1998 Paper 1

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Section A (Answer ALL questions.) 1.

Lithium is a Group I element in the Periodic Table. It occurs naturally in two isotopic forms. The relative abundance of the each of these isotopes is shown in the table below: 6 7 Istope Li Li Relative abundance (%) 7.4 92.6 (a) What is the meaning of the term 'isotope'? (b) Calculate the relative atomic mass of lithium. (c) A piece of freshly cut lithium metal is placed in air. (i) What would be observed on the surface of the metal after some time? Write the relevent chemical equation. (ii) Draw the electronic diagram of the product in (i), showing electrons in the outermost shells only. (6 marks)

1.

(a) (b) (c)

Atoms with the same atomic number but different mass number/atoms with the same no. of protons but different no. of neutrons (Accept any correct definition of isotope.) Relative atomic mass = 6×0.074 + 7×0.926 = 6.93 (Also accept 6.9 and 6.926) (i) The metal surface will tarnish/turn dull 4 Li + O 2 → 2 Li 2 O (ii)

1 1 1 1 1

1 2.

For each of the following experiments, state the expected observation and write a relevent chemical equation. (a) Ethene is passed into an acidified potassium permanganate solution. (b) A mixture of butane and bromine vapour is exposed to diffused sunlight. (c) A sodium sulphite solution is added to an iodine solution (iodine dissolved in aqueous potassium iodide). (6 marks)

2.

(a)

(b)

(c)

3.

potassium permanganate solution changes from purple to colourless / purple colour of potassium permanganate fades. CH 2 = CH 2 + [O] + H 2 O → CH 2 (OH )CH 2 (OH ) or, 5CH 2 = CH 2 + 2 H 2 O + 2 MnO 4 − + 6H + → 5CH 2 (OH )CH 2 (OH ) + 2 Mn 2 + (Also accept equations showing further oxidation of ethane-1,2-diol to ethanedioic acid/carbon dioxide. DO NOT accept half equations.) Brown / red / orange colour of bromine fades slowly/changes to colourless slowly. or, Colourless oily droplets are formed. CH 3CH 2 CH 2 CH 3 + Br2 → CH 3CH 2 CHBrCH 3 + HBr or, C 4 H10 + Br2 → C 4 H 9 Br + HBr (Accept any equation showing substitution of butane by Br2 / equation showing further substitution.) Brown colour of iodine fades/turns colourless. SO 32 − + H 2 O + I 2 → 2I − + SO 4 2 − + 2 H + or, Na 2SO 3 + H 2 O + I 2 → 2 HI + Na 2SO 4

Consider the following substances: sodium benzoate, sodium chloride, sodium hypochlorite, sodium hydrogencarbonate, sodium hydroxide, sodium sulphite and monosodium glutamate

1 1 (1)

1 (1) 1 (1)

1 1 (1)

(a) (b) (c)

3.

(a) (b) (c)

Which substance is commonly used for preserving fish? Briefly explain its action. Which substance is commonly used as an active ingredient in toilet disinfectants? Briefly explain its action. Which substance is commonly used as an active ingredient in oven cleaners? Briefly explain its action. (7 marks) Sodium chloride It dehydrates / removes water from fish and makes the environment unfavourable for the growth of bacteria / germs / micro-organisms Sodium hypochlorite it can kill germs/bacteria or, it is toxic to germs/bacteria Sodium hydroxide it can hydrolyse / react with oil to give soap (which helps cleaning) / soluble products

1 1 1 1 (1) 1 1

For questions 4 and 5, candidates are required to give paragraph-length answers. 3 of the marks for each of these two questions will be awarded for effective communication of knowledge in Chemistry. 4.

On a hot summer day, the engine of a moving bus suddenly caught fire. When the firemen arrived, they used foam, instead of water, to put out the fire. After the fire was extinguished, the firemen sprayed a lot of water onto the hot engine. Discuss the conditions required for the fire to occur and explain the underlying principles of the actions taken by the firemen. (9 marks)

4.

Chemical Knowledge (6 marks) Conditions for the fire to occur diesel / lubricating oil / fuel used in the bus oxygen / air / oxidant the high temperature / heat of the bus engine Water cannot be used because it is immiscible with diesel / lubricating oil / fuel and will spread the fire or, diesel / lubricating oil / fuel floats on/is less dense than water and the fire will spread Foam can keep air away and the fire goes out A lot of water was sprayed onto the engine to remove heat / to lower the temperature, so as to eliminate the possibility of the re-ignition of diesel / lubricating oil / fuel.

3 (1) (1) (1) 1 (1) 1 1

5.

