1995 Paper 1

Section A (Answer ALL questions in this Section.) 1.

Rubidium (Rb) and potassium belong to the same group in the Periodic Table. The relative atomic mass of rubidium is larger than that of potassium. (a) Explain whether rubidium is more reactive than potassium. (b) Write a chemical equation for the reaction between rubidium and water. (State symbols should be given.) (c) Suggest how rubidium can be stored safely in the laboratory. (d) Suggest ONE safety precaution for handling rubidium in the laboratory. (5 marks)

1.

(a)

(b)

(c) (d)

Rb is more reactive than K because Rb can give away/donate/release its (outermost) electron more easily/readily. or, Rb can form ion/cation/positive ion more easily/readily. or, The strength of metallic bond between Rb atoms is weaker than that between K atoms/Rb can atomize more readily than K. (DO NOT accept Rb has one more electron shell than K/Rb atom is larger than K atom/Rb is below K in the Periodic Table.) 2 Rb( s) + 2 H 2 O(l) → 2 RbOH (aq ) + H 2 (g) + − or, 2 Rb( s) + 2 H 2 O(l) → 2 Rb (aq ) + 2OH (aq ) + H 2 (g) (Formulae of reactants and products must be correct; award 1 mark for a balanced equation and 1 mark for correct state symbols.) Store under oil/paraffin oil/kerosene Wear gloves/do not touch directly/use a pair of forceps/wear safety spectacles (goggles)/use a safety screen (DO NOT accept fume cupboard)

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2.

In each of the following groups of substances, there is ONE substance which is different from the others in terms of their properties. In each group, identify the substance which is different from the others and explain your choice. (a) argon, fluorine, helium, neon (b) nylon, perspex, polyethene, urea-methanal (c) milk of magnesia, soap, vinegar, window cleaner (d) carbon monoxide, hydrogen, methane, nitrogen (8 marks)

2.

(a)

(b) (c) (d)

fluorine fluorine is reactive/yellow/coloured gas or, others are inert/unreactive/stable/colourless (gases). (DO NOT accept noble gas/halogens.group VII (0) element) Urea-methanal It is a thermosetting plastic (cannot be softened by heat)/the other substances are thermoplastics (can be softened by heat) Vinegar It is acidic/the others are alkaline Nitrogen It cannot burn in air/the others can burn in air

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3.

Some of the ingredients present in a certain brand of cheese sticks are as follows: benzoic acid, corn meal, cheese, monosodium glutamate (MSG), sunset yellow (E110), vegetable oil Which of the above ingredients are food additives? Suggest ONE function for each of the food additives. (6 marks)

3.

Monosodium glutanate (MSG): flavour enhancer/to enhance (increase) flavour (taste) (DO NOT accept MSG is flavouring) Benzoic acid: preservative/toxic to (can kill) micro-organism (bacteria) Sunset yellow (E110): colouring (3 marks for the correct choice of 3 food additives, if more than 3 substances are chosen, deduct 1 mark for each wrong answer. 3 marks for the correct function of

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

"When atoms combine, they tend to attain noble gas electronic structures." Discuss how atoms can attain the noble gas electronic structures. In your answer, you should give suitable examples and the electronic structures of the products formed. (8 marks)

4.

Chemical knowledge (5 marks) When atoms of non-metals combine, they tend to share their (valence.outermost) electrons to form molecules. Electronic structure of a molecule e.g., HCl/ Cl 2 etc. (Any appropriate example) H

Cl

Cl

Cl

/ When metal and non-metal combine, atoms of the metal donate electrons (to atoms of non-metal) to form cations/positive ions/metallic ions, while atoms of the nonmetal accept electrons to form anions/negative ions/non-metallic ions. (or, award 1 mark for electron transfer, 1 mark for the formation of cation and anion) Electronic structure of an ionic compound e.g. NaCl etc. (Any appropriate example) + + Na

