1999 Paper 1

Section A (Answer ALL questions.) 1.

Each of the tasks listed in the table below can be accomplished by using material A or B. Task

Materials

A (a) To make water pipes polyvinyl chloride (b) To make lenses perspex (c) To make shopping bags polyethene In each case, state an advantage of (i) using A over B to accomplish the task. (ii) using B over A to accomplish the task. (You are not required to consider the price of the materials.)

B iron glass paper

(6 marks) 1.

(a)

(b)

(c)

(i)

(ii) (i) (ii) (i) (ii)

PVC is more corrosion resistant than iron / can be more easily shaped / is chemisically inert (Do NOT accept PVC does not rust.) Iron is stronger / has higher tensile strength than PVC Perspex is not easily broken / lighter Glass cannot be easily scratched / has better light transmittion property Polyethene is more durable / has higher tensile strength / is waterproof Paper is biodegradable / causes less pollution problems when disposed of / is made from renewable materials / is air permeable (Do NOT accept recycle in (i) and in (ii)

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2.

For each of the following experiments, state ONE observable change and write a chemical equation for the reaction involved. (a) Dilute nitric acid is added to magnesium carbonate powder in a beaker. (b) A small piece of calcium is placed in a Bunsen flame. (c) A mixture of copper(II) oxide and carbon powder is heated in a test tube. (6 marks)

2.

When marking the observations, award 0 marks for contradictory answers. (a) Effervescence / (colourless) gas bubbles / magensium carbonate dissolves / heat evolves MgCO 3 + 2 HNO 3 → Mg( NO 3 ) 2 + H 2 O + CO 2 or, MgCO3 + 2 H + → Mg 2 + + H 2 O + CO 2

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(Do NOT accept 2 H + + CO 32 − → CO 2 + H 2 O ) (b)

(c)

Calcium burns with a red (brick red) flame / Formation of white powder (solid) 2Ca + O 2 → 2CaO reddish brown powder (solid) CuO + C → Cu + CO or, 2CuO + C → 2Cu + CO 2 (Do NOT accept evolution of gas)

3.

The illustration below shows the exhaust from a motor car using unleaded petrol:

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(a) (b) (c) (d) (e)

3.

(a) (b)

Explain why the exhaust contains carbon monoxide. (1) Write TWO chemical equations for the formation of acid rain from nitrogen oxides. (2) State ONE undesirable effect of acid rain. State ONE health hazard associated with particulates. Suggest ONE other pollutant that may be found in the exhaust. Suggest a device that can be installed in the motor car to reduce the emission of carbon monoxide and nitrogen oxides. (7 marks) Incomplete combustion of petrol / fuel (1) 2 NO + O 2 → 2 NO 2 2 NO 2 + H 2 O → HNO 2 + HNO 3 or 4 NO 2 + O 2 + 2 H 2 O → 4 HNO 3 Alternative answer: 2 NO 2 + H 2 O → HNO 2 + HNO 3 2 HNO 2 + O 2 → 2 HNO 3 or 4 NO 2 + O 2 + 2 H 2 O → 4 HNO 3 (2)

(c) (d) (e)

damages buildings / statues,

or, increase the rate of corrosion of metals / decrease crop yield / harmful to aquatic life (Accept any correct description of an undesirable effect of acid rain.) Irritates the respiratory system / causes lung cancer (Accept equivalent answer.) (Unburnt) hydrocarbons/ alkanes / sulphur dioxide (Do NOT accept carbon dioxide / lead compounds / dark smoke) Catalytic converter / catalytic oxidizer

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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.

With the help of electronic diagrams, describe the formation of magnesium chloride and tetrachloromethane from atoms of relevant elements. State, with explanation, which of the two compounds has a higher melting point. (9 marks)

4.

Chemical knowledge (6 marks) For MgCl2, each magnesium atom donates / loses (two) electrons and each chlorine atom accepts (one) electron to form an ionic compound. Cl 2+ Mg -

1

Cl 1 In CCl 4 , the carbon atom shares (a pair of) electrons with (each of the four) chlorine atoms to form a covalent compound.

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1 MgCl 2 has a higher melting point than CCl 4 because the attraction / van der Waals’ forces between molecules of CCl 4 is weak and the attraction between ions in MgCl 2 is strong. 1 5.

The diagram below shows a bottle of chemical waste in a school laboratory.

Describe and explain how you would remove kerosene and iron(III) ions from the chemical waste. (You may use any apparatus and chemicals available in a school laboratory.) (8 marks)

5.

