2004 Chemistry Annual Report

2004 Chemistry Annual Report - fi Paper 2 E3 Question No...

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Unformatted text preview: fi : Paper 2 E3 Question No. “OWflP‘EflF‘E’JE‘JT‘ 25. Efi Key c (67) D (54) A (35) D (46) c (60) c (58) A(73) A (56) A (29) B (47) c (60) c (47) A (58) B (66) c (41) A (62) c (49) B (37) A (45) Mm B (36) A (59) c (84) B (41) A (76) Question No. Effififlfi%%%%fl$° Note: Figures in brackets indicate the percentages of candidates choosing the correct answers. 83 7. 3A 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. gfi Key c (83) D (62) B (30) C (67) A(45) B (64) B (42) D (66) B (67) B (59) D (23) B (40) D (43) A (49) B (50) D (67) D (41) D (43) c (58) B (58) C (42) ' C (39) D (69) A (58) A(35) #Efifi #— L__E% gfifififi | *Efifi $4 1 | — %Afi§ 2 — 4§Affifi 3 — E95? 4 — $4 5 — EU? [2% 6 r_ M %Afifi 7 w 44 8 89 9 m J_ %Afi§ (EF'E‘B§EF€IZEVF%S 3 ZEBIEE ’ %%3% °) 5F ’58 1. 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'flfifilfiflélfilfi ‘fififififififiLlTfifiifii‘éfii 16.0) B’Jfifififi O (E H2804 ‘—_“—_—-‘) CH2 EZEEEfiiififiiéfifiEfifl (1‘H¥3‘JE¥EE: H=l.0, c=12.0, N=14.0, » fi % Q.28 (35%) (13%) 5951”; 3mm ’(fl% fi—fimfifiéflflfio ’@% fififi~fimfifiéflfifio 13- Wfifimfiififififi £332 0 fi—fifiifli’flfififi A-* fifiiflt (3) (2) “J (37%) Efio D fififlfifififific fi/m em a g “m )) %% 10 23 (24%) (25%) (1) $1] (2) (1) fl] (3) R75 (2) ft! (3) (1) ‘ (2) *fl (3) RE R75 A. B} C D 92 9! Paper 1 Question Number Section A l Popularity (%) Candidates’ Performance Performance in General Satisfactory Satisfactory Poor Fair Good Satisfactory Fair Fair Satisfactory (All questions in Section A were compulsory. Candidates were required to answer three out of four questions in Section B.) Section A 1. (b) (i) (ii) Many candidates did not read the question carefiilly. They drew set-ups which did not include any beaker. Many of the set—ups proposed did not seem to be workable, for example in one set up, an inverted funnel used to direct the gas produced was placed well above the liquid level. This suggested that candidates nnght not have enough experience in performing such experiments on their own. \- Many candidates correctly correlated the change in reaction rate to the existence of a layer of oxide on the surface of the calcium granules. Not many candidates noticed that the heat evolved could speed up the reaction. Some candidates attributed the change in reaction rate to an increase in surface area of the metal, which is not a sound reason. (c) Most candidates correctly stated the expected observations for the reactions. However, many did not compare or contrast the behaviours of the two metals. Candidates‘ performance was satisfactory on the whole. However, most candidates included in their answers only observations for the substances which gave a positive test result. Such answers were considered incomplete. Some candidates did not read the question carefully. Instead of giving one test for each pair of substances, they gave two different tests for the two substances. (a) Some candidates wrongly stated that table salt does not react with concentrated sulphuric acid. (c) Some candidates wrongly stated that copper reacts with dilute sulphuric acid to give a colourless gas. (a) Very poorly answered. Most candidates did not know what tincture of iodine is. They simply made wild guesses, such as ‘dissolving iodine in tincture’. It appeared that this daily application of chemistry has not been sufficiently attended to. (b) Poorly answered. Possibly, not knowing what tincture of iodine is, many candidates incorrectly attributed the action of sodium sulphite to its bleaching property. In fact, sodium sulphite solution reduces iodine to colourless iodide ions. In deciding which method is the better, most candidates did not provide reasonable answers for their choice. Many wrongly chose method (2) because they thought that the bleaching action of sodium sulphite was not permanent. The performance of candidates reflected that they needed more training in applying their chemical knowledge in unfamiliar situations. The question asked for daily applications of chemistry and the chemical knowledge involved was not demanding; yet the overall performance of candidates was barely satisfactory. Many candidates stated the major air pollutants which lead to the formation of acid rain, and suggested methods to reduce the air pollutants. However, many did not state clearly how the pollutants Were formed, and match the methods with the appropriate pollutants. 