2003 Paper 1 Marking Scheme

Section A 1.

(a)

(b) or, or,

(i) carbon dioxide / CO2 (ii) sodium chloride / NaCl (iii) neon / Ne hydrogen chloride / HCl HCl + H2O → H3O+ + ClHCl(aq) → H+(aq) + Cl-(aq)

Marks 1 1 1 1 1 (1)

carbon dioxide / CO2 CO2(aq) + H2O(l) → HCO3 -(aq) + H+(aq)

(1) (1)

(For each of the substances chosen, marks will be awarded only if the spelling and / or the formula (symbol) are correct.)

2.

(a)

Hydrogen / H2

(b) (c)

It burns with a ‘pop’ sound. redox / reduction-oxidation reaction Reactivity：ZY>Z

(d)

Y is more reactive than Z as Y can displace Cu from CuSO4(aq) but Z cannot. X is more reactive than Y as X can react with cold water but Y cannot / oxide of X cannot be reduced by carbon but oxide of Y can. X is a reactive metal. It reacts with water in the copper(II) sulphate solution and the colourless gas liberated is hydrogen.

___ (5) 1 1 1 1 1 1

1 ___ (7)

correct spelling 3.

(a)

(i)

1

(ii)

1

(b)

(All electrons must be shown in the electronic diagrams.) (i) Two possible methods： Method 1 Mass of Cl per formula unit = 135.2 × 0.525 = 70.98 Mass of S per formula unit = 64.22 No. of Cl atom per formula unit = 70.98 / 35.5 = 2.0 No. of S atom per formula unit = 64.22 / 32.1 = 2.0 Molecular formula：S2Cl2

1*

1*

1

Method 2 (1)* S Cl mole ratio 47.5 / 32.1 ： 52.5 / 35.5 ： = 1.48 1.48 ： = 1 1 Molecular formula = (SCl)x x = 135.2 / (32.1 + 35.5) =2 Molecular formula：S2Cl2 (ii)

(1)* (1) 1

___ (6) * step mark 4.

Chemical Knowledge (6 marks) 2 Similarities in chemical properties： Sulphuric acid as an acid – H2SO4 ionises in water to give H3O+(aq) ions (Both conc. H2SO4 and dil. H2SO4 contain H3O+(aq) ions) Examples：reaction with alkali (base) to give salt and water only (neutralization) / reaction with carbonates (hydrogencarbonates) to give

(1) (1)

carbon dioxide / action on acid-base indicator, etc. e.g. H2SO4 + NaOH → Na2SO4 + 2H2O H2SO4 + Na2CO3 → Na2SO4 + H2O + CO2 Turns blue litmus red (or, action on other indicators) Differences in chemical properties： 4 Notes：Differences in chemical properties cover four areas. Candidates are required to write on any two of these areas. A maximum of 2 marks will be awarded to each area. The oxidizing power of conc. H2SO4 is much stronger than that of dilute

(1)

H2SO4 Examples：conc. H2SO4 can oxidise metal / non-metal / compounds. It is

(1)

commonly reduced to SO2 e.g. Cu + 2H2SO4 → CuSO4 + SO2 + 2H2O C + 2H2SO4 → CO2 + 2SO2 + 2H2O 2HBr + H2SO4 → Br2 + 2H2O + SO2 Conc. H2SO4 can act as a dehydrating agent but dil. H2SO4 cannot. Examples：conc. H2SO4 can dehydrate CuSO4.5H2O / sugar

(1) (1)

Conc. H2SO4 is a non-volatile acid but dil. H2SO4 is not. (1) Example：conc. H2SO4 is used in the preparation of hydrochloric acid and (1) nitric acid. e.g. H2SO4 + NaCl → NaHSO4 + HCl H2SO4 + NaNO3 → NaHSO4 + HNO3 (drying 唔收!) Conc. H2SO4 has a high affinity for water Dilution of conc. H2SO4 is highly exothermic.

(1) (1) (6)

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 candidate who wrote a paragraph which is totally unrelated to the question should deserve zero marks 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, together

with the points to be considered in marking effective communication. Mark for chemical maximum mark for effective points to be considered knowledge communication in paragraph (iii) below 4 or above

3

(A), (B) and (C)

3 or below 2 (B) and (C) (iii) The three marks for effective communication are awarded as follows：

5.

(A) the ability to present ideas in a precise manner, including the proper 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 sentences.