Chemical knowledge (5 marks) Adding the liquid waste to a separating funnel / tap funnel. (Accept using dropper / pipette to remove kerosene. Do NOT accept decantation.) Separate the kerosene layer form the aqueous layer because kerosene and water are immiscible. (Also accept burn kerosene because it is combustible.) Add excess sodium hydroxide solution (any reagent that forms precipitate with Fe 3 + ions e.g. KOH(aq) / NH 3 (aq ) / H 2S( g ) / Na 2 HPO 4 (aq ) ) to the aqueous layer. (Also accept other method that can be used to remove iron(III) ions e.g. ionexchange / displacement) Iron(III) ions will form (reddish) brown precipitate / iron(III) hydroxide precipitate / (iron(III) sulphide / iron(III) phosphate precipitate.) (Award 1 mark for a correct explanation of the method used to remove iron(III) ions e.g. principle of ion-exchange/ displacement.) Filter off the iron(III) hydroxide precipitate (If ion-exchange is used, award 1 mark for the collection of the aqueous solution after passing through the ion-exchange column.

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Section B (Answer any THREE questions.) 6.

(a)

Water is a compound of hydrogen and oxygen. Under suitable conditions, 80.0 cm 3 of hydrogen and 60.0 cm 3 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)

6.

(a)

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)

(i)

(ii)

2H 2 + O 2 → 2H 2O or 2 mol of H 2 reacts with 1 mol of O 2

1 1 (1)

Equal no. of moles of all gases occupy the same volume under the same condition. ∴ 80 cm 3 of H 2 will react with 40 cm 3 of O 2 Volume of O 2 remaining at the end of the reaction = 60 - 40 = 20 cm 3

1

(0 marks for missing the unit.) (iii) (1) To increase the electrical conductivity of water / to provide mobile ions. (2) platinum / carbon (graphite) (3) 4OH − → 2 H 2 O + O 2 + 4e − 1 − − or 2OH → H 2 O + O 2 + 2e 2 (4)

(b) (i)

H 2 : burns with a ‘pop’ sound (For H 2 , do NOT accept using a glowing splint) O 2 : relights a glowing splint

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A teacher prepared an ethanol solution by fermentation of glucose using the following set-up.

(ii)

(1) Suggest what X may be. (2) Explain why the lime water turned milky during the fermentation process. (3) Write the chemical equation for the fermentation of glucose. The teacher used the ethanol solution obtained in (i) to carry out the following experiment on a redox reaction:

(1) (2)

State the observable change in the test tube. Explain, in terms of oxidation number, whether potassium dichromate was oxidized or reduced. (3) Give the structural formula of the product formed from ethanol in the reaction. (iii) Suggest ONE reason for each of the following statements: (1) Drinking a small quantity of wine may be good for health. (2) Excessive drinking of alcoholic beverages may cause health problems. (10 marks)

6.

(b)

(i)

(1)

(2)

(3) (ii)

(1) (2)

yeast / enzyme (Do NOT accept specific enzymes which cannot catalyse the fermentation of glucose.) Fermentation of glucose produces carbon dioxide which reacts with Ca (OH ) 2 in limewater to give insoluble calcium

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carbonate C 6 H 12 O 6 → 2C 2 H 5OH + 2CO 2

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Acidified potassium dichromate solution changes from orange to green O.N. of Cr in Cr2 O 7 2 − is +6 / VI

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1

1 O.N. of Cr in Cr

3+

is +3 / III

Cr2 O 7 2 − is reduced because O.N. of Cr decreases (3)

1

H

or CH 3CO 2 H

H C C H

1

O OH

(Also accept structural formula of ethanal, CH 3CHO ) (iii) (1)

(2)

7.

(a)

drinking a small quantity of wine can reduce the proneness to heart attack. (Also accept causing thinning of blood.) Excessive drinking can cause brain damage / depression / hepatitis / cirrhosis of the live / stomach ulcer / cancer of mouth, throat and gullet.

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The table below lists the key componenets of three alloys. Alloy Key components bronze copper, tin steel iron, carbon solder tin, metal X (i) Suggest an explanation for each of the following statements: (1) Although the amount of iron in the earth crust is greater than that of copper and tin, the Iron Age occurred after the Bronze Age. (2) During the Bronze Age, people used bronze instead of copper to make tools and weapons. (3) After being exposed to air for a period of time, the surface of bronze objects turns green. (ii) Steel is an alloy of iron and carbon. Explain, in terms of structure, why the addition of carbon to iron can enhance the strength of iron. (iii) Solder is an alloy of tin and metal X. (1) What is X? (2) Explain why solder, but not tin or X, is used for joining electrical wires. (9 marks)

7.

