9.3 Past Paper Questions

1. Which statement is correct?

A. Oxidation involves loss of electrons and a decrease in oxidation state.

B. Oxidation involves gain of electrons and an increase in oxidation state.

C. Reduction involves loss of electrons and an increase in oxidation state.

D. Reduction involves gain of electrons and a decrease in oxidation state.

(Total 1 mark)

2. What occurs during the operation of a voltaic cell based on the following reaction?

Ni_(s) + Pb²⁺ _(aq) → Ni²⁺_(aq)+ Pb_(s)

(Total 1 mark)

3. The oxidation number of chromium is the same in all the following compounds except

A. Cr(OH)₃

B. Cr₂O₃

C. Cr₂(SO₄)₃

D. CrO₃

(Total 1 mark)

4. Magnesium is a more reactive metal than copper. Which is the strongest oxidizing agent?

A. Mg

B. Mg²⁺

C. Cu

D. Cu²⁺

(Total 1 mark)

5. Which processes occur during the electrolysis of molten sodium chloride?

I. Sodium and chloride ions move through the electrolyte.

II. Electrons move through the external circuit.

III. Oxidation takes place at the positive electrode (anode).

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

(Total 1 mark)

6. What happens to the Cr³⁺ _(aq) ion when it is converted to CrO₄^2-_(aq)?

A. Its oxidation number decreases and it undergoes reduction.

B. Its oxidation number decreases and it undergoes oxidation.

C. Its oxidation number increases and it undergoes reduction.

D. Its oxidation number increases and it undergoes oxidation.

(Total 1 mark)

7. The following reactions are spontaneous as written.

Fe_(s) + Cd²⁺_(aq) → Fe²⁺_(aq) + Cd_(s)

Cd_(s) + Sn²⁺_(aq) → Cd²⁺_(aq) + Sn_(s)

Sn_(s) + Pb²⁺_(aq) → Sn²⁺_(aq) + Pb_(s)

Which of the following pairs will react spontaneously?

I. Sn(s) + Fe2+(aq)

II. Cd_(s) + Pb²⁺_(aq)

III. Fe_(s) + Pb²⁺_(aq)

A. I only

B. II only

C. III only

D. II and III only

(Total 1 mark)

8. What species are produced at the positive and negative electrodes during the electrolysis of molten sodium chloride?

(Total 1 mark)

9. Consider the following reaction.

H₂SO_3(aq) + Sn⁴⁺ _(aq) + H₂O_(l) → Sn²⁺ _(aq) + HSO₄–_(aq) + 3H⁺ _(aq)

Which statement is correct?

A. H₂SO₃is the reducing agent because it undergoes reduction.

B. H₂SO₃ is the reducing agent because it undergoes oxidation.

C. Sn⁴⁺ is the oxidizing agent because it undergoes oxidation.

D. Sn⁴⁺ is the reducing agent because it undergoes oxidation.

(Total 1 mark)

10. In which change does oxidation occur?

A. CH₃CHO → CH₃CH₂OH

B. CrO₄^2-→ Cr₂O₇^2-

C. SO₄^2-→ SO₃^2-

D. NO^2- → NO₃^-

(Total 1 mark)

11. What happens at the positive electrode in a voltaic cell and in an electrolytic cell?

(Total 1 mark)

12. What are the oxidation numbers of the elements in sulfuric acid, H2SO4?

(Total 1 mark)

13. A voltaic cell is made from copper and zinc half-cells. The equation for the reaction occurring in the cell is

Zn_(s) + Cu²⁺ _(aq) → Zn²⁺ _(aq) + Cu_(s)

Which statement is correct when the cell produces electricity?

