NSS Chemistry Part VII Redox Reactions, Chemical Cells and Electrolysis(HKCE and HKAL Past Paper Questions)1. HKCEE 1994 I Q7

In the above diagram, P and Q are two different metals. When the circuit is closed, a current flows in the external circuit. After some time, 0.36g of copper is deposited on the carbon electrode

(i) (1) What is the direction of (2) Calculate the quantity of electricity that has passed through the circuit.

(ii) After the circuit has been closed for some time, what would be observed (1) at the carbon electrode (2) in the copper(II) sulphate solution? (iii) What is the function of set- (iv) Which of the metals, P or

answer.

(Relative atomic mass: Cu = 63.5; 1 faraday = 96500 coulombs)

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Redox Reactions, Chemical Cells and Electrolysis(HKCE and HKAL Past Paper Questions) Structural Questions

are two different metals. When the circuit is closed, a current flows in the external circuit. After some time, 0.36g of copper is deposited on the carbon electrode

(1) What is the direction of electron flow in the external circuit? Explain your (2) Calculate the quantity of electricity that has passed through the circuit.After the circuit has been closed for some time, what would be observed

at the carbon electrode S ? in the copper(II) sulphate solution?

-up X in this experiment? or Q, occupies a higher position in the electrochemical series? Explain your

(Relative atomic mass: Cu = 63.5; 1 faraday = 96500 coulombs)

Redox Reactions, Chemical Cells and Electrolysis Structural Questions

are two different metals. When the circuit is closed, a current flows in the external circuit. After some time, 0.36g of copper is deposited on the carbon electrode R.

flow in the external circuit? Explain your answer. (2) Calculate the quantity of electricity that has passed through the circuit.#

, occupies a higher position in the electrochemical series? Explain your

(10 marks)

2. HKCEE 1995 I Q9b When the circuit in the set-up shown above is closed, the acidified potassium permanganate solution loses its

colour gradually.

(i) Write a half equation for the reaction that occurs in the acidified potassium permanganate solution. Explain whether the permanganate ion is oxidized or reduced.

(ii) What would be observed in the potassium iodide solution after some time? Write a half equation for the reaction that would occur.

(iii) Identify the direction of electron (iv) Write an ionic equation for the reaction that occurs when an acidified potassium permanganate solution and a

potassium iodide solution are mixed together.(v) (1) What is the function of the salt bridge in the set

(2) Explain whether a sodium sulphite solution can be used instead of a potassium nitrate solution in the salt bridge.

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up shown above is closed, the acidified potassium permanganate solution loses its

reaction that occurs in the acidified potassium permanganate solution. Explain whether the permanganate ion is oxidized or reduced. What would be observed in the potassium iodide solution after some time? Write a half equation for the

flow in the external circuit. Write an ionic equation for the reaction that occurs when an acidified potassium permanganate solution and a potassium iodide solution are mixed together.

hat is the function of the salt bridge in the set-up? (2) Explain whether a sodium sulphite solution can be used instead of a potassium

nitrate solution in the salt bridge.

up shown above is closed, the acidified potassium permanganate solution loses its

reaction that occurs in the acidified potassium permanganate solution. Explain

What would be observed in the potassium iodide solution after some time? Write a half equation for the

Write an ionic equation for the reaction that occurs when an acidified potassium permanganate solution and a

(2) Explain whether a sodium sulphite solution can be used instead of a potassium

(8 marks)

3. HKCEE 1996 I Q9b A student carried out a copper-plating experiment in the laboratory using the set

(i) Explain why copper(II) sulphate solution can conduct electricity.(ii) What would be observed at the carbon anode during the

involved. (iii) In the copper-plating industry, a metal is used as the anode instead of carbon. What is this metal? Explain

your answer. (iv) In a copper-plating factory, the waste water is treated with

copper(II) ions present before discharge. (1) Suggest TWO reasons why it is necessary to remove the copper(II) ions from the waste water before

discharge. (2) 20.0 dm3 of a sample of waste water require 3.5 dm

removal of the copper(II) ions present. Calculate the concentration, in mol dm

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plating experiment in the laboratory using the set-up shown below:

Explain why copper(II) sulphate solution can conduct electricity. What would be observed at the carbon anode during the experiment? Write a half equation for the reaction

plating industry, a metal is used as the anode instead of carbon. What is this metal? Explain

plating factory, the waste water is treated with sodium hydroxide solution to remove the copper(II) ions present before discharge.

reasons why it is necessary to remove the copper(II) ions from the waste water before

of a sample of waste water require 3.5 dm3 of 8.0M sodium hydroxide solution removal of the copper(II) ions present. Calculate the concentration, in mol dm-3, of copper(II) ions in the sample.

up shown below:

experiment? Write a half equation for the reaction

plating industry, a metal is used as the anode instead of carbon. What is this metal? Explain

sodium hydroxide solution to remove the

reasons why it is necessary to remove the copper(II) ions from the waste water before

8.0M sodium hydroxide solution for complete

(10 marks)

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4. HKCEE 1996 I Q6a The table below lists the oxidation number of iron in two compounds:

Compound Iron(II) sulphate Iron(III) sulphate Oxidation number +2 +3

(i) (1) What would be observed when sodium hydroxide solution is added to iron(II) sulphate solution? Write an ionic equation for the reaction involved. (2) Explain whether this reaction is a redox reaction.

