A Level Official Exam Material
FREE Past Paper Questions by Topic
If you need the markscheme, including the examiners' reports for each question, and more exam and teaching materials on this topic, contact me through the contact page
Unit 5: Transition Metals and
Organic Nitrogen Chemistry
Topic 16: Redox Equilibria
For A Level students only
Q1. Answer the questions with a cross in the boxes you think are correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
This question is about alkaline hydrogen-oxygen fuel cells.
(a) What is the half-equation at the negative electrode?
A H2(g) + 2OH− (aq) → 2H2O(l) + 2e−
B 2H2O(l) + 2e− → H2(g) + 2OH− (aq)
C O2(g) + 2H2O(l) + 4e− → 4OH−(aq)
D 4OH− (aq) → O2(g) + 2H2O(l) + 4e−
(b) Which statement is correct for an alkaline hydrogen-oxygen fuel cell when compared with an acidic hydrogen-oxygen fuel cell?
Q2. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
When an alkene is added to a solution of potassium manganate(VII), the purple solution turns colourless.
In terms of electron transfer and oxidation number, how does the manganese change in this reaction?
Q3. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
This question is about the reaction
(a) The electrode potential for the Fe3+/ Fe2+ electrode system is +0.77 V.
What is the electrode potential for the Ti2+/Ti electrode system? (1)
A −3.17 V
B −1.63 V
C +1.63 V
D +3.17 V
(b) What metals should be used for the electrodes in the cell for this reaction? (1)
Q4. For each question, select one answer from A to D and put a cross in the box .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
A group of students carry out an experiment to find the concentration of chlorine, Cl2(aq), in a solution.
Excess potassium iodide solution is added to a 10.0 cm3 sample of the chlorine solution.
Cl2(aq) + 2I− (aq) → 2Cl− (aq) + I2(aq)
The iodine produced is titrated with a solution of thiosulfate ions of known concentration, using starch indicator.
(a) What concentration of thiosulfate ions, in mol dm−3 , is required to give a titre of approximately 20 cm3 ? (1)
A 0.010
B 0.020
C 0.040
D 0.080 (b) What is the most suitable volume of 0.1 mol dm−3 potassium iodide solution, in cm3 , to add to the 10.0 cm3 of chlorine solution? (1)
A 7.6
B 8.0
C 8.4
D 10.0
(c) What is the colour change at the end-point of the titration? (1)
A colourless to pale yellow
B pale yellow to colourless
C colourless to blue-black
D blue-black to colourless
(Total for question = 3 marks)
Q5. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
An electrochemical cell is set up to measure for the reaction
Fe(s) + Sn2+(aq) → Fe2+(aq) + Sn(s) (a)
What is the cell diagram for this cell? (1)
A Fe(s) | Fe2+(aq) || Sn(s) | Sn2+(aq)
B Fe2+(aq) | Fe(s) || Sn2+(aq) | Sn(s)
C Fe(s) | Fe2+(aq) || Sn2+(aq) | Sn(s)
D Fe2+(aq) | Fe(s) || Sn(s) | Sn2+(aq) (b)
The standard electrode potential for the Fe / Fe2+ electrode system is −0.44 V and for the reaction is +0.30 V.
What is the standard electrode potential for the Sn / Sn2+ electrode system? (1)
A −0.74 V
B −0.14 V
C +0.14 V
D +0.74 V
(Total for question = 2 marks)
Q6. For each question, select one answer from A to D and put a cross in the box .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
An electrochemical cell is made from the electrode systems shown by these half-equations.
(a) Which silver compound could be used as the electrolyte in the left-hand half-cell? (1) A silver nitrate B silver hydroxide C silver chloride D silver carbonate (b) The electrolyte in the right-hand half-cell is prepared using equal volumes of (1)
Q7. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
The standard hydrogen electrode uses an electrode of platinum coated in a finely divided form of the metal called platinum black.
What is the purpose of this coating?
A to increase the rate of the equilibrium between the hydrogen gas and the hydrogen ions
B to provide an inert protective coating for the electrode
C to increase the electrical conductivity of the electrode
D to ensure that the conditions remain standard
(Total for question = 1 mark)
Q8. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . Hydrogen-oxygen fuel cells can operate in acidic or alkaline conditions.
What is the reaction at the anode in an alkaline hydrogen-oxygen fuel cell?
A O2(g) + 2H2O(l) + 4e− → 4OH− (aq)
B 4OH− (aq) → O2(g) + 2H2O(l) + 4e−
C 2H2O(l) + 2e− → H2(g) + 2OH− (aq)
D H2(g) + 2OH− (aq) → 2H2O(l) + 2e−
(Total for question = 1 mark)
Q9. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
Which is true of a hydrogen-oxygen fuel cell?