A. Electrons are lost from zinc atoms.

B. The mass of the copper electrode decreases.

C. Electrons flow from the copper half-cell to the zinc half-cell.

D. Negative ions flow through salt bridge from the zinc half-cell to copper half-cell.

(Total 1 mark)

14. What happens when molten sodium chloride is electrolysed in an electrolytic cell?

A. Chlorine is produced at the positive electrode.

B. Sodium ions lose electrons at the negative electrode.

C. Electrons flow through the liquid from negative electrode to the positive electrode.

D. Oxidation occurs at the negative electrode and reduction at the positive electrode.

(Total 1 mark)

15. Which equations represent reactions that occur at room temperature?

I. 2Br^-_(aq)+ Cl_2(aq) → 2Cl^-_(aq) + Br_2(aq)

II. 2Br^-_(aq) + I_2(aq) → 2I^-_(aq)+ Br_2(aq)

III. 2I^-_(aq)+ Cl_2(aq) → 2Cl^-_(aq)+ I_2(aq)

A. I and II only

B. I and III only

C. II and III only

D. I, II and III (Total 1 mark)

16. Which equation represents a redox reaction?

A. KOH_(aq) + HCl_(aq) → KCl_(aq) + H₂O_(l)

B. Mg_(s) + 2HCl_(aq) → MgCl_2(aq) + H₂₍_g₎

C. CuO_(s) + 2HCl_(aq) → CuCl2_(aq) + H2O_(l)

D. ZnCO_3(s)+ 2HCl_(aq) → ZnCl_2(aq) + CO₂₍_g₎ + H₂O_(l)

(Total 1 mark)

17. The following information is given about reactions involving the metals X, Y and Z and solutions of their sulfates.

X(s) + YSO4(aq) → no reaction

Z_(s) + YSO₄^-_(aq) → Y_(s)+ ZSO₄^-_(aq)

When the metals are listed in decreasing order of reactivity (most reactive first), what is the correct order?

A. Z > Y > X

B. X > Y > Z

C. Y > X > Z

D. Y > Z > X

(Total 1 mark)

18. What are the oxidation numbers of the elements in the compound phosphoric acid, H3PO4?

(Total 1 mark)

19. A voltaic cell is made from magnesium and iron half-cells. Magnesium is a more reactive metal than iron. Which statement is correct when the cell produces electricity?

A. Electrons are lost from Magnesium atoms.

B. The concentration of Fe²⁺ ions increases.

C. Electrons flow from the iron half-cell to the Magnesium half-cell.

D. Negative ions flow through the salt bridge from the magnesium half-cell to the iron half-cell. (Total 1 mark)

20. Which are examples of reduction?

I. Fe³⁺ becomes Fe²⁺

II. Cl^- becomes Cl₂

III. CrO₃ becomes Cr³⁺

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

(Total 1 mark)

21. Which statement is correct for the electrolysis of a molten salt?

A. Positive ions move toward the positive electrode.

B. A gas is produced at the negative electrode.

C. Only electrons move in the electrolyte.

D. Both positive and negative ions move toward electrodes.

(Total 1 mark)

22. Which statement about the following reaction is correct?

2Br^-_(aq) + Cl_2(aq) → Br_2(aq) + 2Cl^-_(aq)

A. Br–(aq) is reduced and gains electrons.

B. Cl2(aq) is reduced and loses electrons.

C. Br–(aq) is oxidized and loses electrons.

D. Cl2(aq) is oxidized and gains electrons.

(Total 1 mark)

23. Consider the following spontaneous reactions.

Fe_(s) + Cu²⁺ _(aq) → Fe²⁺ _(aq) + Cu_(s)

Cu_(s) + 2Ag⁺ _(aq) → Cu²⁺ _(aq) + 2Ag_(s)

Zn_(s) + Fe²⁺ _(aq) → Zn²⁺ _(aq) + Fe_(s)

Which is the correct combination?

(Total 1 mark)

24. In which change does nitrogen undergo oxidation?

A. NO₂ → N₂O₄

B. NO₃^-₍ → NO₂

C. N₂O₅→ NO₃^-₍

D. NH₃ → N₂ (Total 1 mark)

25. Which statement is correct?

A. Spontaneous redox reactions produce electricity in an electrolytic cell.

B. Electricity is used to carry out a non-spontaneous redox reaction in a voltaic cell.

C. Oxidation takes place at the negative electrode in a voltaic cell and the positive electrode in an electrolytic cell.

D. Oxidation takes place at the negative electrode in a voltaic cell and reduction takes place at the positive electrode in an electrolytic cell.