(ii) When iron(II) sulphate solution is mixed with dilute sulphuric acid and a small amount of a purple solution, a reaction occurs and the oxidation number of iron changes from +2 to +3.

(1) Suggest what the purple solution may be. (2) What would be observed in this reaction? Write an ionic equation for the reaction involved. (iii) When iron(II) sulphate solution reacts with an element X, the oxidation number of iron changes from +2 to 0. (1) Suggest what X may be. (2) What would be observed in this reaction? Explain whether iron(II) sulphate solution acts as a reducing

agent or an oxidizing agent in this reaction. (10 marks)

5. HKCEE 1997 I Q9b X and Y are different metals. A student studied the reactivity of X, Y and copper by setting up two

electrochemical cells using the following materials and apparatus:

The results of the experiment are tabulated below:

Electrochemical cell

formed by connecting half-cells A and C

formed by connecting half-cells B and C

(i) What is the meaning of the term saturated solution?(ii) Explain the use of the strips of filter paper in the experiment.(iii) The student had to use an additional instrument to determine the direction of electron flow in the external

circuit. (1) What instrument did the student use? (2) Draw a labelled diagram to show the set(iv) Arrange X, Y and copper in the order of increasing reactivity. Explain your answer.(v) What would be observed when a piece of copper foil is immersed in an aqueous solution co

dm-3 of Y2+ ions? Explain your answer.

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X and Y are different metals. A student studied the reactivity of X, Y and copper by setting up two electrochemical cells using the following materials and apparatus:

ent are tabulated below:

Direction of electron flow in the external circuit

Y to Cu

X to Y

What is the meaning of the term saturated solution? Explain the use of the strips of filter paper in the experiment. The student had to use an additional instrument to determine the direction of electron flow in the external

What instrument did the student use? d diagram to show the set-up for the experiment, using half-cells

Arrange X, Y and copper in the order of increasing reactivity. Explain your answer.What would be observed when a piece of copper foil is immersed in an aqueous solution co

ions? Explain your answer.

X and Y are different metals. A student studied the reactivity of X, Y and copper by setting up two

The student had to use an additional instrument to determine the direction of electron flow in the external

cells A and C. Arrange X, Y and copper in the order of increasing reactivity. Explain your answer. What would be observed when a piece of copper foil is immersed in an aqueous solution containing 1 mol

(9 marks)

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6. HKCEE 1997 I Q4 Briefly describe how you would conduct an experiment, using the materials and apparatus listed below, to

nickel-plate a clean metal spoon. (Diagrams are experiment.

a clean metal spoon, a nickel plate, nickel(II) sulphate crystals, a large beaker of distilled water, a d.c. power supply and connecting wires

7. HKCEE 1998 I Q9b Each of the following experiments produces a sodium hydroxide solution.

(i) What should be observed when a small piece of sodium is added to water?(ii) Explain whether experiment I or experiment (iii) During the electrolysis of brine, chlorine and hydrogen are liberated at the anode and cathode respectively. A

sodium hydroxide solution remains in the electrolytic cell after some time. (1) Explain why hydrogen, instead of sodium, is liberated at the cathode. (2) Suppose that 50.0 cm3 of hydrogen is liberated at the cathode at room temperature and

Deduce the theoretical volume of chlorine liberated at the anode under the same (3) Explain why a sodium hydroxide solution remains in the electrolytic cell.(iv) Draw a labeled diagram to show the laboratory set

gaseous products.

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Briefly describe how you would conduct an experiment, using the materials and apparatus listed below, to plate a clean metal spoon. (Diagrams are NOT required.) State the expected observation

a nickel plate, nickel(II) sulphate crystals, a large beaker of distilled water, a d.c. power supply and connecting wires

Each of the following experiments produces a sodium hydroxide solution.

What should be observed when a small piece of sodium is added to water? Explain whether experiment I or experiment II is preferred for preparing a sodium hydroxide solution.During the electrolysis of brine, chlorine and hydrogen are liberated at the anode and cathode respectively. A sodium hydroxide solution remains in the electrolytic cell after some time.