(a)

(i)

(1)

All three metals (Fe / Cu / Sn) occur in nature as cations. Iron is more reactive than copper (tin) / occupies a higher position in the reactivity series than copper (tin). ∴cations of iron are more resistant to reduction than those of copper (tin) / it is more difficult to reduce iron compounds than copper compounds (tin compounds) to the respective elements / iron compounds are more stable. (Accept other equivalent answers.) (2) Bronze is stronger / is more resistant to corrosion than copper (3) The copper in bronze is oxidized / forms copper(II) ions / basic copper(II) carbonate / copper(II) compounds / patina (ii) In iron, the ions(atoms) are regularly arranged. Adding carbon to iron causes a distortion (change) of the regular arrangement. This prevents the smooth sliding of atom layers over each other (iii) (1) Lead / Pb (2) Solder has a lower melting point

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(b) A student carried out the following experiment to determine the concentration of ethanoic acid in a brand of commercial vinegar. ‘25.0 cm 3 of the vinegar was placed in apparatus Y and was then diluted

to 250.0 cm 3 with distilled water. 25.0 cm 3 of the diluted vinegar solution was withdrawn and titrated against 0.10 M sodium hydroxide solution with phenolphthalein as indicator.’ (i) (1) Name apparatus Y. (2) Suggest ONE reason for diluting the vinegar. (ii) State the colour change at the end point of the titration. (iii) The student carried out four titrations altogether. The titration results are listed in the table below: Titration 1 2 3 4 Burette reading 23.90 23.60 23.10 23.60 Final reading/ cm 3 3.00 3.50 3.10 3.40 Initial reading/ cm 3 (1) Based on the titration results, calculate a reasonable average for the volume of the sodium hydroxide solution used. (2) Calculate the concentration, in moldm −3 , of ethanoic acid in the vinegar. (iv) Explain why vinegar can be used as a food preservative. (9 marks) 7.

(b)

(i)

(1) (2)

† Volumetric flask To save chemicals or, the concentration of the titrant should be comparable to that of the titrate (ii) From colourless to pink / purple / red (iii) (1) titration 1: volume used = 23.90 – 3.00 = 20.90 cm 3 3 titration 2: volume used = 23.60 – 3.50 = 20.10 cm titration 3: volume used = 23.10 – 3.10 = 20.00 cm

3

titration 4: volume used = 23.60 – 3.40 = 20.20 cm

3

The result of titration 1 is inaccurate and is discarded. The reasonable average for the volume of NaOH used

(Also accept 20.1 cm 3 ; 0 marks for missing out the unit.) CH 3CO 2 H + NaOH → CH 3CO 2 Na + H 2 O or, 1 mol of CH 3CO 2 H requires 1 mol of NaOH for complete neutralization concentration of CH 3CO 2 H in vinegar 0.1 × 20.1 = × 10 25.0 = 0.804 ( moldm

−3 )

(or 0.80 moldm

−3 )

(iv) The low pH of vinegar can prevent / inhibit the growth of bacteria / micro-organisms germs. (Also accept vinegar can kill bacteria.) 8.

(a)

(1) 1

1 (1) 1

= (20.10 + 20.00 + 20.20)/3 3 = 20.10 cm

(2)

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The diagram below shows the longitudinal section of a zinc-carbon cell.

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(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 NiO 2 + H 2 O → Ni(OH) 2 + OH − (9 marks) 8.

(a)

(i) (ii)

Zn → Zn 2 + + 2e − (1) (2)

as a conductor of electricity/ cathode/ positive pole/ electrode as an oxidizing agent/ to remove hydrogen produced (Part (2) of the question has been deleted) (iii) Warm the paste with NaOH(aq) Evolution of a gas (ammonia) which turns moist pH paper (red litmus) blue / gives white fumes with HCl(g) indicates the presence of NH 4 + (1 mark for using NaOH; 1 mark for evolution of gas; 1 mark for a correct chemical test for ammonia.) (iv) open-ended question: Yes, zinc-carbon cells will produce more (solid) wastes because zinccarbon cells are not rechargeable or, No, cadmium / cadmium compounds are toxic and disposal of NiCd cells can cause pollution problems (Award 1 mark for imprecise answer, such as Ni-Cd cells are toxic.) (v) Cd + 2OH − → Cd(OH ) 2 + 2e − 2e − + NiO 2 + 2 H 2 O → Ni(OH ) 2 + 2OH −

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(b) Manufacturing ammonia by the Haber process involves the following exothermic reaction: 2 NH 3 (g ) N 2 (g ) + 3H 2 (g ) (i) (1) Name a catalyst used in the process. (2) Suggest how the heat liberated in the reaction can be used in the process. (ii) A sufficient amount of hydrogen and 6.0 × 10 9 dm 3 of nitrogen, measured at room temperature and pressure, are allowed to react in the reaction chamber to give ammonia. Suppose that the conversion of nitrogen to ammonia is 15%. Calculate the mass of ammonia formed.