(Total 1 mark)

26. The compound [Co(NH₃)₅Br]SO₄ is isomeric with the compound [Co(NH₃)₅SO₄]Br. What is the oxidation state of cobalt in these compounds?

(Total 1 mark)

27. What happens to vanadium during the reaction VO²⁺ _(aq) → VO₃^-_(aq)?

A. It undergoes oxidation and its oxidation number changes from +4 to +5.

B. It undergoes oxidation and its oxidation number changes from +2 to +4.

C. It undergoes reduction and its oxidation number changes from +2 to –1.

D. It undergoes reduction and its oxidation number changes from +4 to +2.

(Total 1 mark)

28. What occurs during the electrolysis of a molten salt?

A. Electricity is produced by a spontaneous redox reaction.

B. Electricity is used to cause a non-spontaneous redox reaction to occur.

C. Electrons flow through the molten salt.

D. Electrons are removed from both ions of the molten salt. (Total 1 mark)

29. Which statement is correct about an oxidizing agent in a chemical reaction?

A. It reacts with oxygen.

B. It reacts with H⁺ ions.

C. It loses electrons.

D. It undergoes reduction. (Total 1 mark)

30. Which formula represents an aldehyde?

A. CH₃CH₂CHO

B. CH₃COCH₃

C. CH₃CH₂COOH

D. CH₃COOCH₃ (Total 1 mark)

31. What is the reducing agent in this reaction?

Cu_(s) + 2NO₃^-_(aq)+ 4H⁺_(aq) → Cu²⁺ _(aq) + 2NO_2(g) + 2H₂O_(l)

A. Cu(s)

B. NO₃^-_(aq)

C. Cu²⁺ _(aq)

D. H⁺ _(aq)

(Total 1 mark)

32. A particular voltaic cell is made from magnesium and iron half-cells. The overall equation for the reaction occurring in the cell is

Mg_(s) + Fe²⁺ _(aq) → Mg²⁺ _(aq) + Fe_(s)

Which statement is correct when the cell produces electricity?

A. Magnesium atoms lose electrons.

B. The mass of the iron electrode decreases.

C. Electrons flow from the iron half-cell to the magnesium half-cell.

D. Negative ions flow through the salt bridge from the magnesium half-cell to the Iron half-cell.

(Total 1 mark)

33. What process occurs at the cathode in a voltaic cell and at the anode in an electrolytic cell?

(Total 1 mark)

34. Consider the following reaction:

H₂SO_3(aq) + Sn⁴⁺ _(aq) + H₂O_(l) → Sn2²⁺ _(aq) + HSO₄^-_(aq) + 3H⁺ _(aq)

Which statement is correct?

A. H2SO3 is the reducing agent because it undergoes reduction.

B. H2SO3 is the reducing agent because it undergoes oxidation.

C. Sn4+ is the oxidizing agent because it undergoes oxidation.

D. Sn4+ is the reducing agent because it undergoes oxidation.

(Total 1 mark)

35. Which processes occur during the electrolysis of molten sodium chloride?

I. Sodium and chloride ions move through the electrolyte.

II. Electrons move through the external circuit.

III. Oxidation takes place at the anode.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

(Total 1 mark)

36. Tin(II) ions can be oxidized to tin(IV) ions by acidified potassium permanganate(VII) solution according to the following unbalanced equation.

__ Sn²⁺ + __MnO₄– + __ H⁺ →__ Sn⁴⁺ + __Mn²⁺ + __ H₂O

(a) Identify the oxidizing agent and the reducing agent.

Oxidizing agent .........................................................................................................

Reducing agent ..........................................................................................................

(1)

(b) Balance the equation above.

....................................................................................................................................

(1)

(Total 2 marks)

37. Consider the following redox equation.

5Fe²⁺ _(aq) +MnO₄–(aq) +8H⁺ _(aq) → 5Fe³⁺ _(aq) + Mn²⁺ _(aq) + 4H₂O_(l)

(i) Determine the oxidation numbers for Fe and Mn in the reactants and in the products.

………………………………………………………………………………………

(2)

(ii) Based on your answer to (i), deduce which substance is oxidized.

……………………………………………………………………………………(1)

(iii) The compounds CH₃OH and CH₂O contain carbon atoms with different oxidation numbers. Deduce the oxidation numbers and state the kind of chemical change needed to make CH₂O from CH₃OH.