A the cathode has a more positive potential than the anode
B hydrogen is oxidised at the cathode
C oxygen is reduced at the negative electrode
D the cell potential is different when operating under alkaline or acidic conditions
(Total for question = 1 mark)
Q10. For each question, select one answer from A to D and put a cross in the box .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
A hydrogen-oxygen fuel cell is used to provide electrical energy for an electric motor in a car.
(a) The electrolyte in the fuel cell is acidic.
What is the half-equation at the anode? (1)
A 1 ⁄2O2(g) + 2H+ (aq) + 2e− → H2O(l)
B H2O(l) → 1 ⁄2O2(g) + 2H+ (aq) + 2e−
C H2(g) → 2H+ (aq) + 2e−
D 2H+ (aq) + 2e− → H2(g)
(b) Hydrogen-oxygen fuel cells have advantages over methanol-oxygen fuel cells in vehicles.
Which of these is an advantage of the hydrogen-oxygen fuel cell? (1)
A more energy is released per mole of fuel used
B emissions do not contribute to climate change
C hydrogen is easier to store than methanol
D only hydrogen can be obtained from renewable resources
(Total for question = 2 marks)
Q11. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
In acidic conditions, manganate(VII) ions are reduced to manganese(II) ions, Mn2+ .
In neutral conditions, manganate(VII) ions are reduced to manganese(IV) oxide, MnO2. 22.0 cm3 of a solution of manganate(VII) ions were needed to oxidise 25.0 cm3 of a solution of iron(II) ions to iron(III) ions in acidic conditions.
The same solution of manganate(VII) ions is used to oxidise 25.0 cm3 of the same solution of iron(II) ions in neutral conditions.
By considering the oxidation number changes involved, it may be shown that the volume of the manganate(VII) solution required, in neutral conditions, is
A 12.6 cm3
B 13.2 cm3
C 36.7 cm3
D 38.5 cm3
(Total for question = 1 mark)
Q12. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
Excess aqueous sodium thiosulfate is added to an aqueous solution of ammonium vanadate(V).
What colour is the mixture when the reaction is complete?
Refer to page 10 of the Data Booklet. (1)
A yellow
B blue
C green
D violet
Use this space for rough working.
Anything you write in this space will gain no credit.
(Total for question = 1 mark)
Q13. Answer the question with a cross in the box you think is correct . If you change your mind about an answer, put a line through the box and then mark your new answer with a cross . Some standard electrode potentials are shown.
Which of these disproportionation reactions is thermodynamically feasible under standard conditions?
A 2Bk3+ → Bk2+ + Bk4+
B 2Cu2+ → Cu+ + Cu3+
C 2Au+ → Au + Au2+
D 2Ag2+ → Ag+ + Ag3+
(Total for question = 1 mark)
(a) Which material should be used for each electrode?
(b) Solution 1 is (1)
A 0.33 mol dm–3 H3PO4(aq)
B 0.50 mol dm–3 H2SO4(aq)
C 1.00 mol dm–3 HCl(aq)
D 1.00 mol dm–3 CH3COOH(aq)
(c) Solution 2 contains 14.71 g of K2Cr2O7.
What mass of Cr2(SO4)3·18H2O should also be used?
[Mr values: K2Cr2O7 = 294.2 Cr2(SO4)3·18H2O = 716.3] (1)
A 8.95 g
B 17.91 g
C 19.62 g
D 35.82 g
(d) Solution 2 is best acidified with (1)
A H2SO4
B HCl
C HBr
D H2CrO4
(Total for question = 4 marks)
(b) Solution 1 is (1)
A 0.33 mol dm–3 H3PO4(aq)
B 0.50 mol dm–3 H2SO4(aq)
C 1.00 mol dm–3 HCl(aq)
D 1.00 mol dm–3 CH3COOH(aq)
(c) Solution 2 contains 14.71 g of K2Cr2O7.
What mass of Cr2(SO4)3·18H2O should also be used?
[Mr values: K2Cr2O7 = 294.2 Cr2(SO4)3·18H2O = 716.3] (1)
A 8.95 g
B 17.91 g
C 19.62 g
D 35.82 g
(d) Solution 2 is best acidified with (1)
A H2SO4
B HCl \
C HBr
D H2CrO4
(Total for question = 4 marks)
Q14. Brass is a metal alloy containing copper and zinc.
The presence of zinc in the alloy makes brass less malleable than copper alone.
Prince's metal is one type of brass.
It is used to make imitation gold because of its yellow colour.
The copper content of brass can be analysed by first reacting a known sample of the metal with concentrated nitric acid.
The reaction of the copper is: Cu + 4H+ + 2NO3 – → Cu2+ + NO2 + 2H2O.