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Cl

Na

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Cl

or Effective communication (3 marks) General guidelines for marking effective communication: (i) The mark for effective communication should relate to the candidate's knowledge in chemistry. A candidates who wrote a paragraph which is totally unrelated to the question should deserve zero mark both in chemical knowledge and in effective communication. (ii) The table below illustrates the relationship between the mark for chemical knowledge and the maximum mark for effective communication. mark for chemical knowledge maximum mark for effective communication 4 or above 3 3 or below 2 In cases where the mark for effective communication is 3, points (A), (B) and (C) below will be marked. In cases where the mark for effective communication is 2 marks, only points (B) and (C) will be marked. (iii) The three marks for effective communication are awarded as follows: (A) the ability to present ideas in a precise manner, including the correct use of chemical terms (this mark should not be awarded to answers which contained a lot of incorrect/superfluous materials); (B) the ability to present ideas in a systematic manner (i.e., the answer is easy to follow); (C) the ability to present answer in paragraph form and to express ideas using full sentence.

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5.

Describe how large crystals of ammonium sulphate can be prepared from an aqueous solution of ammonia in a school laboratory. (9 marks)

5.

Chemical Knowledge (6 marks) Titrate ammonia solution with (dilute) sulphuric acid until the end-point is reached.

1+1 H SO (DO NOT accept concentrated 2 4 ) or, Add (dilute) sulphuric acid to ammonia solution in correct mole ratio (of 1:2) (1+1) Evaporate excess water/heat to obtain a saturated (more concentrated) solution. 1+1 Cool slowly/add small crystals of ammonium sulphate to the saturated solution to obtain large crystals of ammonium sulphate. 1 Filter/decant the saturated solution to obtain the crystals. 1 or, hang a small crystal of ammonium sulphate in the saturated solution as seed to (1) obtain large crystals. (1) Effective communication (3 marks) General guidelines for marking effective communication: (i) The mark for effective communication should relate to the candidate's knowledge in chemistry. A candidates who wrote a paragraph which is totally unrelated to the question should deserve zero mark both in chemical knowledge and in effective communication. (ii) The table below illustrates the relationship between the mark for chemical knowledge and the maximum mark for effective communication. mark for chemical knowledge maximum mark for effective communication 4 or above 3 3 or below 2 In cases where the mark for effective communication is 3, points (A), (B) and (C) below will be marked. In cases where the mark for effective communication is 2 marks, only points (B) and (C) will be marked. (iii) The three marks for effective communication are awarded as follows: (A) the ability to present ideas in a precise manner, including the correct use of chemical terms (this mark should not be awarded to answers which contained a lot of incorrect/superfluous materials); 1 (B) the ability to present ideas in a systematic manner (i.e., the answer is easy to follow); 1 (C) the ability to present answer in paragraph form and to express ideas using full sentence. 1 END OF SECTION A Section B (Answer any THREE questions.) 6.

(a)

The illustration below shows the plastic bottle of a domestic toilet cleaner and its label.

(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 (l) 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)

6.

(a)

(i)

Name ONE plastic material suitable for making the bottle for the toilet cleaner. (9 marks) +

The H ions/acid (in the toilet cleaner) will react with the bleach ( ClO − and Cl − ) to give chlorine which is poisonous/toxic. Cl − + ClO − + 2 H + → Cl 2 + H 2 O or, NaOCl + 2 HCl → NaCl + H 2 O + Cl 2 − (DO NOT accept OCl + 2 HCl → H 2 O + Cl 2

2− + or, H ions will react with the bleach ( SO 3 in SO 2 bleaches) to give SO 2 which is poisonous/toxic.

(ii)

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SO 32 − + 2 H + → SO 2 + H 2 O chemical: metals such as Fe/Zn...(do not accept Pb, Cu, etc.) change: gas evolves/effervescence occurs/metal dissolves + 2+ equation: Fe + 2 H → Fe + H 2

(1)

or,

(1) (2)

chemical: (strong) alkalis/NaOH/KOH/caustic change: heat evolves/spillage of the alkali + − equation: H + OH → H 2 O

(1) (1) (1)

or,

(1)

chemical: carbonate/hydrogencarbonate change: effervescence/ CO 2 gas evolves

(1) (1)

(1) (2)

(2)

+ 2− equation: CO 3 + 2 H → H 2 O + CO2 + − / HCO 3 + H → H 2 O + CO2

(Accept full or ionic equation; equation should match the chemical suggested.) (iii) hydrochloric acid is corrosive/irritating/can cause acid burn DO NOT accept HCl is harmful) (iv) (1) it is inert/does not react with HCl/the bottle is not easily broken/flexible/light in weight/can be moulded etc. (2) †polyethene/polythene/polypropene/polystyrene/polyvinyl chloride etc. (DO NOT accept short forms e.g., PE, PP, PS...) (DO NOT accpet perspex/name of any thermosetting plastics) †correct spelling is required.