………………………………………………………………………………………

(3)

(Total 6 marks)

38. A part of the reactivity series of metals, in order of decreasing reactivity, is shown below.

magnesium

zinc

iron

lead

copper

silver

If a piece of copper metal were placed in separate solutions of silver nitrate and zinc nitrate

(i) determine which solution would undergo reaction.

………………………………………………………………………………………

(1)

(ii) identify the type of chemical change taking place in the copper and write the

half-equation for this change.

………………………………………………………………………………………

(2)

(iii) state, giving a reason, what visible change would take place in the solutions.

………………………………………………………………………………………

(2)

(Total 5 marks)

39. (i) Solid sodium chloride does not conduct electricity but molten sodium chloride does. Explain this difference, and outline what happens in an electrolytic cell during the electrolysis of molten sodium chloride using carbon electrodes.

………………………………………………………………………………………

(4)

(ii) State the products formed and give equations showing the reactions at each electrode.

………………………………………………………………………………………

(4)

(iii) State what practical use is made of this process.

………………………………………………………………………………………

(1)

(Total 9 marks)

40. Electrolysis can be used to obtain chlorine from molten sodium chloride. Write an equation for the reaction occurring at each electrode and describe the two different ways in which electricity is conducted when the cell is in operation.

(Total 4 marks)

41. Two reactions occurring in the manufacture of bromine from sea water are

I Cl₂₍_g₎ + 2Br^-_(aq) → 2Cl^-_(aq) + Br_2(g)

II Br_2(g) + SO_2(g)+ 2H₂O_(l)→ 2HBr_(g) + H₂SO₄₍_aq₎

(i) Explain, by reference to electrons, why reaction I is referred to as a redox reaction.

(2)

(ii) State and explain whether SO₂ is reduced or oxidized in reaction II by referring to the oxidation numbers of sulfur in this reaction.

(3)

(Total 5 marks)

42. In terms of electron transfer define:

(i) oxidation

....................................................................................................................................

................................................................................................................................(1)

(ii) oxidizing agent

....................................................................................................................................

................................................................................................................................(1)

(Total 2 marks)

43. Deduce the change in oxidation number of chromium in the below reaction. State with a reason whether the chromium has been oxidized or reduced.

CrO₇^2- + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O

..............................................................................................................................................

(Total 2 marks)

44. (a) (i) Define oxidizing agent in terms of electron transfer.

.........................................................................................................................

......................................................................................................................(1)

(ii) Deduce the change in oxidation number of chromium in the reaction below. State with a reason whether the chromium has been oxidized or reduced.

Cr₂O₇^2- + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O

.........................................................................................................................

(2)

(Total 3 marks)

45. Iron in food, in the form of Fe³⁺, reacts with ascorbic acid (vitamin C), C₆H₈O₆, to form dehydroascorbic acid, C₆H₆O₆.

(i) Write an ionic half-equation to show the conversion of ascorbic acid to dehydroascorbic acid in aqueous solution.

.........................................................................................................................

(1)

(ii) In the other ionic half-equation Fe³⁺ is converted to Fe²⁺. Deduce the overall equation for the reaction between C₆H₈O₆ and Fe³⁺.

.........................................................................................................................

(1)

(Total 2 marks)

46. (i) Draw a diagram of apparatus that could be used to electrolyse molten potassium bromide. Label the diagram to show the polarity of each electrode and the product formed.

(3)

(ii) Describe the two different ways in which electricity is conducted in the apparatus.

(2)

(iii) Write an equation to show the formation of the product at each electrode.

(2)

(Total 7 marks)

47. Iodide ions, I^-_(aq), react with iodate ions, IO₃^-_(aq), in an acidic solution to form molecular iodine and water.

(i) Determine the oxidation number of iodine in each iodine-containing species in the reaction.

....................................................................................................................................

................................................................................................................................(2)

(ii) Identify, with a reason, the species that undergoes:

oxidation ....................................................................................................................

....................................................................................................................................

reduction ....................................................................................................................

................................................................................................................................(2)

(Total 4 marks)