Cq Redox
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Redox :1989 4. (a) A student used the set-up below to study the conductivity of aqueous solution of several solutes:
The experimental results were tabulated as follows: Experiment I
Ammeter reading (A)
Solute Sugar
0
II
Sodium sulphate
0.5
III
Copper(II) sulphate
0.5
Other observation No observable change Gas bubbles were liberated at both electrodes Q and R but at different rates. Gas bubbles were liberated from ONE electrode only. 0.22 g of copper was deposited on the other electrode.
(i)
Why was there no ammeter reading in experiment I, while ammeter readings were obtained in experiment II and III? (ii) In experiment II, name the gas liberated at (1) electrode Q. (2) electrode R. Explain why gas bubbles were liberated at different rates. (iii) In experiment III, (1) explain the observations by writing ionic equations for the reactions which occurred at electrodes Q and R respectively. (2) describe briefly how you would confirm that the deposit was copper by a chemical test. State the reagents you would use and give the observations expected. (11 marks) 3. (b) The results of experiments with iron and three other metals X, Y, Z and their oxides are summarized in the following tables: Metal Experiment Action of metal on iron(II) sulphate solution Metal oxide Experiment Action of heat on metal oxide
(i)
Iron No immediate reaction
X
Y
Iron is deposited A gas is evolved
Z No reaction
Fe2O3
XO
Y2O
Z2O
No reaction
No reaction
No reaction
Metal Z is formed
What was the gas evolved in the reaction of Y with iron(II) sulphate solution? Write an equation for the reaction involved. (State symbols should be given.)
Page 1
(ii)
Arrange the four metals in descending order of activity, and briefly explain your answer. (iii) One of the three metals, X, Y or Z was used to form a chemical cell with an iron rod, as shown in the diagram below:
(1) Which of the three metals, X, Y or Z would be the most suitable as the metal strip? Give two reasons for your choice. (2) Suggest one reason why aqueous sodium sulphate solution is used in this cell. (3) Write an ionic equation for the oxidation reaction taking place in the cell. (4) Explain how the principle behind this experiment can be applied to protect iron from rusting. (12 marks) Redox :1990 4. (a)
Diagram I shows a set-up for the electrolysis of 0.5 M potassium bromide solution. After passing electricity for some time, gas bubbles were observed at electrode A, while the solution around electrode B turned yellowish-brown. This colouration gradually extended to the bottom of the U-tube and a steady colour boundary was formed as shown in diagram II. (i) (ii)
which of the electrodes was the cathode? Name the gas produced at electrode A, and suggest a chemical test to identify this gas. (iii) Write half equations for the reaction that occurred during electrolysis at electrodes A and B. (iv) Name the electrolysis product responsible for producing the yellowish-brown colour and explain why the colour extended to the bottom of the U-tube. (v) (1) What ions would migrate from the solution around electrode A towards electrode B during electrolysis? (2) One of these ions reacted with the yellowish-brown solution. Name this ion. (3) Briefly explain, with the aid of an equation, why a steady colour boundary was formed. (12 marks)
Page 2
Redox :1991 2. (c) Iron sheets can be tin-plated by electrolysis of either tin(II) or tin(IV) compounds before they are used to make food cans. (i) (ii)
In the above electrolysis, what material should be used as the anode? Give one reason to explain why iron is first tin-plated before food cans are made from it. (iii) If the tin-plated iron sheet has been scratched to expose the iron, can it still be used to make a food can? (6 marks) 4. (b) The following diagram shows the longitudinal section of a dry cell:
(i)
Name the material commonly used for making (1) P, and (2) Q. (ii) State the oxidizing agent. (iii) A dry cell often leaks after prolong usage. (1) Explain the cause of leakage based on the reaction that occurs at Q. (2) The leaked solution is suspected to contain ammonium ions and chloride ions. Suggest a chemical test to identify each of these ions. (9 marks) Redox :1992 5. (a) Sodium hydroxide can be manufactured by the electrolysis of concentrated sodium chloride solution in the following set-up, where A and B are inert electrodes.
(i) (ii)
Explain which electrode A or B, is the cathode. Using the concept of preferential discharge of ions, explain the electrode reactions and why sodium hydroxide can be manufactured by the above electrolysis. (6 marks)
Page 3
Redox :1993 2. (a) Tuning knobs on radios are often made of plastics plated with metal coatings. (i) (ii)
State two reasons why plastics are used in the manufacture of tuning knobs. What is the purpose of plating the knob with metal?