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(1)

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(b) The table below gives some information about five metals. Metal Abundance in Price per kg Relative resistance of Relative strength of the earth's ($) corrosion metal crust (%) (1 = least resistant (1 = lowest 4 = most resistant) 3 = highest) Al 8.1 170 3 1 Cu 0.0055 140 3 3 Au 0.0000004 1100000 4 2 Fe 5.0 20 1 3 Zn 0.007 160 2 2 (i) Although gold has a very low abundance in the earth's crust, gold was discovered by man a long time ago. Why? (ii) Which of the metals in the above table is the most suitable to make pipes for hot water? Explain your answer. (iii) (1) Aluminium does not corrode easily. Why? (2) Aluminium is a principal material for making aircraft but its strength is relatively low. Suggest how the strength of aluminium can be improved to make it suitable for making aircraft. (iv) (1) Based on the information given in the table, suggest ONE factor that affects the price of a metal.

(2)

6.

(b)

7.

(a)

7.

(a)

Suggest ONE other factor (not indicated in the table) that can also affect the price of a metal. (9 marks)

(i)

Gold is very unreactive/inert/is stable/does not combine with other elements easily/can be found free in nature. (ii) Copper/Cu because it does not corrode easily and has a high metallic strength/is relatively cheap (Award mark for explanation only if Cu is chosen.) (iii) (1) Al reacts with oxygen in air to form a (thin) layer of oxide which is impervious (not permeable) to oxygen/water and prevents the metal from further corrosion. (2) Alloying (with other metals e.g. Cu/Mn/Mg...) (DO NOT accept mix with other metal/alloying with Na, K...) (iv) (1) the price depends on its abundance in the earth's crust. (2) Cost of extraction/cost in mining/supply/demand of the metal.

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The label on a bottle of 'Effervescent Calcium' tablets is shown below. Effervescent Calcium Each bottle contains 10 tablets, Each tablet contains: Calcium carbonate 625 mg Vitamin C 1000 mg Citric acid 1350 mg Dosage: 1 tablet daily Administration: Dissolve one tablet in a glass of water. Warning: (1) Keep out of reach of children. (2) Keep ................................... (i) Effervescence occurs when a tablet of 'Effervescent Calcium' is added to water. Based on the information given on the label, explain why effervescence occurs. Write the ionic equation for the reaction that occurs. (ii) Suppose that a student puts a tablet of 'Effervescent Calcium' into an excess amount of water and collects the gas liberated. (1) Assuming that the tablet completely dissolves, calculate the theoretical volume of gas liberated. (2) It is found that the volume of gas collected in the experiment is less than the theoretical volume calculated in (1). Give ONE reason to explain the difference, assuming that there is no leakage of gas in the experiment. (iii) On the label, some words are missing in the second warning statement. Complete the second warning statement, beginning with the word 'Keep'. Explain your answer. (Relative atomic masses: C = 12.0, O = 16.0, Ca = 40.0; 3 Molar volume of gas at room temperature and pressure = 24.0 dm ) (9 marks) (i)

(ii)

Citric acid/vitamin C (ascorbic acid) when dissolves in water gives H + (aq ) which reacts with calcium carbonate to give gas ( CO 2 ) bubbles. CaCO3 + 2 H + → Ca 2 + + CO 2 + H 2 O

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+ 2− (DO NOT accept CO 3 + 2 H → CO 2 + H 2 O (1) Formula mass of CaCO3 = 40 + 12 + 16×3 = 100

