Cq Bleach
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Bleach
Page 1
Bleach : 1985 2. (a) The diagram below represents a simplified set-up for an industrial method for preparing chlorine : After passing a current through a saturated solution of sodium chloride for some time, chlorine was liberated at electrode P. When the mercury layer was run into cold water, an alkaline solution S and a colourless gas were obtained. (i) Which electrode was the cathode ? (ii) Name a suitable material for electrode P. Explain your choice. (iii) Write ionic equations for the reactions that occurred at (1) electrode P, and (2) the mercury electrode, (iv) Suggest a simple chemical test to show that the gas liberated at electrode P was chlorine and explain the principle involved. (v) (1) Write an equation to show the formation of solution S when the mercury layer was run into the cold water. (2) Briefly describe what would happen if chlorine were bubbled into the cold, dilute solution S, Write an equation for the reaction. (3) Suggest one common use for the reaction mixture obtained in (v) (2). (vi) Suggest two potential hazards in this experiment. (vii) How would the product obtained at electrode P differ if a dilute solution of sodium chloride were used instead of the saturated solution ? (13 marks) Bleach : 1986 4. (a) A saturated solution of chlorine can be obtained by bubbling chlorine into a beaker of water. (i) What is the colour of the solution ? (ii) What would be observed if a piece of blue litmus paper is dipped into the solution ? Explain with the aid of an appropriate equation. (iii) Using the set-up shown in the diagram below, the solution is left under strong sunlight for a long period of time. What would be observed ? Explain your answers with the aid of appropriate equations. (7 marks) Bleach : 1987
Bleach
Page 2
5. (c) (i) Draw a labelled diagram to show how you can prepare an aqueous solution of sulphur dioxide, starting from solid sodium sulphite. (ii) What would be observed when an excess of aqueous sulphur dioxide is added to iodine solution ? Write an appropriate equation for the reaction involved. (iii)Give an example of a reaction in which sulphur dioxide acts as an oxidizing agent. Indicate the change in oxidation number of sulphur in this reaction. (6 marks) Bleach : 1988 2. (b) Hydrogen, chlorine and aqueous sodium hydroxide can be obtained from the electrolysis of brine. They can then be used to prepare useful products as illustrated in the scheme below : (i) Explain the formation of aqueous sodium hydroxide from the electrolysis of brine. (ii) Vegetable oils are unsaturated compounds. (1) State the type of reaction involved when hydrogen reacts with vegetable oils to form margarine. (2) State ONE way in which margarine differs from vegetable oils. (iii) (1) Name product X. (2) Product X dissolves in water and methylbenzene to form two solutions which have different chemical properties. 1. Explain why they have different chemical properties. 2. Describe one test to illustrate how they differ. (iv) Write an equation for the reaction between chlorine gas and aqueous sodium hydroxide. State a major use of product Y in daily life. (v) Explain the cleaning action of soap with reference to the structure of soap, which can be represented as /\/\/\-CO2Na. (13 marks) Bleach : 1992 5. 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.
(a) (i) Explain which electrode, A or B, is the cathode. (ii) Using the concept of preferential discharge of ions, explain the electrode reactions and why sodium hydroxide can be manufactured by the above electrolysis.
Bleach
Page 3
(iii) If 234 g of sodium chloride are used up during the electrolysis, calculate the volume of hydrogen liberated at room temperature and pressure. (9 marks) (b) A domestic bleach can be prepared from the products obtained in the above electrolysis. (i) Write a balanced equation for the formation of the domestic bleach. (ii) Name the active ingredient in the domestic bleach. (iii) The following label is displayed on a container of the domestic bleach : Keep our of the reach of children Store in a cool place Keep away from acids (1) Why should the bleach be kept out of the reach of children ? Suggest TWO potential hazards. (2) Explain why the bleach should be (I) stored in a cool place. (II) kept away from acids. (Relative atomic masses : Na = 23.0; C1= 35.5; Molar volume of gas at room temperature and pressure = 24.0 dm3) (6 marks) Bleach : 1993 5. (b) The diagram below shows an apparatus, which, when it contains water, is part of the setup used in the preparation of chlorine gas.