Explain why hydrogen, instead of sodium, is liberated at the cathode. of hydrogen is liberated at the cathode at room temperature and

Deduce the theoretical volume of chlorine liberated at the anode under the same Explain why a sodium hydroxide solution remains in the electrolytic cell.

Draw a labeled diagram to show the laboratory set-up for the electrolysis of brine and the collection of the

Briefly describe how you would conduct an experiment, using the materials and apparatus listed below, to required.) State the expected observation of the

(8 marks)

II is preferred for preparing a sodium hydroxide solution. During the electrolysis of brine, chlorine and hydrogen are liberated at the anode and cathode respectively. A

of hydrogen is liberated at the cathode at room temperature and pressure. conditions.

up for the electrolysis of brine and the collection of the

(10 marks)

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8. HKCEE 1998 I Q6b The table below includes some information about three types of dry cells. The voltage of each type of cell is

1.5 V. Type Voltage over

discharge Price per cell

/ $ Shelf life / years

Life / minutes

Zinccarbon cell (AA size)

falls quite rapidly

2.5 1.5 70

Alkaline manganese cell (AA size)

remains steady 5.0 3 90

Silver oxide cell (button type)

remains steady 8.0 2 30

(The life of a cell has been determined from its use in a test with a motorized toy.)

(i) Decide and explain which type of cell should be used in a small CD-player (Discman). (ii) A package of 24 zinc-carbon cells is now being offered at a special price of $49.90. Assuming that your radio

consumes one zinc-carbon cell per month, would you buy a package of these specially-priced cells for the use of your radio? Explain your answer.

(iii) The half-equations below show the changes at the two electrodes, A and B, of a silver oxide cell during discharge: A: Zn(s) + 2OH-(aq) ZnO(s) + H2O(l) + 2e- B: Ag2O(s) + H2O(l) + 2e- 2Ag(s) + 2OH-(aq)

(1) Decide and explain which electrode, A or B, is the anode. (2) Write the overall equation for the reaction that would occur in the cell during discharge.

(iv) The following information was found on the packaging of a brand of zinc-carbon cells:

Caution: 1. Do not dispose of used cells in fire. 2. Remove cells when not in use for prolonged periods.

(1) Explain why used cells should not be disposed of in fire. (2) Explain why the cells should be removed when not in use for prolonged periods.

(9 marks)

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9. HKCEE 1998 Q2c For each of the following experiments, state the expected observation and write a relevant chemical equation. A sodium sulphite solution is added to an iodine solution (iodine dissolved in aqueous potassium iodide).

(2 marks)

10 HKCEE 1999 I Q8a The diagram below shows the longitudinal section of zinc

(i) Write a half-equation for the reaction that occurs at (ii) State the function of following substances in a zinc (1) carbon rod (2) manganese(IV) oxide (iii) Suggest a chemical test to show the presence ammonium ions in the moist paste of ammonium chlo(iv) Explain whether you agree with the following statement. Zinc-carbon cells cause more environmental problems than nickel(v) Complete and balance the following half

nickel-cadmium cell. Cd + OH- Cd(OH)2 NiO2 + H2O Ni(OH)

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he diagram below shows the longitudinal section of zinc-carbon cell.

equation for the reaction that occurs at the zinc case of the cell during discharge.State the function of following substances in a zinc-carbon cell.

Suggest a chemical test to show the presence ammonium ions in the moist paste of ammonium chloExplain whether you agree with the following statement.

carbon cells cause more environmental problems than nickel-cadmium cells doComplete and balance the following half-equations for the reactions that occur at the electrodes of

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Ni(OH)2 + OH-

the zinc case of the cell during discharge.

Suggest a chemical test to show the presence ammonium ions in the moist paste of ammonium chloride.

cadmium cells do. equations for the reactions that occur at the electrodes of a

(9 marks)

11. HKCEE 1999 I Q6a Water is a compound of hydrogen and oxygen. Under suitable conditions, 80.0 cm

of oxygen (with one of the reactants in excess) react to give water. The volumes of both gat room temperature and pressure.

(i) Draw the electronic diagram of water, showing electrons in the (ii) Assume that the two gases undergo complete reaction. Deduce the volume of the remaining gas, measured at

room temperature and pressure, at the end of the reaction.(iii) Water can be decomposed by electrolysis with the following set

(1) Explain why a little sulphuric acid has been added to the water used. (2) Suggest a suitable material for the electrodes. (3) Write the half-equation for the formation of oxygen. (4) Suggest a chemical test for

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Water is a compound of hydrogen and oxygen. Under suitable conditions, 80.0 cmof oxygen (with one of the reactants in excess) react to give water. The volumes of both gat room temperature and pressure. Draw the electronic diagram of water, showing electrons in the outermost shells only.Assume that the two gases undergo complete reaction. Deduce the volume of the remaining gas, measured at room temperature and pressure, at the end of the reaction. Water can be decomposed by electrolysis with the following set-up to give hydrogen and oxygen.

