HKDSE Chemistry Paper 2 Q.1 & Q.3

Transcription

1 HKDSE 2017 Chemistry Paper 2 Q.1 & Q.3

2 Focus areas Basic chemical knowledge Question requirement Experimental work Calculations Others

3 Basic Chemical Knowledge Question 1(a)(i) (1) Chemical equation for Haber process? Expected N 2 + 3H 2 2NH 3 Comments: Half of the candidates were able to give a chemical equation for the Haber process.

4 Basic Chemical Knowledge Question 1(a)(i) (1) Exemplars: Version A: The sign is not reversible Version B: [Better]

5 Basic Chemical Knowledge 1. (a) (i) (2) To separate NH 3 from the reaction mixture how? Expected: The mixture is cooled to condense NH 3 to liquid. Comments: Only a small number of candidates were able to suggest that ammonia can be separated from the reaction mixture obtained by cooling.

6 Basic Chemical Knowledge Version A: Version B: 工業上不是這樣處理 Version C: [ 較佳 ] 有冷卻凝結的意思

7 Basic Chemical Knowledge 1. (a) (iii) a potential hazard for storing methanol? Expected: Flammable / toxic / explosive

8 Basic Chemical Knowledge Comments: a high proportion of candidates were able to suggest a potential hazard such as being flammable or toxic for storing methanol in a chemical plant. Exemplar: Version A [ 較佳 ] Version B:

9 Basic Chemical Knowledge 1. (b) (ii) (1) reactants need to be purified Expected: before passing into catalyst? Avoid impurities to poison the catalyst Comments: About one third of the candidates were able to give the reason for purifying the reactants to avoid the impurities in poisoning the catalyst.

10 Basic Chemical Knowledge Exemplars: Version A: Not specific Version B: [Better version] 錯誤概念 by???

11 Basic Chemical Knowledge 1. (b) (ii) (2) (I) Not further increase conversion percentage by lowering the temperature? Expected: Lowering temperature will result with a slower rate Comments: around three quarters of candidates were able to give a reason for not preferring to further increase the conversion percentage by lowering the temperature of the reaction system.

12 Basic Chemical Knowledge Comments: around three quarters of candidates were able to give a reason for not preferring to further increase the conversion percentage by lowering the temperature of the reaction system. Exemplars: [ 較佳 ] 正確陳述

13 Basic Chemical Knowledge 1. (c) (i) chemical equation for obtaining CO from natural gas Expected: CH 4 + H 2 O CO + 3H 2 Comments: half of the candidates were able to write a chemical equation

14 Basic Chemical Knowledge Exemplars Version A: [Better] Version B:

15 Basic Chemical Knowledge 1. (c) (ii) (1) half equation at anode? (2) half equation at cathode? (3) flowing mercury cell phased out? Expected: (1) 2Cl - Cl 2 + 2e - (2) Na + + e - Na (3) Hg is toxic / poisonous.

16 Basic Chemical Knowledge Comments: 2/3 of the candidates were able to write the half equation at the anode and explain why the flowing mercury cell process has been gradually phased out owing to the poisonous nature of mercury. 40% of candidates were able to give the half equation at the cathode.

17 Basic Chemical Knowledge Exemplars: Version A: Not a good representation [i.e.na] in (ii) (2).

18 Basic Chemical Knowledge Version B: 錯誤 未夠精準

19 Basic Chemical Knowledge 3. (a) (i) a test between Na + and K + Expected: Test & difference in observation Perform flame test. Na + : golden yellow flame K + : lilac flame Comments: 80% of the candidates were able to suggest a flame test to distinguish between sodium ions and potassium ions.

20 Basic Chemical Knowledge Exemplars: Version A: [Better] Correct use of flame test with appropriate flame colours Version B: Better write as K + ions

21 Basic Chemical Knowledge (a) (ii) Chemical test : SO 2 Expected: Turns Cr 2 O 7 2- (aq)/h + (aq) from orange to green. Comments: 40% of the candidates were able to suggest a chemical test for detecting sulphur dioxide gas based on its reducing property.

22 Basic Chemical Knowledge Exemplars: Version A: [ 較佳 ] Version B: From to

23 Basic Chemical Knowledge 3. (a) (iii) Compound X + 2,4- dinitrophenylhydrazine gives What is Compound X?

24 Basic Chemical Knowledge Expected: Comments: 40% of the candidates were able to give the structure of X while some candidates misrepresented the structure as the cis-isomer.