(i) Copy the diagram into your answer book, Label clearly (1) the level of water that should be used in the apparatus. (2) the direction of the chlorine gas passing through the apparatus. (ii) Give TWO functions of the apparatus in the preparation of chlorine gas. (iii)The chlorine gas coming out from the apparatus is not dry. Draw a diagram showing how it can be dried and collected. Specify the drying agent used. (6 marks) Bleach : 1995 6. (a) The illustration below shows the plastic bottle of a domestic toilet cleaner and its label.
Bleach
Page 4
LIQUID TOILET CLEANER New formulation contains HC1 Caution : (1) Do not mix with bleaches or other chemicals. (2) Handle with care. (i) Explain, with the help of a chemical equation, why the toilet cleaner should not be mixed with bleaches. (ii) (1) Suggest ONE chemical, other than bleaches, that should not be mixed with the toilet cleaner. (2) If the chemical suggested in (1) and the toilet cleaner are mixed together, what change would be observed ? Write a chemical equation for the reaction involved. (iii) Explain why it is necessary to handle the toilet cleaner with care. (iv) (1) Explain why plastic is used for making the bottle for the toilet cleaner. (2) Name ONE plastic material suitable for making the bottle for the toilet cleaner. (9 marks)
8. (b) Small swimming pools usually use sodium hypochlorite solution rather than chlorine gas to sterilize the pool water. (i) Suggest ONE reason for using sodium hypochlorite solution rather than chlorine gas in small swimming pools. (ii) Suggest ONE chemical test to show the presence of hypochlorite ions in a sample of pool water. State the observable change in the test. (iii) A 100 cm3 sample of pool water contains 5.0 g of sodium hypochlorite. Calculate the concentration, in mol dm-3, of sodium hypochlorite in the sample. (iv) What would be observed if a petal of red flower was put into a sodium hypochlorite solution ? Using an equation, explain this observation. (v) Briefly describe how sodium hypochlorite solution can be produced industrially. (Relative atomic masses : O=16.0, Na=23.0, Cl=35.5) (l0 marks)
Bleach : 1997 8. (a) A class of students visited a chemical plant which manufactures chlorine by the electrolysis of brine. Some ,of the chlorine produced is used to make chlorine bleach. At the end of the visit, each student was given a bottle of chlorine bleach as a gift.
Bleach
Page 5
(i) Explain, in terms of preferential discharge of ions, how chlorine is produced in the electrochemical process. (ii) The students found some metal cylinders containing chlorine in the chemical plant. The students were told that these cylinders would be used in water treatment plants. (1) Which one of the following hazard warning labels should be displayed on the metal cylinders?
A B C D (2) Explain why chlorine is used in water treatment plants. (iii)Suggest ONE product, apart from chlorine and chlorine bleach, likely to be manufactured in the chemical plant. (iv) When the students returned to the school, their teacher asked them to carry out an experiment using the chlorine bleach as illustrated below. (1) Write the ionic equation for the reaction of dilute hydrochloric acid with the chlorine bleach. (2) State the observable change that would occur on the filter paper. Explain whether or not the change would involve a redox reaction.