Explain why a little sulphuric acid has been added to the water used. able material for the electrodes. equation for the formation of oxygen.

Suggest a chemical test for each product obtained in the electrolysis.

Water is a compound of hydrogen and oxygen. Under suitable conditions, 80.0 cm3 of hydrogen and 60.0 cm3 of oxygen (with one of the reactants in excess) react to give water. The volumes of both gases are measured

only. Assume that the two gases undergo complete reaction. Deduce the volume of the remaining gas, measured at

up to give hydrogen and oxygen.

(8 marks)

12. HKCEE 2001 I Q9a, b a. A student used the set-up shown below to prepare hydrogen and chlorine by electrolysis of a very dilute

sodium chloride solution. Contrary to the students expectation, a colourless gas X instead of chlorine was liberated at the anode.

(i) What is X? (ii) Suggest a chemical test for X.

b. The experiment in (a) was then modifanode respectively.

(i) Suggest how the experiment could be modified.(ii) Deduce the ratio of the theoretical volumes of hydrogen and chlorine produced.(iii) With the help of a chemical equation, explain why the volume of chlorine collected is significantly smaller

than the theoretical volume.

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up shown below to prepare hydrogen and chlorine by electrolysis of a very dilute sodium chloride solution. Contrary to the students expectation, a colourless gas X instead of chlorine was

.

The experiment in (a) was then modified so that hydrogen and chlorine were produced at the cathode and

Suggest how the experiment could be modified. Deduce the ratio of the theoretical volumes of hydrogen and chlorine produced.

equation, explain why the volume of chlorine collected is significantly smaller

up shown below to prepare hydrogen and chlorine by electrolysis of a very dilute sodium chloride solution. Contrary to the students expectation, a colourless gas X instead of chlorine was

(2 marks) ied so that hydrogen and chlorine were produced at the cathode and

equation, explain why the volume of chlorine collected is significantly smaller

(6 marks)

13. HKCEE 2002 I Q9c A student used the set-up shown below to conduct a microscale experiment on electrolysis.

(i) (1) The initial colour of the drop shown above was green. State the colour change of the liquid around carbon rod A after a current was passed through the circuit for some time. Explain your answer with the help of a half equation. (2) A gas was liberated at carbon rod B. What was the gas? Explain its information.(ii) Some objects readily available in daily life contain carbon rods which can be used in this Suggest ONE such object.

(iii) The use of microscale experiments in studying chemistry is becoming more popular Suggest TWO advantages of carrying out experiments in mircoscale.

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up shown below to conduct a microscale experiment on electrolysis.

The initial colour of the drop shown above was green. State the colour change of the liquid after a current was passed through the circuit for some time. Explain your

answer with the help of a half equation. A gas was liberated at carbon rod B. What was the gas? Explain its information.

y available in daily life contain carbon rods which can be used in this

The use of microscale experiments in studying chemistry is becoming more popular Suggest TWO advantages of carrying out experiments in mircoscale.

up shown below to conduct a microscale experiment on electrolysis.

The initial colour of the drop shown above was green. State the colour change of the liquid after a current was passed through the circuit for some time. Explain your

A gas was liberated at carbon rod B. What was the gas? Explain its information. y available in daily life contain carbon rods which can be used in this experiment.

The use of microscale experiments in studying chemistry is becoming more popular nowadays.

(8 marks)

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14. HKCEE 2002 I Q4 Using the electrolysis of copper(II) chloride solution as an example, briefly discuss the factors affecting the

discharge of ions in electrolysis. (9 marks)

15. HKCEE 2002 I Q2 For each of the following experiments, state an expected observation and write a chemical equation for the

reaction involved. (a) A magnesium ribbon is placed in a Bunsen flame. (b) Excess iron(II) sulphate solution is added to an acidified potassium permanganate solution. (c) Chlorine gas is bubbled into a sodium bromide solution.

(6 marks)

16. HKCEE 2003 I Q9a The diagram below shows a sodium-sulphur cell connected to an external circuit. This cell operates at a high temperature of about 370oC, which is above the melting points of sodium and sulphur.

(i) State and explain the direction of electron flow in the external circuit when the cell is discharged. Write half equations for the reactions at electrodes (ii) Suggest TWO functions of the porous device.(iii) Suggest why it is necessary for the cell to operate at a high temperature.(iv) Sodium-sulphur cells are rechargeable and are used in power stations to reduce the generated. Suggest why these cells can be used to reduce the wastage of electricity.