(a) Identify the element that is oxidised and the element that is reduced in the reaction shown.
Include relevant oxidation numbers. (2) ............................................................................................................................................. .............................................................................................................................................
(b) Suggest one precaution when carrying out this reaction, other than the use of gloves, goggles and lab coats, clearly stating the hazard concerned. (2)
............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(c) The copper ions are then reacted with excess potassium iodide.
2Cu2+ + 4I– → 2CuI + I2 The iodine formed is analysed by titration with sodium thiosulfate.
I2 + 2S2O3 2– → 2I– + S4O6 2– A 5.000 g sample of Prince's metal was analysed.
After reaction with concentrated nitric acid, the sample was diluted to 250 cm3 and then 10.0 cm3 aliquots or portions were titrated with 0.100 mol dm–3 sodium thiosulfate solution.
The mean titre was 22.65 cm3 .
Calculate the percentage of copper, by mass, in this sample of Prince's metal to an appropriate number of significant figures. (6)
(f) Explain, in terms of their structures, why brass is less malleable than pure copper. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 12 marks)
Q15. This question is about manganese compounds. Some data are given below
(a) (i) Write the ionic equation for the disproportionation of manganate(VI) ions, in acidic conditions, using relevant half-equations from the table.
State symbols are not required. (2)
(ii) Calculate for the disproportionation of manganate(VI) ions in acidic conditions, stating whether or not the reaction is thermodynamically feasible. (2) (iii) Using the standard electrode potentials in the table, assess the thermodynamic feasibility of preparing manganate(VI) by reacting manganate(VII) and manganese(IV) oxide in alkaline conditions. (4)
............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(b) Steel is an alloy of iron and carbon.
A group of students determined the iron content of a sample of steel wire by a titration method.
A known mass of the wire was dissolved in dilute sulfuric acid and the resulting solution made up to 250.0 cm3 with more dilute sulfuric acid and mixed thoroughly.
Fe + H2SO4 → FeSO4 + H2 25.0 cm3 samples of the resulting solution were titrated with 0.0195 mol dm−3 potassium manganate(VII) solution.
(i) State the colour change at the end-point of the titration. (1)
.............................................................................................................................................
(ii) One student used 1.53 g of the wire (weighed directly on the balance pan) and obtained a mean titre of 27.35 cm3 .
Using half-equations 3 and 5 from the table, calculate the percentage of iron in the steel wire.
Give your answer to three significant figures. (5)
(iii) A second student carried out the same experiment but used distilled water to make up the solution in the volumetric flask.
A brown suspension formed during the titration.
Explain how, if at all, the titre value would be affected by this student's error. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(c) The uncertainties of the apparatus used in the experiment in (b) are shown.
(i) Complete the table. (2)
(ii) A third student obtained a value of 95.863% for the proportion of iron in the wire. State whether or not this student has given their answer to an appropriate number of significant figures. Justify your answer in terms of the total percentage uncertainty of the experiment. (2)
............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 21 marks)
Q16. The hydride of arsenic, arsine, is a toxic gas used in the production of semiconductors.
(a) Draw a dot-and-cross diagram for arsine, AsH3 . (1)
(b) Arsine is a reducing agent and reacts with cerium(IV) sulfate solution, forming arsenic.
The data from an experiment are shown.
Volume of arsine gas = 350 cm3 at 115 000 Pa and 20 °C Volume of cerium(IV) sulfate solution = 488 cm3
Concentration of cerium(IV) sulfate solution = 0.102 mol dm−3
(i) Complete the half-equation. (1) AsH3 → As
(ii) Calculate the final oxidation state of the cerium ion formed in the reaction. (6)
(Total for question = 8 marks)
Q17. This question is about electrochemical cells.
(a) The half-equation for an iron(III) / iron(II) half-cell is
A Salt bridge containing a solution of ........................................................................................................................................
B Electrode made of .............................................................................................................................................
C Solution containing .............................................................................................................................................
(Total for question = 8 marks)
Q18. Standard electrode potentials can be used to show whether or not a reaction is feasible.
(a) State the conditions required, in addition to 1 mol dm–3 , for obtaining standard electrode potentials. (1) .............................................................................................................................................
(b) A Daniell cell is a combination of standard zinc and copper electrodes.
(ii) Give three observations that would be made when current flows for several hours in the Daniell cell. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(c) Some standard reduction potentials are:
State and justify, in terms of cell values, whether copper(I) ions and iron(II) ions will be disproportionate.
Include any equations for reactions which occur. (3)
............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(d) State one reason why the feasibility determined from standard electrode potentials does not necessarily result in a reaction. (1)
............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 8 marks)
If you need the markscheme, including the examiners' reports for each question, and more exam and teaching materials on this topic, contact me through the contact page