94 In their answers, many candidates quoted a detailed account of the harmfiil effects of acid rain, but discussed its formation and reduction in brief. Most of the ideas were not presented systematically and logically. More training on essay writing is needed. Other mistakes included: 0 making incorrect suggestions that carbon dioxide leads to the formation of acid rain; 0 assuming that catalytic converters can be used to remove sulphur dioxide; and 0 wrongly recognising that electrostatic precipitators are used to remove nitrogen oxides and sulphur dioxide. Generally well answered. Most candidates were able to present ideas in paragraphs according to the three types of structures of the substances given, and were able to correlate the structure of the substances to their melting points. However, many candidates did not use the terminology correctly and properly when describing the structures and the nature of forces holding the particles of the substances. A more concrete grasp on this aspect is needed. Section B 6. (a) (i) & (ii) Well answered. (iii) The addition of sulphuric acid can increase the electrical conductivity of water. Many candidates wrongly mentioned that sulphuric acid acts as a catalyst in the electrolysis. (iv) Very few candidates were able to score full marks. Some candidates correctly brought out the relationship between mole ratio and volume ratio of gases, i.e. the Avogadro’s Law. Most failed to include in their deduction that the atomicity of oxygen and that of hydrogen are both 2 (b) (i) The oxidation number of O in H202 is —l, but many candidates wrongly stated that it is —2. (ii) Many candidates provided an incorrect electronic diagram of hydrogen peroxide. 95 (iii) (ii) (iii) (iv) (a) (i) (b) (i) (ii) Candidates were weak in writing chemical equations for reactions involving an unfamiliar Situation. Most candidates did not notice that to balance the half equation, it is necessary to include the appropriate numbers of H+(aq} and H300). The plastic wastes will burn if the pyrolysis takes place in the presence of air. Many candidates wrongly mentioned that it is the burning of the products, methane and ethene. Many candidates did not read the question carefully. They wrongly suggested ‘cracking’ as a separation method. The methane produced can be used as a fuel. Many candidates incorrectly mentioned that it is used to make towu gas or liquefied petroleum gas {LPG}. Generally well answered except that some candidates did not use the terminology correctly. Some candidates wrote ‘hurning‘ for ‘incineration‘, and ‘burying underground‘ for ‘Iandfilling‘. some candidates wrongly suggested that in the dilution .process, a pipette needs to be used for transferring the acid to the volumetric flask. Many candidates overlooked the point that only one-tenth of the total amount of the acid was used for titration. They were unable to calculate the baSIcity of the solid acid. The performance of candidates in this traditional type of question was good. However, a few candidates failed to point out the hydrophilic and hydrophobic parts of the detergent. Some candidates failed to recognise that sea water contains CaiTaq) and Mg2+(aq) ions, which can form scum with soapy detergents. Others wrongly suggested that the detergent is suitable for treating oil spill because it is biodegradable. 96 (C) (a) (b) (c) (i) (ii) (ii) (iii) (i) (iii) (iv) (0 Some candidates wrongly suggested that concentrated H2804 would undergo decomposition to give SOZ(g) and H20(l). Some confused the terms ‘hygroscopic’ and ‘dehydrating’. The reaction gives a white insoluble thermosetting plastic, namely urea-methanal. However, many candidates wrongly stated that carbon is formed. Some candidates appeared not to know the purpose of the experiment, that is, to determine the percentage by mass of calcium carbonate in the coral sample. They wrongly calculated the number of moles of calcium carbonate from the mass of the impure sample. Many candidates did not realise that carbon dioxide is