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sulphur cell connected to an external circuit. This cell operates at a high C, which is above the melting points of sodium and sulphur.

State and explain the direction of electron flow in the external circuit when the cell is discharged. Write half equations for the reactions at electrodes A and B.

functions of the porous device. Suggest why it is necessary for the cell to operate at a high temperature.

sulphur cells are rechargeable and are used in power stations to reduce thecells can be used to reduce the wastage of electricity.

sulphur cell connected to an external circuit. This cell operates at a high

State and explain the direction of electron flow in the external circuit when the cell is discharged.

sulphur cells are rechargeable and are used in power stations to reduce the wastage of electricity cells can be used to reduce the wastage of electricity.

(8 marks)

17. HKCEE 2003 I Q7a The set-up shown below is used to investigate the electrical conductivity of lead(II) bromide.

When the lead(II) bromide becomes molten, the bulb lights up.(i) What would be observed at electro(ii) State ONE potential hazard when carrying out the experiment.(iii) State what will happen to the bulb when heating is stopped and the molten lead(II) down gradually to room temperature. Explain your answer.

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up shown below is used to investigate the electrical conductivity of lead(II) bromide.

When the lead(II) bromide becomes molten, the bulb lights up. What would be observed at electrode X? Write the half equation for the reaction involved.State ONE potential hazard when carrying out the experiment. State what will happen to the bulb when heating is stopped and the molten lead(II)

to room temperature. Explain your answer.

up shown below is used to investigate the electrical conductivity of lead(II) bromide.

? Write the half equation for the reaction involved.

State what will happen to the bulb when heating is stopped and the molten lead(II) bromide is allowed to cool

(6 marks)

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18. HKCEE 1997 I Q8b Read the following paragraph concerning chromium and answer the questions that follow:

The Greek word maochr means colour. Many chromium-containing compounds and chromium-containing gemstones are beautifully coloured. The oxidation number of chromium in its compounds can be +2, +3 and +6.

(i) Jade, a green-coloured gemstone, is chromium-containing. Suggest what chromium ion present in jade is responsible for its green colour.

(ii) Potassium dichromate is an oxidizing agent. The oxidation number of chromium in potassium dichromate is +6.

(1) Name ONE compound which can be oxidized by potassium dichromate. (2) State the condition(s) under which the compound reacts with potassium dichromate. (3) What product is formed from the compound in the redox reaction? (iii) In the presence of a dilute acid, chromium(II) ions react with atmospheric oxygen to form

chromium(III) ions and water. (1) Write the half equation for the formation of chromium(III) ions. (2) Write the half equation for the formation of water. (3) Write the overall equation for the reaction. (iv) Suggest TWO ways in which chromium can be used to prevent the corrosion of iron.

(9 marks)

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19. HKCEE 2004 I Q3 (a) Suggest how iodine tincture can be prepared in a school laboratory.

(b) A student split some iodine tincture on his laboratory coat. His classmate suggested the following two methods to remove the iodine stain from the laboratory coat:

(1)treating the stain with sodium sulphite solution

(2)treating the stain with 1,1,1-trichloroethane

State the principle underlying each method. Decide and explain which method is better. (5 marks)

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20. HKCEE 2004 I Q6a, b (a) Water (H2O) is an oxide of hydrogen. Electrolysis of water in the presence of sulphuric acid gives hydrogen

and oxygen in a volume ratio of 2:1.

(i) Suggest suitable electrodes to be used in the electrolysis.

(ii) Write the half equation for the reaction at the cathode and that at the anode during the electrolysis.

(iii) What is the function of sulphuric acid in the electrolysis?

(iv) Is it possible to deduce the formula of water from the results of the electrolysis? Explain your answer. (6 marks)

(b) Hydrogen peroxide (H2O2) is another oxide of hydrogen.

(i) What is the oxidation number of oxygen in hydrogen peroxide?

(ii) Draw the electronic diagram of a molecule of hydrogen peroxide, showing electrons in the outermost shells only.

(iii) In the presence of a dilute acid, hydrogen peroxide oxidises iron(II) ions and it is reduced to water. (1) Write the half equation for the reduction of hydrogen peroxide.

(2) State the expected observation and write a chemical equation for the reaction involved. (5 marks)

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21. HKCEE 2005 I Q2a

Upon strong heating, silver oxide (Ag2O) undergoes decomposition as represented by the following word equation :

silver oxide silver + oxygen

(i) Transcribe the word equation into a chemical equation.

(ii) Explain why the decomposition is a redox reaction.

