3- Chemical Cells And Electrolysis

CE : Section 3 Chemical Cells and Electrolysis LQ P.1

CE : Section 3 Chemical Cells and Electrolysis 1.

93 2(a) Tuning knobs on radios are often made of plastics plated with metal coatings. (i) State TWO reasons why plastics are used in the manufacture of tuning knobs. (ii) What is the purpose of plating the knobs with metals ? 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. (5 marks) 2. 94 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. (7 marks) 3. 95 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 ions is oxidized or reduced.

CE : Section 3 Chemical Cells and Electrolysis LQ P.2

(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) 4. 96 6(a) The table below lists the oxidation number of iron in two compounds: Compound Oxidation number

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? 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 oxidation agent in this reaction. (10 marks) 5. 96 9(b) A student carried out a copper-plating experiment in the laboratory using the set-up shown below:

(i) Explain why copper(II) sulphate solution can conduct electricity. (ii) 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 dm3 of a sample of waste water require 3.5 dm 3 of 8.0 M sodium hydroxide solution for complete removal of the copper(II) ions present. Calculate the concentration, in mol dm-3, of copper(II) ions in the sample. (10 marks)

CE : Section 3 Chemical Cells and Electrolysis LQ P.3

6. 97 4 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 any 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) 7. 97 8(b) Read the following paragraph concerning chromium and answer the questions that follow: The Greek work "chroma" 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) 8. 97 9(b) 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

Direction of electron flow in the external circuit Y to Cu X to Y

(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-up for the experiment, using half-cells A and C. (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 containing 1 mol dm-3 of Y 2+ ions? Explain your answer.

CE : Section 3 Chemical Cells and Electrolysis LQ P.4

(9 marks) 9. 98 9(b) Each of the following experiments produces a sodium hydroxide solution.

(i) What would be observed when a small piece of sodium is added to water? (ii) 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 labelled diagram to show the laboratory set-up for the electrolysis of brine and the collection of the gaseous products. (10 marks) 10. 99 6(a) Water is a compound of hydrogen and oxygen. Under suitable conditions, 80.0 cm 3 of hdyrdogen 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) 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 each product obtained in the electrolysis. (8 marks) 11. 99 8(a) The diagram below shows the longitudinal section of a zinc-carbon cell.

CE : Section 3 Chemical Cells and Electrolysis LQ P.5

(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 statements. '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(9 marks)

12. 01 9 (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. (2 marks) (b) The experiment in (a) was then modified so that hydrogen and chlorine were produced at the cathode and anode 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. (6 marks) 13. 02-2 For each of the following experiments, state an expected observation and write a chemical equation for the reaction involved. (a) A magnesium ribbon in 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) 14. 02-4

CE : Section 3 Chemical Cells and Electrolysis LQ P.6

Using the electrolysis of copper(II) chloride solution as an example, briefly discuss the factors affecting the discharge of ions in electrolysis. (6 + 3 marks) 15. 02-9(c) 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 formation. (ii) Some objects readily available in daily life contain carbon rods which can be used in this experiment. Suggest ONE such object. (iii)The use of microscale experiments in studying chemistry in becoming more popular nowadays. Suggest TWO advantages of carrying out experiments in microscale. (8 marks)