The plastic knobs are first coated with copper and then electroplated with nickel. The electroplating can be conducted using the following set-up:
(iii) Why is the plastic knob first coated with copper before electroplating? (iv) Write an ionic equation for the reaction that occurs at the cathode during electroplating. (6 marks) Redox :1994 7. (a)
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, 0.36 g of copper is deposited on the carbon electrode R. (i)
What is the direction of electron flow in the external circuit? Explain your answer. (ii) After the circuit has been closed for some time, what would be observed (1) at the carbon electrode S? (2) in the copper(II) sulphate solution? (iii) What is the function of set-up X in this experiment? (iv) Which of the metals, P or Q, occupies a higher position in the electrochemical series? Explain your answer. (8 marks)
Page 4
Redox :1995 9. (b)
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 flow in the external circuit. (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-up? (2) Explain whether a sodium sulphite solution can be used instead of a potassium nitrate solution in the salt bridge. (8 marks) Redox :1996 6. (a) The table below lists the oxidation number of iron in two compounds: Compound Oxidation no.
Iron(II) sulphate +2
Iron (III) sulphate +3
(i)
(1) What would be observed when sodium hydroxide solution is added to iron(II) sulphate solution? (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. Suggest what the purple solution may be. (1) What would be observed in this reaction? (2) 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)
Page 5
9. (b) A student carried out a copper-plating experiment in the laboratory using the set-up shown below:
(i) (ii)
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 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 sodium hydroxide solution to remove the 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 cm3 of a sample of waste water require 3.5 cm3 of 8.0 M sodium hydroxide solution for complete removal of the copper(II) ions present. (3) Calculate the concentration, in mol dm–3, of copper(II) ions in the sample. (10 marks) Redox :1997 8. (b) Read the following paragraph concerning chromium and answer the questions that follow: The Greek word "chrōma" 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) Page 6
12. Essay Question Briefly describe how you would conduct an experiment, using the materials and apparatus listed below, to nickel-plate a clean metal spoon. (Diagrams are not required.) State the expected observation of the 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 (8 marks) Redox :1998 6. (b) The table below includes some information about three types of dry cells. The voltage of each type of cell is 1.5 V. Voltage over discharge Zinc-carbon cell (AA size) falls quite rapidly Alkaline manganese cell remains steady (AA size) Silver oxide cell (button remains steady type) Type
Price per cell /$ 2.5
Shelf life /years
Life /minutes
1.5
70
5.0
3
90
8.0
2
30
(The life of a cell has been determined from its use in a test with a motorized toy.) (i)
Describe 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 ofeered 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() + 2eB : Ag2O(s) + H2O() + 2e- → 2Ag(s) + 2OH-(aq) (1) Describe and explain which electrode, A or B, is the anode. (2) Write the overall equation for the reason that would occur in the cell during discharge. (iv) The following information was found on the package 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) 2. For each of the following experiment, state the expected observation and write a relevant chemical equation. (c) A sodium sulphite solution is added to an iodine solution (iodine dissolved in aqueous potassium iodide.)
Page 7
(2 marks) 9. (b) Each of the following experiments produces a sodium hydroxide solution. Experiment I Action of sodium on water Sodium hydroxide solution Electrolysis of brine
Experiment II
(i) (ii)
What would 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. (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 pressure. Deduce the theoretical volume of chlorine liberated at the anode under the same conditions. (3) Explain why a sodium hydroxide solution remains in the electrolytic cell. (iv) Draw a labeled diagram to show the laboratory set-up for the electrolysis of brine and the collection of the gaseous products. (10 marks) Redox :1999 8. (a) The diagram below shows the longitudinal section of a zinc-carbon cell.
(i)
Write a half-equation for the reaction that occurs at the zinc case of the cell during discharge. (ii) State the function of following substances in a zinc-carbon cell. (1) carbon rod (2) manganese(IV) oxide (iii) Suggest a chemical test to show the presence of ammonium ions in the moist paste of ammonium chloride. (iv) Explain whether you agree with the following statement. ‘Zinc-carbon cells cause more environmental problems than nickel-cadmium cells do.’ (v) Complete and balance the following half-equations for the reactions that occur at the electrodes of a nickel-cadmium cell. Cd + OH- → Cd(OH)2 NiO2 + H2O → Ni(OH)2 + OH-
Page 8
(9 marks) 6. (a) 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 at room temperature and pressure. (i)
Draw the electronic diagram of water, showing electrons in the outermost shells only. (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-up to give hydrogen and oxygen.