(9 marks)
Bleach : 1998 3. Consider the following substances: Sodium benzoate, sodij chloride, sodium pypochlorite, Sodium hydrogencarbonate, sodium hydroxide, Sodium sulphite and monsodium glutamate (b) Which substance is commonly used as an active ingredient in toilet disinfectants? Briefly explain its action. (2 marks) 8. (a) A student added concentrated sulphuric acid dropwise to some sugar cubes as illustrated in the diagram below:
Bleach
Page 6
(i) (1) State the observable changes in the above experiment and write the chemical equation for the c o n c e n tra te d reaction involved. s u lp h u ric a c id (2) What property of concentrated sulphuric acid is responsible for the reaction? (ii) When there was no further change, the student added su g ar cu b es excess concentrated sulphuric acid to the reaction mixture. Upon heating, carbon dioxide, sulphur dioxide and water were produced. (1) Write the chemical equation for the reaction that occurred during heating. (2) What property of concentrated sulphuric is responsible for this reaction? (3) State ONE safety precaution that should be taken. Explain your answer. (iii) Draw ONE hazard warning label which should be displayed on a bottle of concentrated sulphuric acid. (9 marks)
Bleach : 1999 9. (a) In an experiment to prepare sulphur dioxide, a mixture of copper turnings and concentrated sulphuric acid was heated in a test tube as shown in the diagram below: (i) Write a chemical equation for the reaction that occurred in the test tube. (ii) Decide which of the following set-ups, (I), (II) or (III), should be connected to the delivery tube to collect the sulphur dioxide produced. Explain your decision.
(I)
(II)
c o n c e n tra te d s u lp h u ric a c id
d e liv e ry tu b e
copper tu rn in g s
H EAT
(III) g a s ja r
g a s ja r w a te r
(iii) (1) State and explain the observation when a piece of wet blue litmus paper is added to a gas jar of sulphur dioxide. (2) State the observation when a few drops of bromine water are added to a gas jar of sulphur dioxide. Write a chemical equation for the reaction that occurs. (8 marks) Bleach : 2000 6. (b)
Bleach
Page 7
In Hong Kong, the supply of fresh water mainly comes from Dong Jiang (東江). After some preliminary treatment of the river water in the water treatment plants, chlorine and calcium hydroxide are successively added. (i) Why are the following substances added to the river water? (1) chlorine (2) calcium hydroxide (ii) Chlorine can react with organic matter present in the river water to give trichlromethane which is harmful to human beings. (1) Draw the structure of trichloromethane. (2) Why is trichloromethane harmful to human beings? (3) Suggest ONE preventive measure to reduce the amount of organic matter in the river water. (5 marks)
Page 1
Bleach : 1985 2. (a) The diagram below represents a simplified set-up for an industrial method for preparing chlorine : After passing a current through a saturated solution of sodium chloride for some time, chlorine was liberated at electrode P. When the mercury layer was run into cold water, an alkaline solution S and a colourless gas were obtained. (i) Which electrode was the cathode ? (ii) Name a suitable material for electrode P. Explain your choice. (iii) Write ionic equations for the reactions that occurred at (1) electrode P, and (2) the mercury electrode, (iv) Suggest a simple chemical test to show that the gas liberated at electrode P was chlorine and explain the principle involved. (v) (1) Write an equation to show the formation of solution S when the mercury layer was run into the cold water. (2) Briefly describe what would happen if chlorine were bubbled into the cold, dilute solution S, Write an equation for the reaction. (3) Suggest one common use for the reaction mixture obtained in (v) (2). (vi) Suggest two potential hazards in this experiment. (vii) How would the product obtained at electrode P differ if a dilute solution of sodium chloride were used instead of the saturated solution ? (13 marks) Bleach : 1986 4. (a) A saturated solution of chlorine can be obtained by bubbling chlorine into a beaker of water. (i) What is the colour of the solution ? (ii) What would be observed if a piece of blue litmus paper is dipped into the solution ? Explain with the aid of an appropriate equation. (iii) Using the set-up shown in the diagram below, the solution is left under strong sunlight for a long period of time. What would be observed ? Explain your answers with the aid of appropriate equations. (7 marks) Bleach : 1987
Bleach
Page 2
5. (c) (i) Draw a labelled diagram to show how you can prepare an aqueous solution of sulphur dioxide, starting from solid sodium sulphite. (ii) What would be observed when an excess of aqueous sulphur dioxide is added to iodine solution ? Write an appropriate equation for the reaction involved. (iii)Give an example of a reaction in which sulphur dioxide acts as an oxidizing agent. Indicate the change in oxidation number of sulphur in this reaction. (6 marks) Bleach : 1988 2. (b) Hydrogen, chlorine and aqueous sodium hydroxide can be obtained from the electrolysis of brine. They can then be used to prepare useful products as illustrated in the scheme below : (i) Explain the formation of aqueous sodium hydroxide from the electrolysis of brine. (ii) Vegetable oils are unsaturated compounds. (1) State the type of reaction involved when hydrogen reacts with vegetable oils to form margarine. (2) State ONE way in which margarine differs from vegetable oils. (iii) (1) Name product X. (2) Product X dissolves in water and methylbenzene to form two solutions which have different chemical properties. 1. Explain why they have different chemical properties. 2. Describe one test to illustrate how they differ. (iv) Write an equation for the reaction between chlorine gas and aqueous sodium hydroxide. State a major use of product Y in daily life. (v) Explain the cleaning action of soap with reference to the structure of soap, which can be represented as /\/\/\-CO2Na. (13 marks) Bleach : 1992 5. 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.