(iii) Calculate the mass of silver that would be obtained when 3.50 g of silver oxide undergoes complete decomposition.

(5 marks)

22. HKCEE 2005 I Q4

The wastewater generated from an electroplating factory contains dichromate ions. Before the wastewater is discharged, it is treated in two stages as described below to remove the chromium-containing substances.

Stage 1: Treat the wastewater with excess sodium sulphite solution in the presence of acid to reduce the dichromate ions to chromium(III) ions.

Stage 2: Add a suitable chemical to the treated wastewater from Stage 1 to precipitate the chromium(III) ions.

(a) Why is it necessary to remove chromium-containing substances from the wastewater ?

(b) In Stage I, the sulphite ions are oxidised to sulphate ions by the dichromate ions.

(i) Write the half equation for the oxidation of sulphite ions.

(ii) Write the half equation for the reduction of dichromate ions.

(iii) Write the overall equation for the redox reaction.

(c) Suggest a suitable chemical for the precipitation of chromium(III) ions in Stage 2.

(5 marks)

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23. HKCEE 2005 I Q7

A chemical cell can be made from two metal strips and a lemon. Given the following materials and equipment, outline how you can set up a chemical cell with the maximum output voltage.

a lemon, a copper strip, a magnesium strip, a zinc strip, a multimeter and several connecting wires

(Your answer should include variables that need to be controlled.)

(9 marks)

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24. HKCEE 2005 I Q9 An experiment was carried out to study the electrolysis of a concentrated sodium chloride solution using

several zinc-carbon cells as a source of electricity. The following diagram shows the set-up used:

(a) (i) What is gas X? (ii) Give ONE use of X in industry.

(2 marks) (b) (i) What is gas Y? (ii) If the electrolysis is repeated using a very dilute sodium chloride solution, another gas will be

liberated at carbon electrode B. Suggest an explanation for this phenomenon. (3 marks)

(c) With reference to the longitudinal section of a zinc-carbon cell shown below, suggest how chemical energy is converted to electrical energy when the cell is producing a current.

(3 marks)

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25. HKCEE 2006 I Q10

A student used an aluminium can, a carbon rod and household bleach to make a chemical cell. The diagram below shows the set-up of the cell connected to a load and an ammeter.

(a) The materials used by the student to make the cell are readily available at home. Suggest ONE household item (i) which contains a carbon rod.

(ii) which includes an aluminium can. (2 marks) (b) When the cell is producing a current, the aluminium can undergoes oxidation to give aluminate ions,

Al(OH)4- (aq), while at the carbon rod the hypochlorite ions undergo reduction in the presence of water to give chloride ions and hydroxide ions. (i) the oxidation of the aluminium metal, and

(ii) the the reduction of the hypochlorite ions. (2 marks) (c) The student also used the above set-up to investigate the relation between the current produced by the cell

and the concentration of hypochlorite ions in the bleach. (i) Suggest TWO conditions which should be kept constant when conducting this investigation.

(ii) The student noticed that the current produced by the cell increases with the concentration of hypochlorite ions in the bleach. Suggest an explanation for the phenomenon. (3 marks)

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26. HKCEE 2007 I Q4

A student learnt from a book that an ancient chemical cell could be made by immersing an iron rod in a liquid placed inside a copper can. The liquid used was vinegar but not wine. The diagram below shows the set-up designed by him in simulating the cell.

(a) Explain, in terms of structure and property of particles, why the liquid inside the ancient chemical cell was vinegar but not wine.

(2 marks)

(b) The student found that the iron rod dissolved gradually, and colourless gas bubbles were given out on the inner wall of the copper can. (i) Write a half equation, involving iron, for the reaction that occurred at the iron rod.

(ii) Write a half equation for the reaction that occurred on the inner wall of the copper can. (2 marks)

(c) The student found that colourless gas bubbles were also given out at the surface of the iron rod that immersed in vinegar. Explain the observation.

(1 mark)

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27. HKCEE 2007I Q11 In a chemical plant, extraction of copper from its ores involves roasting copper(I) sulphide with air inside a high temperature furnace. Copper(I) sulphide reacts with oxygen in air according to the following equation:

Cu2S + O2 2Cu + SO2 The copper so extracted contains impurities including metals such as silver, iron, zinc and gold. The impure copper is then purified by electrolysis as illustrated in the diagram below:

(a) With reference to the reaction between copper(I) sulphide and oxygen, identify the species undergoing oxidation and the species undergoing reduction. Explain your answers in terms of changes in oxidation numbers.

(2 marks)

(b) Explain briefly how impure copper can be purified by electrolysis as illustrated in the diagram above. (2 marks)

(c) Insoluble impurities deposit under the impure copper anode as anode sludge'. According to the information given, suggest what substances the anode sludge would contain. Explain your answer.