(1) (2) (3) (4)
Explain why a little sulphuric acid has been added to the water used. Suggest a suitable material for the electrodes. Write the half-equation for the formation of oxygen. Suggest a chemical test for each product obtained in the electrolysis. (8 marks)
Page 9
The experimental results were tabulated as follows: Experiment I
Ammeter reading (A)
Solute Sugar
0
II
Sodium sulphate
0.5
III
Copper(II) sulphate
0.5
Other observation No observable change Gas bubbles were liberated at both electrodes Q and R but at different rates. Gas bubbles were liberated from ONE electrode only. 0.22 g of copper was deposited on the other electrode.
(i)
Why was there no ammeter reading in experiment I, while ammeter readings were obtained in experiment II and III? (ii) In experiment II, name the gas liberated at (1) electrode Q. (2) electrode R. Explain why gas bubbles were liberated at different rates. (iii) In experiment III, (1) explain the observations by writing ionic equations for the reactions which occurred at electrodes Q and R respectively. (2) describe briefly how you would confirm that the deposit was copper by a chemical test. State the reagents you would use and give the observations expected. (11 marks) 3. (b) The results of experiments with iron and three other metals X, Y, Z and their oxides are summarized in the following tables: Metal Experiment Action of metal on iron(II) sulphate solution Metal oxide Experiment Action of heat on metal oxide
(i)
Iron No immediate reaction
X
Y
Iron is deposited A gas is evolved
Z No reaction
Fe2O3
XO
Y2O
Z2O
No reaction
No reaction
No reaction
Metal Z is formed
What was the gas evolved in the reaction of Y with iron(II) sulphate solution? Write an equation for the reaction involved. (State symbols should be given.)
Page 1
(ii)
Arrange the four metals in descending order of activity, and briefly explain your answer. (iii) One of the three metals, X, Y or Z was used to form a chemical cell with an iron rod, as shown in the diagram below:
(1) Which of the three metals, X, Y or Z would be the most suitable as the metal strip? Give two reasons for your choice. (2) Suggest one reason why aqueous sodium sulphate solution is used in this cell. (3) Write an ionic equation for the oxidation reaction taking place in the cell. (4) Explain how the principle behind this experiment can be applied to protect iron from rusting. (12 marks) Redox :1990 4. (a)
Diagram I shows a set-up for the electrolysis of 0.5 M potassium bromide solution. After passing electricity for some time, gas bubbles were observed at electrode A, while the solution around electrode B turned yellowish-brown. This colouration gradually extended to the bottom of the U-tube and a steady colour boundary was formed as shown in diagram II. (i) (ii)
which of the electrodes was the cathode? Name the gas produced at electrode A, and suggest a chemical test to identify this gas. (iii) Write half equations for the reaction that occurred during electrolysis at electrodes A and B. (iv) Name the electrolysis product responsible for producing the yellowish-brown colour and explain why the colour extended to the bottom of the U-tube. (v) (1) What ions would migrate from the solution around electrode A towards electrode B during electrolysis? (2) One of these ions reacted with the yellowish-brown solution. Name this ion. (3) Briefly explain, with the aid of an equation, why a steady colour boundary was formed. (12 marks)
Page 2
Redox :1991 2. (c) Iron sheets can be tin-plated by electrolysis of either tin(II) or tin(IV) compounds before they are used to make food cans. (i) (ii)
In the above electrolysis, what material should be used as the anode? Give one reason to explain why iron is first tin-plated before food cans are made from it. (iii) If the tin-plated iron sheet has been scratched to expose the iron, can it still be used to make a food can? (6 marks) 4. (b) The following diagram shows the longitudinal section of a dry cell:
(i)
Name the material commonly used for making (1) P, and (2) Q. (ii) State the oxidizing agent. (iii) A dry cell often leaks after prolong usage. (1) Explain the cause of leakage based on the reaction that occurs at Q. (2) The leaked solution is suspected to contain ammonium ions and chloride ions. Suggest a chemical test to identify each of these ions. (9 marks) Redox :1992 5. (a) Sodium hydroxide can be manufactured by the electrolysis of concentrated sodium chloride solution in the following set-up, where A and B are inert electrodes.
(i) (ii)
Explain which electrode A or B, is the cathode. Using the concept of preferential discharge of ions, explain the electrode reactions and why sodium hydroxide can be manufactured by the above electrolysis. (6 marks)
Page 3
Redox :1993 2. (a) Tuning knobs on radios are often made of plastics plated with metal coatings. (i) (ii)
State two reasons why plastics are used in the manufacture of tuning knobs. What is the purpose of plating the knob with metal?