(a) (i) Explain which electrode, A or B, is the cathode. (ii) Using the concept of preferential discharge of ions, explain the electrode reactions and why sodium hydroxide can be manufactured by the above electrolysis.
Bleach
Page 3
(iii) If 234 g of sodium chloride are used up during the electrolysis, calculate the volume of hydrogen liberated at room temperature and pressure. (9 marks) (b) A domestic bleach can be prepared from the products obtained in the above electrolysis. (i) Write a balanced equation for the formation of the domestic bleach. (ii) Name the active ingredient in the domestic bleach. (iii) The following label is displayed on a container of the domestic bleach : Keep our of the reach of children Store in a cool place Keep away from acids (1) Why should the bleach be kept out of the reach of children ? Suggest TWO potential hazards. (2) Explain why the bleach should be (I) stored in a cool place. (II) kept away from acids. (Relative atomic masses : Na = 23.0; C1= 35.5; Molar volume of gas at room temperature and pressure = 24.0 dm3) (6 marks) Bleach : 1993 5. (b) The diagram below shows an apparatus, which, when it contains water, is part of the setup used in the preparation of chlorine gas.
(i) Copy the diagram into your answer book, Label clearly (1) the level of water that should be used in the apparatus. (2) the direction of the chlorine gas passing through the apparatus. (ii) Give TWO functions of the apparatus in the preparation of chlorine gas. (iii)The chlorine gas coming out from the apparatus is not dry. Draw a diagram showing how it can be dried and collected. Specify the drying agent used. (6 marks) Bleach : 1995 6. (a) The illustration below shows the plastic bottle of a domestic toilet cleaner and its label.
Bleach
Page 4
LIQUID TOILET CLEANER New formulation contains HC1 Caution : (1) Do not mix with bleaches or other chemicals. (2) Handle with care. (i) Explain, with the help of a chemical equation, why the toilet cleaner should not be mixed with bleaches. (ii) (1) Suggest ONE chemical, other than bleaches, that should not be mixed with the toilet cleaner. (2) If the chemical suggested in (1) and the toilet cleaner are mixed together, what change would be observed ? Write a chemical equation for the reaction involved. (iii) Explain why it is necessary to handle the toilet cleaner with care. (iv) (1) Explain why plastic is used for making the bottle for the toilet cleaner. (2) Name ONE plastic material suitable for making the bottle for the toilet cleaner. (9 marks)
8. (b) Small swimming pools usually use sodium hypochlorite solution rather than chlorine gas to sterilize the pool water. (i) Suggest ONE reason for using sodium hypochlorite solution rather than chlorine gas in small swimming pools. (ii) Suggest ONE chemical test to show the presence of hypochlorite ions in a sample of pool water. State the observable change in the test. (iii) A 100 cm3 sample of pool water contains 5.0 g of sodium hypochlorite. Calculate the concentration, in mol dm-3, of sodium hypochlorite in the sample. (iv) What would be observed if a petal of red flower was put into a sodium hypochlorite solution ? Using an equation, explain this observation. (v) Briefly describe how sodium hypochlorite solution can be produced industrially. (Relative atomic masses : O=16.0, Na=23.0, Cl=35.5) (l0 marks)
Bleach : 1997 8. (a) A class of students visited a chemical plant which manufactures chlorine by the electrolysis of brine. Some ,of the chlorine produced is used to make chlorine bleach. At the end of the visit, each student was given a bottle of chlorine bleach as a gift.