soluble in water, and incorrectly suggested answers such as ‘it is necessary to measure the volume of air present in the conical flask’. Most candidates did not precisely point out that rust is hydrated iron(lII) oxide. (1) It is common sense that the moving parts of machinery should be oiled. But some candidates wrongly suggested that painting or sacrificial protection should be used to prevent bicycle gear wheels from corrosion. (2) Some candidates gave an incorrect description for the sacrificial protection, such as ‘placing a more reactive metal near the water pipe’. Aluminium oxide is impermeable to air and water, and hence can prevent aluminium from corrosion. Some candidates gave answers, which were not detailed enough. They simply wrote ‘aluminium oxide is a protective layer’. Most candidates correctly cited the colour change in the breathalyser. Many did not explain the change in terms of the reduction of dichromate ions or the oxidation of ethanol. 97 xx (3) ('3) (ii) This question set on the application of scientific knowledge was badly answered. Many candidates did not read the question carefully and Overlooked the phrase ‘without using other instruments‘. They suggested testing the ethanol content in blood or urine sample of the driver. Some suggested unrealistic answers, such as ‘measuring the degree of redness of the face of the driver’. (i) & (ii) Well answered. (iii) Many candidates were unable to give a precise description of metallic bonding and to account for the ductility of metals with reference to their bonding and structure. (v) (1) Many candidates were unable to deduce the electronic arrangement of bromine to be 2, 8, 18, 7. Well answered. (ii) (1) Many candidates were not able to work backwards from the structure of salicylic acid to deduce the structure of the sodium salt. This indicated that their line of drinking was linear and were unable to apply their knowledge to solve problems involving an unfamiliar situation. (2) Few candidates correctly suggested the reagent to be HCl(aq) or HZSO4(aq). (3) Fair performance. 98 General comments and recommendations 1. Many candidates were unable to give precise and accurate description of chemical terms, such as ‘metallic bonding’. Thrs reflected their weakness in communicating ideas. Reading more science-related articles and even science fiction can help candidates improve in this respect. When answering questions set on unfamiliar situations, many candidates simply regurgitated material learnt from textbooks. These candidates were weak in applying their knowledge to solve problems in real life situations. Candidates should appreciate that many scientific principles are derived from daily life experience. 99 Paper 2 The paper consisted of 50 multiple-choice questions. The mean percentage score was 52.4 and the standard deviation was 23.0. Post- 1. examination analysis revealed the following: Candidates were weak in the nomenclature of organic compounds. In Q.15, the systematic name of the monomer is methylpropene. Many candidates mistakenly thought that it is 1,1-dimethylethene. Q. 15 The structure of polymerX can be represented by What is the monomer of X ? A. 1, l -dimethylethene (40%) B. 1 ,2-dimethylethene ( l 2%) C.* methylpropene (41%) D. but-l-ene (7%) In general. candidates failed to demonstrate a good mastery of the properties of chemicals commonly encountered in the laboratory and in daily life. It is a well-unknown fact that chlorine can displace bromine from an aqueous solution of sodium bromide. However, in Q.18, many candidates wrongly chose 1,1,l-u-ichlorethane, which is a solvent for non~polar substances, to distinguish sodium bromide solution from sodium chloride solution. Q.18 Which of the following reagents can be used to distinguish sodium bromide solution from sodium chloride solution ? A. bromine water (11%) B.* chlorine water (37%) C. 1,1,l-trichloroethane (39%) D. potassium fluoride solution (13%) 100 In Q.28, each of the distracters attracted more than 20% of the responses. Many candidates wrongly conSIdered that set-up (2) could be used to collect the ethene produced. They dld not reahse that the density of ethene is close to that of air, and that ethene should not be collected by downward displacement of air. Q.28 Ethene can be prepared by heating ethanol with excess concentrated sulphuric acid. The reaction involved can be represented by the equation : conc. H2804 -—-—’ CH2=CH2 + H20 Which of the set-ups shown below can be used to collect the ethene produced ? (Relative atomic masses : H = 1.0, C = 12.0, N = 14.0, O = 16.0) (i) (2) (3) ‘ syringe gas —b 835 —> A. (1) and (2) only (21%) B.