1 mole of CaCO3 gives 1 mole of CO 2 No. of moles of CO 2 evolved = No. of moles of CaCO3 present 625 × 10−3 = 100 = 6.25 × 10−3

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−3 3 Theoretical volume of gas = 6.25 × 10 × 24 dm 3 3 = 150 cm (0.15 dm ) (Deduct 1 mark for wrong/no units) (2) Some of the CO 2 produced dissolved in water/ CO 2 is (fairly) soluble in water (iii) (Keep) out of moisture (water)/(keep) in a dry place Reason: The amount of active ingredients will decrease/the tablet will lose function/the active ingredients of the tablet will react in the presence of water. or, (Keep) out of heat/(keep) in a cool place Reason: at high temperatures, vitamin C deteriorates/ CaCO3

undergoes decomposition/the amount of active ingredients will decrease/the tablet will lose function. or, (Keep) away from sunlight Reason: vitamin C may decompose CaCO3 can be decomposed by sunlight

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(The warning statement and the reason must match each other.

7.

(b)

The following flow diagram shows the conversion of a compound X to an acid Y. oxidation X Ethanol Y X can rapidly decolourize a solution of bromine in 1,1,1-trichloroethane. (i) What is X? Name the industrial process by which X is converted to ethanol. (ii) Write a chemical equation for the reaction between X and bromine. (iii) (1) Give the systematic names of Y. (2) Draw a labelled diagram of the laboratory set-up for the conversion of ethanol to Y. (iv) Ethanol can be detected in the breath of a drunken driver. Suggest ONE chemical test to show the presence of ethanol in his breath and state the observable change produced by the test. (9 marks)

(b)

(i) (ii)

ethene/ CH 2 = CH 2 / C2 H 4 †catalytic hydration CH 2 = CH 2 + Br2 → CH 2 BrCH 2 Br or, C2 H 4 + Br2 → CH 2 BrCH 2 Br

(iii) (1) (2)

†Ethanoic acid

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3 or (If more than 1 diagram is drawn, only the first diagram will be marked.) deduct 1 mark for wrong reagents/incomplete indication of reagents. deduct 1 mark for closed/open system deduct 1 mark for wrong/no indication of direction of water flow deduct 1 mark for no indication of heat (maximum mark deducted = 3) (iv) pass the breath into acidified potassium dichromate (solution). The colour of the solution will change from orange to green. †correct spelling is required

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8.

(a)

The fumes emitted from a factory using diesel fuel contain several gaseous pollutants. One of these pollutants, Z, has a choking smell and can decolourizes bromine water. (i) (1) What is Z? (2) What is the effect of Z on the environment? (3) Suggest ONE way reduce the amount of Z in the fumes. (ii) (1) Suggest ONE other pollutant that is present in the fumes. (2) Explain how this pollutant is formed. (3) What is the effect of this pollutant on the environment? (4) Suggest ONE way to reduce the amount of this pollutant in the fumes. (iii) If a fire is caused by the burning of diesel fuel, what type of fire extinguisher should not be used to put out the fire? (8 marks)

8.

(a)

(i)

(1) (2) (3)

(ii)

(1) (2)

(3)

(4)

Sulphur dioxide/ SO 2 attacks respiratory system/produces acid rain/causes smog/toxic (DO NOT accept harmful/air pollutant) installation of scrubbers (pass fumes through alkalis)/use fuel of low sulphur content carbon monoxide/CO/particulate/carbon particles/nitrogen oxides/ NO x /unburnt hydrocarbons CO/carbon particles (particulates)/unburnt HCs: incomplete combustion (of fuel) NO x : combination of N 2 and O 2 at high temperature CO: poisonous/toxic or, unburnt HCs/carbon particles: causes smog or, unburnt HCs/particulates: carcinogenic/causes smog or, NO x : poisonous/toxic/produces acid rain/(photochemical) smog CO/C particles/unburnt HCs: ensure that there is sufficient supply of air during combustion of fuel/installation of catalytic converter. or, particulates: installation of electrostatic precipitator/bag filtration/centrifugation or, NO x : installation of scrubber/catalytic converter

(iii) water type fire extinguisher (Award 0 mark if fire extinguisher other than water type is given) (b)

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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 pool. (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. 3 (iii) A 100 cm sample of pool water contains 5.0 g of sodium hypochlorite.