The plastic knobs are first coated with copper and then electroplated with nickel. The electroplating can be conducted using the following set-up:
(iii) Why is the plastic knob first coated with copper before electroplating? (iv) Write an ionic equation for the reaction that occurs at the cathode during electroplating. (6 marks) Redox :1994 7. (a)
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, 0.36 g of copper is deposited on the carbon electrode R. (i)
What is the direction of electron flow in the external circuit? Explain your answer. (ii) After the circuit has been closed for some time, what would be observed (1) at the carbon electrode S? (2) in the copper(II) sulphate solution? (iii) What is the function of set-up X in this experiment? (iv) Which of the metals, P or Q, occupies a higher position in the electrochemical series? Explain your answer. (8 marks)
Page 4
Redox :1995 9. (b)
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 flow in the external circuit. (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-up? (2) Explain whether a sodium sulphite solution can be used instead of a potassium nitrate solution in the salt bridge. (8 marks) Redox :1996 6. (a) The table below lists the oxidation number of iron in two compounds: Compound Oxidation no.
Iron(II) sulphate +2
Iron (III) sulphate +3
(i)
(1) What would be observed when sodium hydroxide solution is added to iron(II) sulphate solution? (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. Suggest what the purple solution may be. (1) What would be observed in this reaction? (2) 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)
Page 5
9. (b) A student carried out a copper-plating experiment in the laboratory using the set-up shown below:
(i) (ii)
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 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 sodium hydroxide solution to remove the 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 cm3 of a sample of waste water require 3.5 cm3 of 8.0 M sodium hydroxide solution for complete removal of the copper(II) ions present. (3) Calculate the concentration, in mol dm–3, of copper(II) ions in the sample. (10 marks) Redox :1997 8. (b) Read the following paragraph concerning chromium and answer the questions that follow: The Greek word "chrōma" 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) Page 6
12. Essay Question Briefly describe how you would conduct an experiment, using the materials and apparatus listed below, to nickel-plate a clean metal spoon. (Diagrams are not required.) State the expected observation of the 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 (8 marks) Redox :1998 6. (b) The table below includes some information about three types of dry cells. The voltage of each type of cell is 1.5 V. Voltage over discharge Zinc-carbon cell (AA size) falls quite rapidly Alkaline manganese cell remains steady (AA size) Silver oxide cell (button remains steady type) Type
Price per cell /$ 2.5
Shelf life /years
Life /minutes
1.5
70
5.0
3
90
8.0
2
30
(The life of a cell has been determined from its use in a test with a motorized toy.) (i)
Describe 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 ofeered 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() + 2eB : Ag2O(s) + H2O() + 2e- → 2Ag(s) + 2OH-(aq) (1) Describe and explain which electrode, A or B, is the anode. (2) Write the overall equation for the reason that would occur in the cell during discharge. (iv) The following information was found on the package 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) 2. For each of the following experiment, state the expected observation and write a relevant chemical equation. (c) A sodium sulphite solution is added to an iodine solution (iodine dissolved in aqueous potassium iodide.)
Page 7
(2 marks) 9. (b) Each of the following experiments produces a sodium hydroxide solution. Experiment I Action of sodium on water Sodium hydroxide solution Electrolysis of brine
Experiment II
(i) (ii)
What would 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. (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 pressure. Deduce the theoretical volume of chlorine liberated at the anode under the same conditions. (3) Explain why a sodium hydroxide solution remains in the electrolytic cell. (iv) Draw a labeled diagram to show the laboratory set-up for the electrolysis of brine and the collection of the gaseous products. (10 marks) Redox :1999 8. (a) The diagram below shows the longitudinal section of a zinc-carbon cell.
(i)
Write a half-equation for the reaction that occurs at the zinc case of the cell during discharge. (ii) State the function of following substances in a zinc-carbon cell. (1) carbon rod (2) manganese(IV) oxide (iii) Suggest a chemical test to show the presence of ammonium ions in the moist paste of ammonium chloride. (iv) Explain whether you agree with the following statement. ‘Zinc-carbon cells cause more environmental problems than nickel-cadmium cells do.’ (v) Complete and balance the following half-equations for the reactions that occur at the electrodes of a nickel-cadmium cell. Cd + OH- → Cd(OH)2 NiO2 + H2O → Ni(OH)2 + OH-
Page 8
(9 marks) 6. (a) 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 at room temperature and pressure. (i)
Draw the electronic diagram of water, showing electrons in the outermost shells only. (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-up to give hydrogen and oxygen.
(1) (2) (3) (4)
Explain why a little sulphuric acid has been added to the water used. Suggest a suitable material for the electrodes. Write the half-equation for the formation of oxygen. Suggest a chemical test for each product obtained in the electrolysis. (8 marks)
Page 9
