Electrochemistry Written Response

January 1999 7. Balance the following redox reaction in acidic solution:

RuO4 + P → Ru(OH)2

2+ + H3PO3 (acid) (3 marks) 8. A technician tests the concentration of methanol, CH3OH, in diluted windshield washer fluid using a redox titration. A 25.00mL sample is titrated with 14.50mL of 0.0200M KMnO4. Determine the concentration of methanol in the sample given the following redox reaction:

6H+ + 2MnO4− + 5CH3OH → 5CH2O + 2Mn2+ + 8H2O (3 marks)

9. An electrolytic cell can be used to plate a copper penny with a silver coating. Sketch a diagram of the electrolytic cell. Label the cathode and show the direction of electron flow. (2 marks) April 1999 9. Balance the following redox reaction in acid:

N2O4 + In → HNO2 + In3+ (acid) (3 marks) 10. What is an electrolytic cell? (2 marks) 11. A solution contains either acidified IO3

− or acidified SO4 2−. Why could the solution be

identified using I-(aq) ? Provide equations to support your answer. (3 marks)

June 1999 9. The data below were obtained in a redox titration of a 25.00mL sample containing Sn2+ ions using 0.125M KMnO4 according to the following reaction:

2MnO4− + 16H+ + 5Sn2+ → 2Mn2+ + 8H2O + 5Sn4+

Calculate the [Sn2+] in the original sample. (4 marks)

10. Consider the following electrochemical cell:

a) Calculate the initial cell voltage. (1 mark) b) What is the purpose of the salt bridge? (1 mark) 11. Consider the electrolysis of 1.0M H2SO4 using inert platinum electrodes. a) Write the oxidation half-reaction. (1 mark) b) Write the reduction half-reaction. (1 mark) August 1999 9. Balance the following redox reaction in basic solution. (3 marks)

MnO4− + C2O4

2− → MnO2 + CO2 (basic)

10. Describe two chemically different methods that can be used to prevent corrosion of iron and explain why each method works. (2 marks) Method 1: Explanation: Method 2: Explanation:

11. Consider the following diagram for the electro refining of lead:

a) On the diagram above, label the anode and the cathode. (1 mark) b) Write the formula for a suitable electrolyte. (1 mark) c) Write the equation for the reduction half-reaction. (1 mark) January 2000 10. a) Indicate in the blank spaces on the following chart whether or not a reaction will occur when the metals are added to aqueous ions. (1 mark)

b) List the oxidizing agents in order of strongest to weakest. (1 mark) 11. Balance the following redox reaction in basic solution: (4 marks)

Au + Cl- + O2 → AuCl4- + OH − (basic)

12. Draw and label a simple electrolytic cell capable of electroplating an inert electrode with silver. (2 marks) April 2000 10. Balance the following redox reaction in a basic solution. (4 marks)

As → AsH3 + H2AsO4− (basic)

11. Consider the following electrochemical cell:

a) Write the equation for the half-reaction that occurs at the anode. (1 mark) b) Calculate the reduction potential of Ti 2+ (1 mark) 12. Consider the following reaction for the formation of rust:

Fe(s) + O2 (g) + H2O(l) →Fe(OH)2 (s) Describe and explain two methods, using different chemical principles, to prevent the formation of rust. (2 marks)

June 2000 8. The metals Rh, Ti, Cr and Pd are individually placed in 1.0M solutions of Rh2+, Ti2+, Cr2+ and Pd2+ and the cell voltages of the spontaneous reactions are determined.

a) Arrange the metals in order of increasing strength as reducing agents. (2 marks)

b) Determine the cell voltage for Ti in a 1.0M solution of Cr+2 (2 marks) 9. Consider the following reactions for a fuel cell:

a) Write the reaction at the anode. (1 mark) b) Discuss the advantage of a fuel-cell powered vehicle over an internal combustion powered vehicle by comparing the products formed. (1 mark) 10. Draw and label an electrochemical cell using a copper anode and having an E° value > 1.00V. (2 marks)

August 2000 9. During the production of magnesium metal from sea water, magnesium ions are first precipitated from sea water as magnesium hydroxide.a) The magnesium hydroxide is then neutralized by hydrochloric acid, producing magnesium chloride. Write the neutralization reaction. (1 mark) b) The salt produced, magnesium chloride, is dried, melted and undergoes electrolysis. Write the reaction occurring at each electrode. (2 marks) Anode: Cathode: c) It is not possible to use electrolysis to remove Mg from a 1.0M MgCl2 solution. Why?

