bridge and a high-resistance voltmeter, and measures an emf of 1.05 V, slightly lower than the theoretical value of 1.10 V.
(a)Explain why a high-resistance voltmeter, rather than an ammeter or a low-resistance meter, must be used to measure the emf of the cell.(2)
(b)State the purpose of the salt bridge in this cell and name one substance that could be used to make it.(2)
(c)Suggest two reasons, other than the voltmeter resistance, why the measured emf (1.05 V) might be lower than the theoretical value (1.10 V).(2)
(d)The concentration of the CuSO4(aq) in the copper half-cell is decreased to 0.0010 mol dm-3, while the zinc half-cell remains at standard conditions. State and explain the effect this has on the emf of the cell.(3)
(Total for Question 4 is 9 marks)
5
Explain, using ideas about standard electrode potentials and reaction kinetics, why a reaction between two half-cells that is predicted to be thermodynamically feasible from a positive E cell value may not actually be observed to occur, or may occur only extremely slowly, when the two solutions are mixed.
(Total for Question 5 is 6 marks)
6
This question uses practical techniques from the required practical on redox titrations. A student wants to determine the percentage by mass of iron in an iron supplement tablet. One tablet, of total mass 2.50 g, is dissolved in dilute sulfuric acid and the resulting solution made up to exactly 250 cm3 in a volumetric flask.
25.0 cm3 portions of this solution are titrated against 0.0150 mol dm-3 potassium manganate(VII) solution. The mean titre required is 16.00 cm3.
Equation: MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 5Fe3+ + 4H2O
(a)State the colour change observed at the end point of this titration, and explain why no external indicator is required.(2)
(b)Calculate the amount, in mol, of MnO4- used in the titration of one 25.0 cm3 sample.(1)
(c)Calculate the amount, in mol, of Fe2+ present in the 25.0 cm3 sample.(1)
(d)Calculate the total amount, in mol, of Fe2+ in the whole 250 cm3 solution, and hence the mass, in g, of iron in the tablet.(3)
(e)Calculate the percentage by mass of iron in the tablet.(2)
(Total for Question 6 is 9 marks)
7
Copper(I) ions, Cu+(aq), are unstable in aqueous solution and undergo disproportionation:
2Cu+(aq) -> Cu2+(aq) + Cu(s)
Standard electrode potentials: E(Cu2+/Cu+) = +0.15 V; E(Cu+/Cu) = +0.52 V
(a)Write the two half-equations that combine to give this disproportionation reaction, and confirm that they combine to give the overall equation shown above.(2)
(b)Calculate the E cell value for this disproportionation reaction.(2)
(c)State, with a reason, whether this disproportionation reaction is thermodynamically feasible under standard conditions.(2)
(Total for Question 7 is 6 marks)
8
A hydrogen-oxygen fuel cell operating under alkaline conditions is used to power a small electric vehicle.
E(O2/OH-, alkaline) = +0.40 V; E(H2O/H2, alkaline) = -0.83 V
(a)Write the half-equation at the negative electrode of this fuel cell.(2)
(b)Write the half-equation at the positive electrode of this fuel cell.(2)
(c)Combine your two half-equations to give the overall equation for the reaction in this fuel cell.(1)
(d)Calculate the standard emf of this fuel cell.(2)
(e)State two advantages and one disadvantage of using a hydrogen-oxygen fuel cell, rather than a rechargeable lithium-ion cell, to power a vehicle.(3)
(Total for Question 8 is 10 marks)
9
A 1.20 g sample of brass (a copper-zinc alloy) is dissolved completely in excess dilute nitric acid. The resulting solution is treated with excess potassium iodide solution, which reacts with the copper(II) ions present:
2Cu2+(aq) + 4I-(aq) -> 2CuI(s) + I2(aq)
The mixture is made up to exactly 250 cm3 in a volumetric flask. A 25.0 cm3 sample of this solution is titrated against 0.100 mol dm-3 sodium thiosulfate solution:
I2(aq) + 2S2O32-(aq) -> 2I-(aq) + S4O62-(aq)
The mean titre of sodium thiosulfate solution required is 12.30 cm3.
(a)Deduce the change in oxidation state of copper and of iodine in the equation 2Cu2+(aq) + 4I-(aq) -> 2CuI(s) + I2(aq), and identify the oxidising agent.(3)
(b)Calculate the amount, in mol, of thiosulfate ions used to react with the iodine in the 25.0 cm3 sample.(1)
(c)Deduce the amount, in mol, of I2 formed and hence the amount, in mol, of Cu2+ that was present in the 25.0 cm3 sample.(2)
(d)Calculate the total amount, in mol, of Cu2+ in the original 250 cm3 solution, and hence the percentage by mass of copper in the brass sample.(3)
(Total for Question 9 is 9 marks)
10
A steel spoon is electroplated with silver. The spoon (cathode) and a silver anode are placed in a solution of AgNO3(aq), and a constant current of 0.500 A is passed through the cell for 45.0 minutes.
Faraday constant, F = 96500 C mol-1; Ar(Ag) = 108
(a)Write the half-equation for the reduction of silver ions at the cathode.(1)
(b)Calculate the total charge, in coulombs, passed through the cell.(1)
(c)Calculate the amount, in mol, of electrons that passed through the cell.(1)
(d)Calculate the mass, in g, of silver deposited on the spoon.(2)
(e)The silver coats the spoon evenly over a total surface area of 12.0 cm2, and solid silver has a density of 10.5 g cm-3. Calculate the thickness, in mm, of the silver coating.(3)
(Total for Question 10 is 8 marks)
11
A breathalyser test uses the reaction between acidified potassium dichromate(VI) and ethanol vapour from a person's breath:
2Cr2O72-(aq) + 3CH3CH2OH(aq) + 16H+(aq) -> 4Cr3+(aq) + 3CH3COOH(aq) + 11H2O(l)
E(Cr2O72-/Cr3+) = +1.33 V; E(CH3COOH/CH3CH2OH) = +0.19 V (data given for this question)
(a)State the oxidation state of chromium in Cr2O72- and in Cr3+, and state whether chromium is oxidised or reduced in this reaction.(2)
(b)State the colour change observed during this reaction and name the type of reaction that ethanol undergoes.(2)
(c)Using the E values given, calculate the E cell value for this reaction and state whether the reaction is feasible under standard conditions.(3)
(Total for Question 11 is 7 marks)
12
Standard electrode potentials: E(Cl2/Cl-) = +1.36 V; E(Br2/Br-) = +1.09 V; E(I2/I-) = +0.54 V; E(Fe3+/Fe2+) = +0.77 V
(a)Use the E values to deduce, with a calculation, whether chlorine gas will oxidise bromide ions to bromine under standard conditions.(2)
(b)Use the E values to deduce, with a calculation, whether iodide ions will reduce Fe3+ ions to Fe2+ under standard conditions.
(Overall equation: 2Fe3+(aq) + 2I-(aq) -> 2Fe2+(aq) + I2(aq))(3)
(c)A student mixes potassium iodide solution with iron(III) chloride solution and observes a colour change. Describe the colour change and explain how it supports your answer to part (b).(2)
(Total for Question 12 is 7 marks)
Mark scheme · AC5 Redox and Electrochemistry

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12