Bleach
Page 5
(i) Explain, in terms of preferential discharge of ions, how chlorine is produced in the electrochemical process. (ii) The students found some metal cylinders containing chlorine in the chemical plant. The students were told that these cylinders would be used in water treatment plants. (1) Which one of the following hazard warning labels should be displayed on the metal cylinders?
A B C D (2) Explain why chlorine is used in water treatment plants. (iii)Suggest ONE product, apart from chlorine and chlorine bleach, likely to be manufactured in the chemical plant. (iv) When the students returned to the school, their teacher asked them to carry out an experiment using the chlorine bleach as illustrated below. (1) Write the ionic equation for the reaction of dilute hydrochloric acid with the chlorine bleach. (2) State the observable change that would occur on the filter paper. Explain whether or not the change would involve a redox reaction.
(9 marks)
Bleach : 1998 3. Consider the following substances: Sodium benzoate, sodij chloride, sodium pypochlorite, Sodium hydrogencarbonate, sodium hydroxide, Sodium sulphite and monsodium glutamate (b) Which substance is commonly used as an active ingredient in toilet disinfectants? Briefly explain its action. (2 marks) 8. (a) A student added concentrated sulphuric acid dropwise to some sugar cubes as illustrated in the diagram below:
Bleach
Page 6
(i) (1) State the observable changes in the above experiment and write the chemical equation for the c o n c e n tra te d reaction involved. s u lp h u ric a c id (2) What property of concentrated sulphuric acid is responsible for the reaction? (ii) When there was no further change, the student added su g ar cu b es excess concentrated sulphuric acid to the reaction mixture. Upon heating, carbon dioxide, sulphur dioxide and water were produced. (1) Write the chemical equation for the reaction that occurred during heating. (2) What property of concentrated sulphuric is responsible for this reaction? (3) State ONE safety precaution that should be taken. Explain your answer. (iii) Draw ONE hazard warning label which should be displayed on a bottle of concentrated sulphuric acid. (9 marks)
Bleach : 1999 9. (a) In an experiment to prepare sulphur dioxide, a mixture of copper turnings and concentrated sulphuric acid was heated in a test tube as shown in the diagram below: (i) Write a chemical equation for the reaction that occurred in the test tube. (ii) Decide which of the following set-ups, (I), (II) or (III), should be connected to the delivery tube to collect the sulphur dioxide produced. Explain your decision.
(I)
(II)
c o n c e n tra te d s u lp h u ric a c id
d e liv e ry tu b e
copper tu rn in g s
H EAT
(III) g a s ja r
g a s ja r w a te r
(iii) (1) State and explain the observation when a piece of wet blue litmus paper is added to a gas jar of sulphur dioxide. (2) State the observation when a few drops of bromine water are added to a gas jar of sulphur dioxide. Write a chemical equation for the reaction that occurs. (8 marks) Bleach : 2000 6. (b)
Bleach
Page 7
In Hong Kong, the supply of fresh water mainly comes from Dong Jiang (東江). After some preliminary treatment of the river water in the water treatment plants, chlorine and calcium hydroxide are successively added. (i) Why are the following substances added to the river water? (1) chlorine (2) calcium hydroxide (ii) Chlorine can react with organic matter present in the river water to give trichlromethane which is harmful to human beings. (1) Draw the structure of trichloromethane. (2) Why is trichloromethane harmful to human beings? (3) Suggest ONE preventive measure to reduce the amount of organic matter in the river water. (5 marks)
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