25 Focus areas Basic chemical knowledge Question requirement Experimental work Calculations Others

26 Question Requirement 1. (b) (ii) (2) 96% conversion (I) Not further increase conversion percentage by increasing the pressure? Expected: The conversion percentage will not be much higher by increase in pressure as the conversion percentage has already been high => not cost effective

27 Question Requirement Comments: A very small number of candidates were able to suggest: not cost effective for increasing the pressure of the reaction system as the conversion percentage will not increase to a high extent.

28 Question Requirement Exemplars: Version 1: (Better version) Version 2: 如能 提及高轉化率提升的轉化百分比程度不夠高 更佳

29 Question Requirement 1. (b) (ii) (3) increase conversion percentage which reactant in slight excess? Why? Expected: O 2 in slight excess Reason: it is cheaper / more readily available [A comparative sense]

30 Question Requirement Comments: 1/4 of the candidates were able to explain that it is preferable to make O 2 in slight excess for the conversion of SO 2 (g) to SO 3 (g).

31 Question Requirement Exemplars: Version A: Not comparing SO 2 and O 2

32 Question Requirement Version B: [ 較佳 ] 有比較意思

33 Question Requirement- Reflux 3. (c) (i) (1) use reflux for carrying out Expected: this kind of extraction? Prevent the loss of solvents Comments: 1/4 of the candidates were able to state an advantage of heating under reflux in the context of the question as without the loss of volatile solvent.

34 Question Requirement - Reflux Exemplars: Version A: Not mentioning the loss of products / solvents can be prevented Version B: [ 較佳 ] 如能寫下物質為溶劑更佳

35 Focus areas Basic chemical knowledge Question requirement Experimental work Calculations Others

36 Experimental Work - Titration 3. (b) (i) colour change at the end point. Expected: From pale yellow to pale pink Comments: A very small number of candidates were able to state the colour change at the end point of the titration.

37 Experimental Work Titration 3. (b) (ii) reasonable average? 38.34, 37.62, 37.58, cm 3 Expected: ( ) / 3 = cm 3 Comments: 1/4 of the candidates were able to calculate a reasonable average volume used in titration with two decimal places.

38 Experimental work Distillation set-up 3. (c) (i) (2) Labelled diagram: simple distillation Expected: Not accept D to represent heat Note: position of thermometer, water in/out & delivery tube

39 Experimental work Distillation set-up Comments: 1/3 of the candidates were able to draw a labelled diagram for the set-up for simple distillation. Some candidates were not aware of the correct position of the thermometer and the use of a heat source.

40 Experimental work Distillation set-up Exemplars: 溫度計? Fractional distillation?

41 Experimental work - others 3. (c) (i) (3) a method to separate S from Expected: Column chromatography Comments: a small number of the candidates were able to suggest column chromatography for separating S from the impurities. Some of the candidates wrongly suggested the use of liquid-liquid extraction or distillation as the separation method.

42 Focus areas Basic chemical knowledge Question requirement Experimental work Calculations Others

43 Calculations order of a reaction 1. (c) (iii) [CO] doubled, [Cl 2 ] unchanged new rate becomes 2.83 times the original rate. Order w.r.t. CO? Expected: rate = k [CO] m [Cl 2 ] n 2.83 = 2 m m = 1.5

44 Calculations order of a reaction Comments: just under half of the candidates were able to deduce the order of reaction from the given information Exemplars: Version A: [Better]

45 Calculations order of a reaction Exemplars: Version B: 2.83/2 = 1.415?

46 Calculations - Titration 3. (b) (ii) (2) mole ratio HONH 2 : Fe 3+?

47 Calculations - Titration Expected:

48 Calculations - Titration Comments half of the candidates were able to calculate the mole ratio of HONH 2 (aq) : Fe 3+ (aq)

49 Calculations - Titration Exemplars: Version A: Correct calculation Better if the ratio can be represented as 1 : 2

50 Calculations - Titration Version B: 能計算 HONH 2 的摩爾數但未能計算正確摩爾比

51 Calculations - Titration 3. (b) (ii) (3) deduce the oxidation number of N in the oxide (iii) suggest the empirical formula of the oxide Expected: (b) (ii) (3) oxidation number = = +1 [Better with elaboration] (iii) N 2 O