Each of five unlabelled bottles contains one of the following chemicals: 2 M hydrochloric acid 2 M nitric acid 2 M sodium chloride solution 2 M sodium hydroxide solution distilled water Suggest how you would carry out tests to identify the contents of each bottle, using the materials and apparatus listed below. Your answer should include the observation of each test. copper foil, solid copper(II) carbonate, 2 M copper(II) chloride solution, test tubes and a Bunsen burner (You are NOT required to write chemical equations. Answers in the form of flow diagrams will NOT be marked.) (8 marks)

5.

Chemical Knowledge (5 marks) (For this question, candidates may give answers in an order different from that illustrated below. In such cases, award one mark for each test which can identify / eliminate a substance in the list.) Add copper foil to each chemical (and heat). Only nitric acid can dissolve copper / gives a blue solution / brown fumes

1

Add copper(II) chloride solution to the remaining four chemicals. Only sodium hydroxide gives (blue) precipitate Add copper(II) carbonate to the remaining three chemicals. Only hydrochloric acid gives (colourless) gas bubbles / effervescence Evaporate the remaining two solution to dryness. Only sodium chloride solution will leave (white) residue The remaining one is distilled water

1 1 1 1

Section B (Answer any THREE questions.) 6.

(a)

(i)

A student prepared sodium nitrate solution by reacting 1 M sodium hydroxide solution with dilute nitric acid. The student carried out a titration to determine the amount of dilute nitric acid required to react with a known volume of 1 M sodium hydroxide solution. (1) Write the chemical equation for the reaction. (An ionic equation will NOT be accepted for this question.) (2) Draw a labelled diagram for the set-up of the titration. (3) Phenolphthalein can be used to determine the end point of the titration. State the colour change at the end point. (4) Suggest how the student can prepare a sodium nitrate solution using the titration results. (ii) Sodium nitrate is a nitrogenous fertilizer. (1) Calculate the percentage by mass of nitrogen in sodium nitrate. (2) Explain why nitrogen is essential for the growth of plants. (Relative atomic masses: N = 14.0, O = 16.0, Na = 23.0) (9 marks)

6.

(a)

(i)

(ii)

(1) (2)

NaOH + HNO 3 → NaNO 3 + H 2 O (1 mark for a diagram showing the set-up for the titration experiment; 2 marks for labelling the apparatus and reagents)

(Criteria for marking the labels: 2 marks for all four labels; 1 mark for three labels / two labels of which one should be a reagent.) (3) from pink / purple / red to colourless (4) Add dilute nitric acid to 1 M sodium hydroxide solution in the same volume ratio so that in the titration result, without adding the indicator or, repeat the titration procedure without adding the indicator (1) Formula mass of NaNO 3 = 23 + 14 + 16×3 = 85 14 % by mass of N = × 100% 85 = 16.5% (or 16.47%) (2) Nitrogen is used in plants to produce amino acids / proteins / cytoplasm / chlorophyll / nucleotide

1 1+2

1 1 (1) 1 (1)

1 1

(b)

The table below includes some information about three types of dry cells. The voltahge of each type of cell is 1.5 V. Type Voltage over Price per Shelf Life/minutes discharge cell/$ life/years Zinc-carbon cell falls quite 2.5 1.5 70 (AA size) rapidly Alkaline remains steady 5.0 3 90 manganese cell (AA size) Silver oxide cell remains steady 8.0 2 30 (button type) (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: Zn ( s) + 2OH − ( aq ) → ZnO ( s) + H 2 O ( l ) + 2e − A: Ag 2 O ( s) + H 2 O ( l ) + 2e − → 2Ag ( s) + 2OH − ( l ) B: (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) 6.

7.

(b)

(a)

(i)

Alkaline-manganese cell. Silver oxide cell is not used because it is not of the right size. Zinc-carbon cell is not used because its voltage drops quite rapidly. (Award 1 mark only for the explanation: alkaline-manganese cell is of the right size and has a steady voltage over discharge.) (ii) No. The shelf life of zinc-carbon cell is 1.5 y and only 18 pieces can be consumed/6 pieces will be wasted. The average price per cell used = $49.9/18 = $2.77 which is more expensive than the normal price of a zinc-carbon dry cell. or, the price for 18 zinc-carbon cells = $2.5×18 = $45 which is cheaper than the price of the package (iii) (1) Electrode A/zinc metal because an oxidation occurs (2) Zn + Ag 2 O → ZnO + 2 Ag (iv) (1) An explosion may occur or, Liberation of mercury/toxic substances (2) The zinc-carbon cell may leak and damage the electrical item.