(2 marks)

(d) The concentration of copper(II) ions in copper(II) sulphate solution remains UNCHANGED in the above electrolysis. Is this statement correct? Explain your answer.

(2 marks)

(e) State TWO advantages of building a factory in which contact process is carried out near the chemical plant mentioned above.

(2 marks)

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28. HKCEE 2008 I Q5 The diagram below shows a set-up with metal strips inserted in fresh potatoes. The multimeter reading in the

set-up is +0.75 V.

(a) State, with explanation, the direction of electrons flow across the connecting wire between zinc strip and magnesium strip.

(1 mark)

(b) (i) Which metal strip in Potato B is anode? Why? (ii) Write the half-equation for the change occurred at the anode in Potato B. (2 marks)

(c) Which two metal strips should be interchanged in order to increase the multimeter reading? (1 mark)

(d) Explain why fresh potatoes should be used in the set-up. (1 mark)

(e) What will the multimeter reading be if the zinc strip in potato A is replaced by another magnesium strip, while other three metal strips remain unchanged?

(1 mark)

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29. HKCEE 2009 I Q13

Electrolysis can be applied to enhance the corrosion resistance of iron. Describe the chemical principle involved in this application. Your description should include the chemical reactions involved, and the use of appropriate electrodes and electrolyte.

(Diagrams are NOT required.) (9 marks)

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30. HKCEE 2010 I Q9

The following diagram shows a system used in some piers for slowing down the rusting of steel pillars:

Design an experiment performed in a laboratory to show that such a system can slow down the rusting of steel in sea water. Labelled diagrams of the set-up, expected observation and the chemical principle involved should be included in your answer.

(9 marks)

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31. HKCEE 2010 Q11

The diagram below shows a kind of traditional button cell making from mercury(Il) oxide, zinc powder and sodium hydroxide paste :

When the cell is producing a current, the overall cell reaction can be represented by the chemical equation : Zn + HgO ZnO + Hg

(a) Explain whether mercury(Il) oxide or zinc powder should be at the region labelled X. (1 mark)

(b) What is the function of the sodium hydroxide paste in the cell ? (1 mark)

(c) Why should this kind of button cell be banned in the market ? (1 mark)

(d) Explain whether the cell can work if mercury(Il) oxide is replaced by magnesium oxide, while other materials remain unchanged.

(1 mark) (e) Explain the change of the maximum voltage supplied by the cell if zinc powder is replaced by copper

powder, while other materials remain unchanged. (1 mark)

(f) The following diagram shows the electrolytic cell used in the manufacture of sodium hydroxide :

(i) Write a half equation for the anodic reaction. (ii) Write a half equation for the cathodic reaction. (iii) It is known that only cations can pass through the membrane. Explain why sodium hydroxide solution of

higher concentration is eventually obtained. (4 mark)

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32. [HKAL 1991 II Q2b]

(i) CrO42- ions react with S2O32- ions to form Cr(OH)4- and SO42- ions in basic medium. Write balanced

equations for each half reaction and for the overall reaction.

(ii) What volume of 0.5 M Na2CrO4 solution is needed to react completely with 40.0 cm3 of 0.20 M Na2S2O3

solution in basic medium?

(5 marks)

(i) CrO42- (aq) + 4H2O(l) + 3e- Cr(OH)4-(aq) + 4OH-(aq) (1) S2O32-(aq) + 10OH-(aq) 2SO42-(aq) + 5H2O(l) + 8e- (1)

Combining the two half ionic equations 8CrO42-(aq) + 3S2O32-(aq) + 17 H2O(l) 6SO42-(aq) + 8Cr(OH)4-(aq) + 2OH- (1)

(ii) Number of moles of S2O32-(aq) = (0.2)(0.04) = 810-3mol Number of moles of CrO42-(aq) = (810-3mol)(8/3) = 0.02133 mol (1) Volume of Na2CrO4 = 0.02133 mol 0.5M = 42.67 cm3 (1)

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33. [HKAL 1993 I Q1d] Write balanced equations for: a. The reaction between MnO2(s) and PbO2(s) in acidic solution to give MnO4-(aq) and Pb2+(aq). b. The reaction between H2O2(aq) and Cr(OH)4-(aq) in alkaline solution to give CrO42-(aq).

(3 marks)

34. [HKAL 1993 II Q3a] (i) Write balanced half equations for the redox reaction of Crn+ and MnO4- in acidic solution. In terms of n, how

many moles of electrons per mole of Crn+, are involved in the oxidation reaction? Write the balanced overall chemical equation for the above reaction.