1 ___ (3)

Chemical Knowledge (6 marks) Possible ways of treating plastic wastes： (Candidates are required to write on any three of the following. In each case, 1 mark for an advantage and 1 mark for a disadvantage.) Incineration (2) Advantage：operation cost is low / volume of solid waste can be greatly reduced / energy can be recycled / reduce / land wastage Disadvantage：incineration produces toxic gases / the cost of operating a controlled incineration plant is high / air pollution Recycling (2) Advantage：save materials / plastic wastes can be converted to useful products Disadvantage：the cost of operating a recycling plant is high / separation of the different types of plastics in the waste is costly / low quality plastics are produced by melting and remoulding plastic wastes Landfilling (2) Advantage：does not cause much air pollution / a lot of plastic waste can be treated in a short period of time Disadvantage：land wastage / it takes a long time for plastic wastes to degrade / may cause pollution of underground water / slow release of toxins from landfill sites Pyrolysis (2) Advantage：save materials / useful products (e.g. methane, ethane) can be obtained

Disadvantage：requires a lot of energy

___ (6)

(reuse / reduce 不接受) (depolymerization no mark) 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 candidate who wrote a paragraph which is totally unrelated to the question should deserve zero marks 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, together with the points to be considered in marking effective communication. Mark for chemical maximum mark for effective points to be considered knowledge communication in paragraph (iii) below 4 or above

3

(A), (B) and (C)

3 or below 2 (B) and (C) (iii) The three marks for effective communication are awarded as follows：

6.

(a)

(A) the ability to present ideas in a precise manner, including the proper 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 sentences.

1 ___ (3)

(i)

1

(ii)

2NH4Cl + CaO → CaCL2 + H2O + 2NH3

The water vapour produced will condense near the mouth of the test tube. The test tube will crack when the cold water flows back to the hot test tube. (sucking back no mark) (iii) (II) Ammonia is less dense than air And is very soluble in water

1 1

1 1 1

☆(I) × but NH3 solution in water ! 1 ☆(II) ×× (iv) Formula mass of NH4Cl = 14 + 4 + 35.5 or no. of mole × 24 = 53.5 Theoretical volume of NH3(g) = 1 / 53.5 × 24 3

1 3

= 0.45 dm (450 cm )

1* 1

(wrong unit answer no mark) (Acceptable range：0.44 dm3 to 0.46 dm3)

(b)

(i)

(2-5 sig. fig.)

___ (9)

The reaction is reversible.

1 1

(ii) as a catalyst / to speed up the reaction (iii) No. Air contains (about 20% of) oxygen A mixture of oxygen and hydrogen is explosive. or,

At elevated temperatures, N2 reacts with O2 to give nitrogen oxides. This makes the purification of NH3 complicated. (Accept other reasonable answers.) (iv) Separate NH3 from the gas mixture. Pass the unreacted N2 and H2 back into the reaction chamber

(c)

(i)

(ii)

Yes O.N. of Cu：decreases from +2 to 0 O.N. of N ：increases from –3 to 0 3CuO + 2NH3 → 3Cu + 3H2O + N2

1 1 (1)

1 1 (6) 1+1 (1+1) 1 (3)

* step mark 7. (a) (i)

Brown / orange / red fumes evolved(gas / vapour / bubbles) 2Br- → Br2 + 2e-

(liquid no mark) (ii) Bromine / lead(II) bromide / lead is toxic / harmful (Accept bromine vapour is corrosive.) (iii) The light bulb gradually goes out / becomes dim. At lower temperatures, movement of ions slows down. ∴ A smaller current flows through the external circuit.

1 1 1 1 1

When molten lead(II) bromide becomes solid, there is no translational motion of ions / ions are no longer mobile.∴no

1

current flow through the external circuit.

___

(b)

(i) (ii)

cracking – breaking down of large (hydrocarbon) molecules into smaller ones. Cracking can help produce extra petrol / LPG Taxi / Domestic Fuel which is used as fuel for motor vehicles

(6) 1 1,1

or,

Cracking produces unsaturated hydrocarbons which can be converted to other useful organic compounds

(1) (1)

or,

Cracking can produce extra petrol and unsaturated hydrocarbons

(1,1)

(iii)

3

(gas jar no mark) (1 mark for the set-up used for cracking octane; 1 mark for collection of gaseous product; 1 mark for the labels of an appropriate catalyst and heat.) (iv) (1) C8H18 → C4H8 + C4H10 1

(c)

(2)

Treat compounds with Br2 in CH3CCl3 (or Br2 in an 1 appropriate solvent) The unsaturated HC readily turns Br2 in CH3CCl3 from 1 brown / orange to colourless.

or,

Treat compounds with acidified KMnO4 (1) The unsaturated HC turns acidified KMnO4 from purple (1) to colourless.

or,

Treat compounds with neutral KMnO4 The unsaturated HC gives a brown ppt. (K2Cr2O7 / H+ no mark) (Alkaline KMnO4 no mark) (burning no mark) (Accept other tests for unsaturated hydrocarbons.)