−3 Calculate the concentration, in moldm , 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) (10 marks)

8.

(b)

(i)

(ii)

sodium hypochlorite solution is easy to handle/leakage of toxic chlorine gas may require the evacuation of people living nearby/chlorine is stored in heavy steel cylinder which is difficult to transport. Test with litmus paper (solution)/dye/pH paper, the colour of the indicator/dye will be bleached

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(DO NOT accpet blue litmus turn red and then white.) or, Add dilute acid to the sample, greenish yellow chlorine gas will be given off. or, add some pool water to a test tube containing potassium iodide solution, brown colour of iodine will appear (iii) Formula mass of NaOCl = 23 + 16 + 35.5 = 74.5 Molarity of sodium hypochlorite = (5 ÷ 74.5) × 10 = 0.67 (M) (Accept also 0.7 M, 0.671 M, 0.6711 M) (iv) the red colour of the petal is bleached/faded/turn pink/turn white/decolourized OCl − + dye → Cl − + ( dye + O) or, NaOCl + dye → NaCl + ( dye + O) (v)

9.

(a)

(DO NOT accept HOCl + dye → HCl + (dye + O) Electrolysis of brine/concentrated solution of NaCl, produces chlorine gas (at the anode) and NaOH solution (from the electrolysis). Allow chlorine and NaOH to react to give NaOCl.

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Sodium hydroxide can be used as a raw material in the manufacture of both soapy and soapless detergents. (i) Briefly describe how a soapy detergent can be prepared from a vegetable oil in a school laboratory. (ii) The formula of a certain soapy detergent is C n H 2 n +1COONa and its formula mass is between 300 and 310. Calculate the value of n. (iii) The structure of a certain soapless detergent is shown below:

CH3

CH3 CH3 CH3 CH3 H H H C C C C + C C C SO3 Na H H H H CH3 CH3 CH3

(1)

What other raw materials, apart from sodium hydroxide, are required in the manufacture of this soapless detergent? (2) Give ONE advantage and ONE disadvantage of using this soapless detergent for domestic cleaning compared with using a soapy detergent. (Relative atomic masses: H = 1.0, C = 12.0, O = 16.0, Na = 23.0) (10 marks) 9.

(a)

(b)

(i)

Heat/boil vegetable oil with sodium hydroxide solution. Add concentrated NaCl solution/salt solution/brine/salt out the soap. Separate (filter) the soap from the solution. (ii) Formula mass of the soap = 12(n+1) + (2n+1) + 2×16 + 23 = 14n + 68 300 < 14n + 68 < 310 16.6 < n < 17.3 ∴ n = 17 (iii) (1) petroleum (fraction) (concentrated) sulphuric acid/oleum (DO NOT accept dilute sulphuric acid) (2) advantage: the soapless detergent can be used in hard water/acidic solution. (DO NOT accept can be used in sea water.) disadvantage: the soapless detergent is non-biodegradable/may causes water pollution/may cause skin allergies/can kill marine lives

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1

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) 9.

(b)

(i)

(ii)

MnO 4 − + 8H + + 5e − → Mn 2 + +4 H 2 O

MnO 4 − is reduced because it receives electrons/the oxidation number of Mn changes from +7 to +2/the oxidation number of Mn decreases The solution turns (pale) brown/yellow. (DO NOT accept iodine/brown solid/violet (gas) is formed.) 2I − → I 2 + 2e −

1

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− − or, 2I − 2e → I 2

(iii) From KI solution to KMnO 4 solution/from RHS to LHS/counterclockwise (DO NOT accept from -ve carbon electrode to +ve carbon electrode) (iv) 2 MnO − + 16H + + 10I − → 2 Mn 2 + +8H O + 5I 4 2 2 (v) (1) To allow migration of ions between the two beakers. (Accept to complete the circuit.) (DO NOT accept migration of electrons/conduction of electricity) (2) No. Sodium sulphite can be oxidized/react with permanganate ions. END OF PAPER

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