(1 mark)

10. In the process of extracting tin from a sample of ore, the tin is removed as Sn 2+ ions. A titration requires 21.43mL of 0.0170M K2Cr2O7 to reach the equivalence point with the Sn 2+ + in a 0.750g sample of the ore.

3Sn2+(aq) + Cr2O7

2-(aq) + 14H+

(aq) → 3Sn4+(aq) + 2Cr3+

(aq) + 7H2O(l) Using the reaction, calculate the percent mass of tin in the ore sample. (4 marks) January 2001 9. Balance the following redox reaction in basic solution. (5 marks)

SO32- + MnO4

- → SO42- + MnO2 (Basic)

10. Draw and label the parts of an operating electrochemical cell using a zinc anode that will produce an electric current having a voltage of 1.56V at standard conditions.

(4 marks)

April 2001 9. Balance the following redox reaction in basic solution. (5 marks)

MnO4- + C2O4

2- → MnO2 + CO32- (Basic)

10. a) Draw and label the parts of an operating electrolytic cell during the electrolysis of molten potassium chloride KCl(l). (3 marks) b) Define the term oxidizing agent. (1 mark) June 2001 10. Balance the following redox reaction in acidic solution. (3 marks)

MnO4- + Fe2+ → Mn2+ + Fe3+ (Acidic)

11. Aluminum is a stronger reducing agent than copper. What is meant by the term stronger reducing agent? (2 marks) 12. Consider the electrolysis of water. a) Draw and label the parts of an electrolytic cell capable of decomposing water.

(3 marks)

b) Identify the gas produced at the anode. (1 mark) August 2001 10. Consider the following redox reaction in acidic solution:

MnO4- + H2O2 → Mn2+ + O2 (Acidic)

a) Write a balanced equation for the above reaction. (4 marks) b) The above reaction was used for a redox titration. At the equivalence point 5.684 X10-4 mol KMnO4 was required to titrate 5.00mL of H2O2 solution. Calculate the [H2O2]. (2 marks)

11. Cathodic protection is one method used to inhibit the corrosion of iron. a) Explain the principle of cathodic protection. (2 marks) b) Identify two methods, other than cathodic protection, that could be used to inhibit the corrosion of iron. (2 marks) January 2002 11. A titration is performed to determine the concentration of Fe2+ in 25.00 mL of an

FeSO4 solution. It requires 22.52 mL of 0.015M KMnO4 to reach the equivalence

Point according to the following equation:

MnO4- + 5Fe2+ + 8H+ Mn2+ + 5Fe3+ + 4H2O

Calculate the [Fe2+].

12. Consider the following diagram:

Students are asked to produce hydrogen and oxygen gas by the electrolysis of water.

They are given three substances (CuSO4, K2SO4 and NaI) to choose from to

Prepare an electrolytic solution that will only produce hydrogen and oxygen.

a) Which substance should be selected? Explain why.

b) Write the equation for the half-reaction that occurs at the anode in the

electrolytic cell.

c) Explain why it would not be acceptable to use a copper anode in this cell.

April 2002

11. Balance the following redox reaction:

Ag2O + Si → Ag + SiO32- (acidic)

12. Sodium metal is produced commercially by the electrolysis of molten NaCl(l).

Explain why sodium metal, Na(s), cannot be produced by electrolysis of

Aqueous NaCl(aq).

13. Blister copper is an impure sample of copper containing small amounts of zinc and

gold. Blister copper is purified using electrolysis.

Sufficient voltage is supplied to oxidize copper at the anode.

a) What happens to the zinc at the anode? Explain.

b) Write the equation for the half-reaction that occurs at the cathode.

June 2002

11. Balance the following redox equation:

Ag + NO3- → Ag+ + NO (acidic)

12. Draw a diagram of an operating electrolytic cell used to extract pure lead from an

impure lead sample. Identify the electrolyte and the material used for the anode.

13. A sample of copper is placed in HNO3(aq) and another sample of copper is placed in

HCl(aq).

a) In which acid does the copper react?

b) Calculate E° for the reaction that occurs.

August 2002

11. Balance the following redox equation:

ClO3- + S2O3

2- → S4O62- + Cl- (acidic)

12. State two characteristics of the overall reaction in an electrochemical cell.

13. Describe two chemically different methods of preventing the corrosion of iron.

Explain how each method works.

January 2003

11. Balance the following redox reaction.

Sb + NO3- Sb2O5 + NO (acidic)

12. A 1.0M HCl solution is electrolyzed using a copper anode and an inert carbon

cathode. Predict the half-reactions that will occur and describe what you would

observe at each electrode.