(iii) Upon reaction with a suitable chemical, ammonia gives a nitrogeneous fertilizer. Write a chemical equation for such a reaction. (iv) Most window cleaners contain ammonia solution as an active ingredient. (1) Explain why ammonia solution is used in window cleaners. (2) State ONE reason for using ammonia solution instead of sodium hydroxide solution in window cleaners. (Relative atomic masses: H = 1.0, N = 14.0; molar volume of gas at room temperature and pressure = 24.0 dm 3 ) (9 marks) 8.

(b)

(i)

(ii)

(1) (2)

†Iron / iron(III) oxide To heat up the reactant gases before passing into the reaction chamber. (Also accept heat exchanging) 6 × 10 9 No. of moles of N 2 = 24 = 2.5 × 108 No. of moles of NH 3 produced

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1

= 2 × 2.5 × 108 × 0.15 7 = 7.5 × 10

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Mass of NH 3 produced = (14 + 3) × 7.5 × 10 7 g 6 9 = 1.275 × 10 kg (or 1.275 × 10 g)

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(Also accept 1.3 × 10 6 kg and 1.28 × 10 6 kg; 0 marks for missing out the unit.) (iii) 2 NH 3 + H 2SO 4 → ( NH 4 ) 2 SO 4 (Accept any correct chemical equation for the formation of an ammonium salt from NH 3 ) (iv) (1)

(2)

9.

(a)

Ammonia solution is alkaline and can assist the hydrolysis of oily dirt / can react with oily dirt to form soluble compounds (soaps / detergents) Ammonia is volatile / will not leave stain on glass or, sodium hydroxide is corrosive to skin / can attack glass

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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) (ii)

Write a chemical equation for the reaction that occurred in the test tube. 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.

(iii) (1) (2)

9.

(a)

(i)

State and explain the observation when a piece of wet blue litmus paper is added to a gas jar of sulphur dioxide. 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)

Cu + 2 H 2SO 4 → CuSO 4 + 2 H 2 O + SO 2 +

or, Cu + H 2SO 4 + 2 H → Cu

2+

+ 2 H 2 O + SO 2

(ii)

(I) (II) is not used because SO 2 is denser than air (III) is not used because SO 2 is soluble in water (iii) (1) The blue litmus paper turns red because SO 2 is acidic (2) Bromine water changes from orange / brown / yellow / red to colourless SO 2 + Br2 + 2 H 2 O → 2 HBr + H 2SO 4

1 (1) 1 1 1 1+1

or, SO 32 − + Br2 + H 2 O → 2 Br − + SO 4 2 − + 2 H +

1 1 (1)

or, HSO 32 − + Br2 + H 2 O → 2 Br − + SO 4 2 − + 3H +

(1)

(b) Cracking of naphtha gives alkane X (relative molecular mass 44), alkene Y (relative molecular mass 42) and other products. (i) What is the meaning of the term ‘cracking’? (ii) Suggest a chemical test to distinguish between X and Y. (iii) Deduce the molecular formula of Y. (iv) Y can be used as a starting material for the production of plastic Z. (1) Write the chemical equation for the formation of Z from Y. (2) Suggest how plastic cups can be made from Z. (v) Suggest an advantage and a disadvantage of using plastic wastes as an energy source. (Relative atomic masses: H = 1.0, C = 12.0) (10 marks)

9.

(b)

(i)

Breaking down of large hydrocarbon (molecules) to small hydrocarbon (molecules) by heat and with the help of a catalyst or, Breaking down of heavy petroleum fractions (long chain carbon compounds) into light petroleum fractions (short chain carbon

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(ii)

compounds) (Accept equivalent answers.) Treat compounds with bromine in 1,1,1-trichloroethane / bromine water. Y can cause the bromine solution to change from brown / orange to colourless rapidly or, treat compounds with acidified KMnO 4 Only Y can cause the KMnO 4 solution to change from purple to

(1)

colourless or, treat compounds with neutral KMnO 4

(1) (1) (1)

Only Y can give a brown precipitate (Do NOT accept combustion.) (iii) General formula of alkene is C n H 2 n 12n + 2n = 42 n=3 Y is C 3 H 6

1 1 (1)

1 (1) 1

(iv) (1) CH3 n

H C C

H

H

CH3 H C

C

H

H

n

(2)

(v)

Apply heat to Z until it softens/ melts Compress molten Z to the shape of a cup in a mould and allow it to cool. Advantage: to reduce the consumption of non-renewable energy sources / fossils fuels (Also accept reduction of solid waste for landfill.) Disadvantage: Burning plastic wastes produces air pollutants / toxic gases or, the cost to remove the pollutants produced by burning plastic wastes is high. END OF PAPER

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