Sea water is a solution while muddy water is a suspension. The size of ions in sea water is much smaller than that of the

(1) (1)

___ (9) 1 1

mud particles in muddy water. Ions in sea water can pass through pores in filter device (filter paper / filter bed etc.) while mud particles cannot. (NaCl is soluble, mub is insoluble have 1 mark) (correct comparison have 1 mark) (Accept equivalent answers.) 8.

(a)

(i)

(1)

glucose / fructose C6H12O6 → 2C2H5OH + 2CO2

___ (3) 1 1

or,

sucrose C12H22O11 + H2O → 4C2H5OH + 4CO2

(1) (1)

(2)

yeast 1 Yeast provides enzymes (zymase) / organic catalysts for 1 the fermentation of glucose (fructose / sucrose).

or, Yeast carries out anaerobic respiration to give ethanol. (ii) Prevent oxygen (air) from entering the jar / prevent the ethanol produced from being oxidised. / entrance of O2 / bacteria Prevent building up of pressure in the jar. (iii) (1) When the concentration of ethanol exceeds 18%, the yeast will not function / die and fermentation stops. (2) distillation / fractional distillation (iv) Ethanol is the wine undergoes oxidation to give ethanoic acid which is sour. (b)

(i)

1

Ni2+ + 2OH- → Ni(OH)2

(ii) yellow to orange / pink / red (iii) (1) H+ + OH- → H2O 1 mole of H+ reacts with 1 mole of OHNo. of moles of OH- in the filtrate

(2)

(3)

= 0.251 × 18.5 × 10-3 = 4.64 × 10-3 No. of moles of NaOH used = 0.503 × 25 × 10-3 = 0.0126 No. of NaOH that has reacted with Ni2+ = 0.012575 – 4.6435 × 10-3 = 7.93 × 10-3 Ni2+ + 2OH- → Ni(OH)2

(1) 1 1 1 1 1 ___ (9) 1 1 1 (1)

1

1

1

Concentration of Ni2+ ions = 7.9315 × 10-3 / 2 × 25 × 10-3 = 0.158 mol dm-3 (3 to 5 sig. fig.) (Acceptable range：0.158 to 0.160 mol dm-3) (iv) To remove, as far as possible, OH- ions (NaOH) which have

1* 1 1

-

adhered to the surface of the residue / to transfer OH ions to the filtrate. ___ (9) * step mark 9. (a) (i)

From A to B / Clockwise B to A (Contradiction) Sodium has a higher tendency to donate electrons than sulphur sodium is more electropositive than sulphur.

1

or,

Sulphur has a lower tendency to donate electrons than sodium electronegative than sodium.

(1)

or,

Sodium donates electrons and sulphur accepts electrons. At electrode A： Na → Na+ + e-

(1) 1

At electrode B： S + 2e- → S23S + 2e- → S32nS + 2e- → Sn2-

1 (1) (1)

or, or,

(ii)

(iii) (iv)

or,

(Na undergoes oxidation (RA), S undergoes reduction (OA)) (Na has a stronger reducing power than S have 1 mark) To separate sodium from sulphur so as to prevent them from direct reaction. To allow the passage of ions between the two compartments / to complete the circuit To keep sodium and sulphur in molten state / to keep mobility of particles / atoms / ions inside the cell Sodium-sulphur cells can store up electricity produced in power station. When there is a surplus of electricity generated, the cell is charged up.

1 1 1 1 1

When the consumption of electricity is greater than its (1) production, the electricity that has been stored up in the cell will be used. ___ (8)

(b)

(i) (ii)

zinc Any ONE of the following：

brass is more corrosion resistant than copper brass is more appealing than copper (Accept other reasonable answers.) (iii) Any TWO of the following： A blue / green solution is formed. The alloy dissolves. A colourless gas (NO) is evolved. A brown gas (NO2) can be seen. Relevant equation： 3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O / 3Cu + 8H+ + 2NO3- → 3Cu2+ + 2NO + 4H2O 3Zn + 8HNO3 → 3Zn(NO3)2 + 2NO + 4H2O / 3Zn + 8H+ + 2NO3- → 3Zn2+ + 2NO + 4H2O

(c)

(i) (ii)

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

2NO + O2 → 2NO2 3CuZn + 4NO3- + 16H+→ 3Cu2+ + 3Zn2+ + 4NO + 8H2O

(1) (1)

(The equations should tally with the observations given.)

___ (6) 1

sewage sludge / manure / dead plant / dead animal (Accept other reasonable answers.) CH4 + 2O2 → CO2 + 2H2O

(iii) Save fossil fuels / as a renewable energy source / reduce solid wastes (iv) Any ONE of the following： Methane produced in biogas plants cannot meet the huge demand of domestic fuel. Investment in the construction of biogas plants may be great. Biogas plants release air pollutants. Difficult to collect large amount of organic wastes. (Accept other reasonable answers.)

1 1 1 (1) (1) (1) (1) (1) ____ (4)