April 2003

9. Consider the following experimental results:

Ce4+ + Pd → Pd2+ + Ce3+

In3+ + Cd → no reaction

Pd2+ + In2+ → In3+ + Pd

Cd2+ + Pd → no reaction

Use these results to complete the table of reduction half-reactions below.

10. Balance the following equation.

Cr2O72- + C2O4

2- → Cr3+ + CO2 (acidic)

11. Consider the following electrolytic cell which contains a porous barrier to prevent

general mixing of solutions.

a) Label the anode and cathode in the space provided on the diagram above.

b) Write an equation for the overall cell reaction.

c) Calculate the minimum theoretical voltage required for this reaction under

standard conditions.

June 2003

10. Balance the following redox equation:

H2S + CrO42- → S8 + Cr3+ (acidic)

11. An excess of copper solid is dropped into a solution which contains

AgNO3, Fe(NO3)3 and Zn(NO3)2. Write the equations for any reduction

Half-reactions that occur over time under standard conditions.

12. Consider the following diagrams:

a) Predict what should happen to the Fe in Beaker A.

b) Predict what should happen to the Fe in Beaker B. Explain.

August 2003

11. A reaction occurs when copper metal is dropped into a solution of silver nitrate.

Write the balanced formula equation and the balanced net ionic equation for this reaction.

12. When setting up the apparatus to electroplate a zinc object with copper, the object is

suspended in a Cu2+ solution. Explain why it is a good idea to turn on the power

supply before immersing the electrodes in the solution.

13. Consider the following apparatus consisting of an electrochemical cell joined to an

electrolytic cell:

a) On the diagram above, indicate the direction of electron flow in the top wire.

b) Which metal in cell A is the cathode?

c) Write the anode and cathode half-reactions for cell B.

Electrochemistry Written Response Answers

January 1999 7)

8)

9)

April 1999 9)

10)

11)

June 1999 9)

10a)

b)

11a)

b)

August 1999 9)

10)

11a)

b)

c)

January 2000 10a)

b)

11)

12)

April 2000 10)

OR

OR

11) a)

b)

12)

June 2000 8a

b)

9a)

b)

10)

August 2000 9a)

b)

c)

10)

January 2001 9)

OR

10)

April 2001 9)

10a)

b)

June 2001 10)

11)

12a)

b)

August 2001 10a

b)

11a)

b)

January 2002 11.

12. a) Substance: K2SO4

Explanation: It is the only one of the three substances that will neither oxidize

nor reduce before water does.

b)

c) A copper anode would oxidize.

April 2002

11.

12. H2O is more easily reduced than Na+.

13. a) Zn is oxidized.

It is an even stronger reducing agent than the copper.

b) Cu2+ + 2e- → Cu

June 2002

11.

12.

13. a) Copper reacts in HNO3(aq).

b) E° is + 0.62 volts

August 2002

11.

12. Any two of the following for 1 mark each:

- redox reaction

- spontaneous

- +E° value

- exothermic

13. - Coating with paint or oil prevents contact between iron and oxygen.

- Attaching a more readily oxidized metal such as zinc – cathodic protection –

turns the iron into a cathode, preventing oxidation.

January 2003

11.

12. Anode half-reaction: Cu(s) → Cu2+ + 2e-

Anode observations: Electrode is eaten away and solution turns blue.

Cathode half-reaction: SH+ + 2e- → H2(g)

Cathode observations: Bubbles form, but no change to electrode.

April 2003

9.

10.

11. a) see diagram below

b) Sn2+ + Cu(s) → Cu2+ + Sn(s)

c) 0.48V

June 2003

10)

11) Ag+ + e- → Ag

Fe3+ + e- → Fe2+

12) a) Prediction: The iron is oxidized.

b) Prediction: Nothing happens to the Fe.

Explanation: Zn is oxidized and protects the Fe.

OR

The Fe is cathodically protected by the Zn.

August 2003

11) Formula equation:

Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)

Net ionic equation:

Cu(s) + 2Ag+(aq) Cu2+

(aq) + 2Ag(s)

12) If you did not turn on the power supply before immersing the electrodes in the

solution, the Cu2+ would react spontaneously with the zinc to be plated,

oxidizing the zinc.

13)a)

b) Cathode: Ag

c) Anode: H2O → 2H+ + 1/2O2 + 2e-

Cathode: 2H2O + 2e- → H2 + 2OH-