1 1 1

1 1 (1) 1 1 1 (1) 1

Both carbon and silicon are Group IV elements in the Periodic Table. The diagram below show the structures of dry ice (solid carbon dioxide) and quartz (a form of silicon dioxide).

dry ice quartz With reference to the structures of the two substances, explain why quartz is a solid which melts at a high temperature, while carbon dioxide is a gas at room temperature. (ii) With the help of a labelled diagram, suggest how to show experimentally that dry ice sublimes to give gaseous carbon dioxide. (iii) Sand (an impure form of quartz) and limestone are raw materials used for making glass. (1) Name the main chemical constituent of limestone. (2) Suggest ONE reason why glass had been used by mankind for a long time. (3) Suggest ONE reason why glass bottles are preferred to plastic bottles for the storage of champagne. (9 marks) (i)

7.

(a)

(i)

(ii)

Quartz is a giant covalent network / covalent crystal. or, The attraction between (Si and O) atoms in quartz is covalent bond. Carbon dioxide has a simple molecular structure and the attraction between molecules is van der Waals’ forces Covalent bond is stronger than van der Waals’ forces / intermolecular force ∴Quartz is solid and CO 2 is gas diagram of a workable set-up

observation: lime water turns milky, no liquid residue remains (in the test tube) (iii) (1) †calcium carbonate (2) the materials for making glass are easily available / abundant in the earth crust. or, Glass can easily be manufactured by heating sand, limestone and sodium hydroxide. (3) champagne contains a pressurized carbon dioxide solution. Glass can withstand the pressure. or, the ethanol solution (champagne) can dissolve unpolymerized monomers in plastic (b)

1 (1) 1 1 1

1+1 1 1 (1) 1 (1)

Polyvinyl chloride (PVC) is a plastic which has a wide range of uses. (i) Write the chemical equation for the formation of PVC from its monomers. (ii) Plastic products made of PVC may vary greatly in rigidity. (1) Give ONE flexible product made of PVC. (2) Give ONE rigid product made of PVC. (3) Explain whether PVC is suitable for making electric sockets or not. (iii) Incineration of PVC wastes produces hydrogen chloride which causes air pollution. (1) State ONE harmful effect of the discharge of hydrogen chloride into the atmosphere.

(2) (3)

Suggest how hydrogen chloride can be removed from incinerator flue gas prior to its discharge to the atmosphere. Suppose that all the chlorine in PVC is converted to hydrogen chloride upon incineration. Calculate the volume of hydrogen chloride produced, measured at room temperature and pressure, when a plastic waste containing 1000 kg of PVC is incinerated.

(You may assume that no other chlorine-containing compounds are preseent in the waste.) (Relative atomic masses: H = 1.0, C = 12.0, Cl = 35.5; 3 molar volume of gas at room temperature and pressure = 24.0 dm ) (9 marks) 7.

8.

(b)

(a)

nCH 2 = CHCl → [CH 2 CHCl]n (For (1) and (2), accept other correct uses of PVC) (1) garment / surface of sofa / hose / cable sheathing / foot wear / tiles / curtains / rain coats / bottles (2) pipes / bottles / record (3) No. PVC is a thermoplastic / melts upon heating (iii) (1) acid rain / damage to the respiratory system (2) washing the flue gas with alkali / water or, passing flue gas through scrubber (3) No. of moles of HCl produced = no. of moles of PVC repeating units = 1000000/(12×2 + 1×3 + 35.5) = 16000 Volume of HCl produced = 16000×24 = 384000 dm 3 (i) (ii)

1

1 1 1 1 1 (1) 1 (1) (1) 1 1

A student added concentrated sulphuric acid dropwise to some sugar cubes as illustrated in the diagram below:

State the observable changes in the above experiment and write the chemical equation for the reaction involved. (2) What property of concentrated sulphuric acid is responsible for the reaction? (ii) When there was no further change, the student added 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 acid 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) 8.

(a)

(i)

(1)

(i)

(1)

(ii)

(2) (1)

sugar chars / turns black / brown misty fumes (steam) / heat is evolved C12 H 22 O11 → 12C + 11H 2 O dehydration / dehydrating C + 2 H 2SO 4 → CO 2 + 2SO 2 + 2 H 2 O

1 1 1 1 1

(2) (3)

oxidation / oxidizing The experiment should be carried out in a fume cupboard because the experiment produces sulphur dioxide which is a toxic gas or, wearing safety spectacles / safety goggles / face shield because concentrated sulphuric acid is corrosive (Award 1 mark only for wearing gloves because concentrated sulphuric acid is corrosive) (iii) hazard warning label: corrosive:

1 1 1 (1) (1)

1

(1) C O R R O S IV E

(The label should show the corrosive property of H 2SO 4 on skin and / or on materials such as metal or wooden surface.) or, oxidizing

(1)

O X ID IS IN G

(b)

The photograph below shows a can of fruit juice. The body of the can is made of iron coated with another metal. The top of the can and the ring-pull are made of aluminium.