(ii) An acidified solution containing 1.5010-3 mol of Crn+is titrated with 0.0250 M KMnO4 solution. The equivalence point of the reaction is reached after 48.00 cm3 of KMnO4 has been added.

Calculate the value of n. (9 marks)

(i) 2MnO2(s) + PbO2(s) + 4H+(aq) 2MnO4-(aq) + 3Pb2+(aq) + 2H2O(l) (1.5) (ii) 3H2O2(aq) + 2Cr(OH)4-(aq) + 2OH-(aq) 2CrO42-(aq) + 8H2O(l) (1.5)

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35. [HKAL 1994 I Q1e] Write a balanced equation for each of the following reactions.

a. The reaction between BrO3-(aq) and Br-(aq) in acidic solution to give Br2(aq).

b. The reaction between MnO4-(aq) and SO2(g) in alkaline solution to give MnO2(s) and SO42-(aq).

(3 marks)

36. [HKAL 1995 II Q3c] The electromotive force of a new zinc-carbon dry cell is 1.5V. When it is producing an electric current, the

following changes occur at the two electrodes:

Anode : Zn(s) reacts to give Zn2+(aq). Cathode : MnO2(s) and NH4Cl(aq) react to give Mn2O3(s) and NH3(g). (i) Write half equations for the reactions at the anode and at the cathode, and the equation for the overall

reaction that occurs in the dry cell.

(ii) Write the cell diagram for the dry cell, using the IUPAC convention. # (iii) Explain why the electromotive force of the dry cell drops,

(1) after it has been used for some time; (2) after it has been stored for a long time without being used.

(8 marks)

(i) BrO3-(aq) + 5Br-(aq) + 6H+(aq) 3Br2(aq) + 3H2O(l) (1.5) (ii) 2MnO4-(aq) + 3SO2(g) + 4OH-(aq) 2MnO2(s) + 3SO42-(aq) + 2H2O(l) (1.5)

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37. [HKAL 1997 II Q4b] Discuss and write relevant equations for the electrochemical processes involved when broken surfaces of the

following are exposed to moist air.

(i) galvanized iron (ii) tin-plated iron

(4 marks)

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38. [HKAL 2002 II Q3b] A hydrogen-oxygen fuel cell uses concentrated potassium hydroxide solution as electrolytes and nickel as

electrodes.

(i) Draw a labeled diagram to show the design of the above fuel cell. (ii) Briefly describe how the cell works, giving the equations for the electrode half reactions. (iii) State one advantage of using fuel cells over using batteries.

(5 marks)

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39. [HKAL 2003 II Q4b] Carbon monoxide is a highly toxic gas and is also an indoor air pollutant. The level of indoor carbon monoxide

can be monitored by the use of carbon monoxide detectors. One type of carbon monoxide detectors uses an

electrochemical sensing method. The detector contains two inert electrodes coated with platinum which

catalyses the reaction of carbon monoxide with oxygen in the atmosphere. The anodic and cathodic reactions

are as follows:

Anode: CO(g) + H2O(l) CO2(g) + 2H+(aq) + 2e- Cathode: 1/2 O2(g) + 2H+(aq) + 2e- H2O(l)

(i) Why is carbon monoxide toxic? (ii) Outline how the carbon monoxide detector can detect the indoor carbon monoxide level. (iii) Describe one situation in which the indoor carbon monoxide level would increase suddenly.

(5 marks)

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40. [HKAL 2006 II Q4a] The overall reaction for the discharging process of a lead-acid cell can be represented by the following

equation:

PbO2(s) + Pb(s) + 4H+(aq) + 2SO42-(aq) 2PbSO4(s) + 2H2O(l) (i) Write the half equation of the cathodic reaction and that of the anodic reaction during discharge for a

lead-acid cell.

(ii) Using the IUPAC conventions, write the cell diagram of a lead-acid cell.# (iii) Based on the above information, explain why a lead-acid cell is rechargeable. (iv) A lead-acid accumulator used in automobiles consists of six lead-acid cells connected in series. Suggest why

(I) the state of charge of a lead-acid accumulator can be estimated by measuring the density of the acid in the accumulator, and

(II) an excessively high voltage should not be used to charge a lead-acid accumulator. (7 marks)

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41. [HKAL 2008 I Q2b] The diagram below shows the design of a hydrogen-oxygen fuel cell:

+ -

O2(g)

unreacted O2(g)

H2(g)

unreacted H2(g) + H2O(g)

Pt-containing porous electrodePt-containing porous electrode

concentrated NaOH(aq)

(i) Describe the working principle of the fuel cell. (ii) One advantage of using hydrogen-oxygen fuel cells is that they do not emit air pollutants. Suggest

ONE other advantage of using hydrogen-oxygen fuel cells. (4 marks)