* (l) and (3) only (30%) C. (2) and (3) only (24%) D. (1), (2) and (3) (25%) “3| Candidates’ performance on questions set on chemical calculations was barely satisfactory. In Q.12 and Q21, the percentages of candidates who chose the correct answer were 48% and 36% respectively. Q.12 The relative atomic mass of element X is 74.9. It forms an oxide containing 24.3% of oxygen by mass. What is the mole ratio of X to oxygen in the oxide ? (Relative atomic mass : O = 16.0) A. 1 . 2 (8%) B. 1 : 3 (32%) c.* 2 - 3 (48%) D. 2 5 (12%) Many candidates did not pay attention to the application of chemicals in daily life. Many did not know that sulphur dioxide is commonly used to preserve dry fruits and hence they were unable to choose the key in Q50. Candidates should realise that reading information on the package of chemicals commonly encountered in daily life such as food, drugs and cosmetics can help enhance scientific literacy. Q.50 lst statement 2nd statement Sulphur dioxide is used Sulphur dioxide is toxic to to preserve dried fruits. micro-organisms. A.* Both statements are true and the 2nd statement is a correct explanation of the lst statement. B. Both statements are true but the 2nd (13%) statement is NOT a correct explanation of the lst statement. C. The lst statement is false but the 2nd (37%) statement is true. Both statements are false. (35%) In Q.46, many candidates wrongly thought that methanoic acid is a non-electrolyte. They did not know the definition of ‘electrolyte’ — a compound which, in aqueous solution or when molten, conducts electricity and is decomposed in the process. 102 Mi'l'fi'fil Examination Statistics 1. %$%5§EZX§%ELBUB’JE5}$ Percentages of candidates awarded each grade tflfikm $fi¥$¥fl§fifi§fifl$ Rfiififiifi No sat Percenta awarded each 9 I: based on no. sat Absentee ' “an we (%) EWM:§ E Pkg‘i Day school first one u ters 26 750 El +32%; Day school 3.4 20.3 candidates I fag-9i. 34551 3.8 7.8 25.2 2L4 3.4 Allcandidates 2. ¢§Cfifiifi%$$lfik§fl Attendance statistics of the two language versions 2 qulilfli’y'fih‘éfi Percentage sitting Chinese version LX933ZM¥E5§$ Percentage sitting En ' h version Da school first attemters Elliefii D. Emmet...“ um All candidates fll‘ififwkm Total no. sat 3. %$%fi A ‘ C J52 E tELBZE ' E§E§Efififlfifilfifi§k Minimum score of candidates in the multiple-choice (m.c.) paper for the award of grades A, C and E EHEMW’S‘HJ’XTWLSU' ' #iiid’flg‘flfififl‘fl Minimum score to achieve the followin y de ‘ glfiifllflfififififi Maximum score ofm.c. items " EEQEEE-fiffll‘fimfifl’flfiififltflm ° smeaemaaseaaaraaaaem fiifi ' lfifiéflfi¥$7£¥fi€rflfiflfifi$flfiflfl€ ° This is an estimate based on the percentage of all candidates awarded that grade in the subject. The percentage is applied to the multiple-choice paper mark distribution. 103 4. El Fi%it%$fi2§§fifi§+ Day school candidates' results statistics by gender iii/“ill Candidate category Gender Hilfikfit No sat gauge Hist/13$ Day school first attemptcrs HERVE Day school candidates " IS 45 EA!!! “0361299 42 3972 5. Elfi%$§$%2l§fillfilfl Statistics on day school candidates sitting this subject 91$ 5%; Male Female Eifitflfifilfifigi‘ fififitfi£$ EJXlE-fi‘EllQ—‘g‘i wattage All day school first All day school All day school first All day school 81': rm candidates . ' éflé¢$fi¥fldfikfl J? an ters candidates No. ofcandidam sittin ancee 3' 393 35 193 33 109 mas-mum ( Efléfi) No. 0%.) ofcandidates sitting this subject '5 5 '8 (49-6) '3 “4 (49-3) “ 232 (33-9) 13 012 (34. l) E = fi'fifltfitifififlfiififlfial‘ifiiiififi? ' flfiéfiffiflfifi’fl r%§“fifi%i E‘JgfififiT E a Note: The examinationstatistics in this publication do not include the appeals results. Thus they may differ slightly from those in the Examination Report to be published later. 104 ...
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2004 Chemistry Annual Report - fi Paper 2 E3 Question No...

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