(i)

(1) Suggest ONE reason why the iron body is coated with another metal. (2) Name ONE metal commonly used for coating the iron body. (ii) Suggest ONE reason why aluminium, rather than iron, is used for making the top of the can and the ring-pull. (iii) Explain why it is not advisable to buy cans of fruit juice (1) if the cans have scratches on the iron body; (2) if the cans are swollen. (iv) There is an increasing tendency for manufacturers to use cans made entirely of aluminium for the storage of fruit juice. Suggest ONE advantage and ONE disadvantage of using aluminium cans for the storage of fruit juice. (9 marks) 8.

(b)

(i)

(1) To prevent iron from rusting / corrosion (2) †Tin (ii) Al is softer than iron. The ring pull can be pulled off more easily. (iii) (1) Tin is less reactive / weaker reducing agent / occupies a lower position in the electrochemical series than iron. Iron exposed to air will corrode faster / rust faster. (2) Fruit juice in swollen cans has already deteriorated. Gas generated by (anaerobic) respiration of bacteria causes the can to swell.

1 1 1 1 1 1 1

(iv) Adantage: Al is lighter / is more resistant to corrosion than Fe / can be recycled more easily / can be dyed more easily Disadvantage: Al is more expensive / is not so strong as Fe 9.

(a)

1 1

A student used the following set-up to prepare propanoic acid:

water out X

water in

pumice stones

a mixture of propan-1-ol and acidified potassium dichromate solution

heat (i) Name apparatus X. (ii) Explain why some pumice stones were added to the reaction mixture before heating. (iii) Write the chemical equation for the reaction involved. (iv) Suggest a method to obtain propanoic acid from the reaction mixture. The student used the propanoic acid obtained to carry out the following experiment:

(v)

Why is a water bath, instead of a naked flame, used for heating the test tube and its contents? (vi) (1) State TWO observable changes when the contents of the test tube were added to the sodium carbonate solution. (2) Give the systematic name of the carbon compound formed in the experiment. (8 marks) 9.

(a)

(i) †condenser (ii) to prevent bumping / to ensure uniform heating / to smooth boiling (iii) CH 3CH 2 CH 2 OH + 2[O] → CH 3CH 2 CO 2 H + H 2 O or, 3CH 3CH 2 CH 2 OH + 2Cr2 O 7 2 − + 16H + → 3CH 3CH 2 CO 2 H + 4Cr 3 + + 11H 2 O O] or, CH 3CH 2 CH 2 OH [ → CH 3CH 2 CO 2 H + H 2 O (iv) (fractional) distillation (v) The methanol in the reaction mixture is flammable (vi) Any TWO of the following: (1) effervescence / gas bubbles two layers of liquids resulted pleasant / sweet smell is detected (2) †methyl propanoate

(b)

Each of the following experiment produces a sodium hydroxide solution.

1 1 1 (1) (1) 1 1 2 (1) (1) (1) 1

Action of sodium on water experiment I 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. 3 (2) Suppose that 50.0 cm 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) 9.

(b)

(i)

Any TWO of the following sodium melts into a silvery ball / dashes around on the surface of water / floats on surface of water sodium burns with a yellow flame colourless gas evolved (ii) Open-ended question Experiment II because sodium metal is highly reactive / it is dangerous to handle sodium metal / the reaction in experiment I is too violent and difficult to control or, Experiment I because a sodium hydroxide solution with high purity can be obtained (DO NOT accept explanation in terms of cost.) (iii) (1) Na occupies a higher position than H in the e.c.s. / H + is more readily reduced than Na + (2) At anode: 2Cl − → Cl 2 + 2e −

(3)

At cathode: 2 H + + 2e − → H 2 Equal number of moles of Cl 2 and H 2 will be liberated during electrolysis Under the same temperature and pressure, equal no. of moles of gases occupy the same volume. ∴Theoretical volume of chlorine liberated = 50.0 cm 3 After the removal of Cl − and H + , only Na + (aq ) and

OH − (aq ) ions remain in the solution. (iv) Labelled diagram of laboratory set-up (Marks will only be awarded to set-ups which are workable.)

(1 mark for labelling the two electrodes; 1 mark for showing the collection of gaseous products at the electrodes; 1 mark for

2 (1) (1) (1)

1 (1) 1 1 (1) 1 1 1 3

labelling the correct products.) END OF PAPER

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