52 Calculations - Titration Comments: a small number of the candidates were able to deduce the oxidation number of N in the oxide 20% of the candidates were able to suggest a reasonable empirical formula for the oxide as N 2 O

53 Calculations - Titration Exemplars: Version A: [Better] Correct deduction. Better with more elaboration Correct suggestion

54 Calculations - Titration Version B: 解釋正確但未能寫下 N 的氧化數 建議正確

55 Focus areas Basic chemical knowledge Question requirement Experimental work Calculations Others

56 Others: Interpretation from graph 1. (a) (ii) order from given graph with explanation Expected: Order of reaction w.r.t. A is 0. Reason: From graph - The rate of change of [A] is a constant.

57 Others: Interpretation from graph Comments: Just below half of the candidates were able to deduce the order of reaction from the given information

58 Others: Interpretation from graph Exemplars: Version A: [Better version] State the correct order with explanation

59 Others: Interpretation from graph Version B: Incorrect order Correct interpretation Better if mention the rate of change of [A] is indepentant of [A]

60 Others: Draw sketches of energy profiles 1. (b) (i) 2 labelled energy profiles in the Expected: same sketch. (1) exothermic (2) 2 peaks for catalysed pathway (3) indicating the correct reactants and product

61 Others: Draw sketches of energy profiles Comments: half of the candidates were able to draw the energy profiles for the catalysed pathway and uncatalysed pathway for the conversion of sulphur dioxide to sulphur trioxide in the same sketch. Some represented the two pathways in two sketches. Some wrongly labeled the axis.

62 Others: Draw sketches of energy profiles Exemplars: Version A [Better] Correct sketches. Better use only information given and not attempt to write the transition state

63 Others: Draw sketches of energy profiles Version B: 若能 1) 標籤反應物 (SO 2 & O 2 ) 和生成物 (SO 3 ) 並 2) 在加入催化劑的能線上顯示兩個峰更佳

64 Others: Green Chemistry 1. (c) (iv) CO(g) + Cl 2 (g) COCl 2 (g) green / non-green, why? Expected: Green: Atom economy is 100% Non-green: Toxic reactants (CO & Cl 2 )

65 Others: Green Chemistry Comments: about sixty percent of the candidates were able to explain why the given process would be green based on its high atom economy while about half of the candidates were able to explain why the given process would not be green as the reaction involves poisonous reactants.

66 Others: Green Chemistry Exemplars: Version A: [ 較佳 ] 概念正確

67 Others: Green Chemistry Version B: Color related with Green Chemistry?

68 Others: Artemisinin ( 青蒿素 ) structure 3. (c) (ii) Given Artemisinin - not react with NaHCO 3 (aq) - an absorption peak at 1700 cm -1 in IR spectrum?

69 Others: Artemisinin ( 青蒿素 )structure Expected: not react with NaHCO 3 (aq) no carboxyl group i.e. not Y absorption peak at 1700 cm -1 in IR spectrum presence of carbonyl group i.e. not X & Z W is the correct structure.

70 Others: Artemisinin ( 青蒿素 )structure Comments: two thirds of the candidates were able to deduce the possible structure of artemisinin. Some candidates did not relate the given information to confirm the presence of the carbonyl group and the absence of the carboxyl group.

71 Others: Artemisinin ( 青蒿素 )structure Exemplars: Version A: [Better version] Correct interpretation from NaHCO 3 reaction Correct choice Correct interpretation from IR spectrum

72 Others: Artemisinin ( 青蒿素 )structure Version B: Incorrect choice Correct interpretation from IR spectrum Cannot fully make use of the information from NaHCO 3 reaction

73 Focus areas Basic chemical knowledge Question requirement Experimental work Calculations Others

74 HKDSE 2017 化學卷 2 第 2 題物料化學

75 討論範圍 基本化學知識 : 例 : 液晶分子結構與液晶性的關係 金屬晶體不同裝填 HDPE 與 LDPE 分子間引力的相異之處 化學計量學 : 例 : 晶胞內原子數目 配位數 傳意 : 例 : 繪畫有機玻璃單體的結構 例 : 解說 熱塑性 的意思 / 縮合作用 的特徵 理解關係 : 例 : 解釋為什麽 PHB 的生產和棄置是對環境友善

76 考生整體表現 : (a) 表現欠佳 (b) 表現尚可 (c) 表現尚可

77 基本化學知識 (1) 考生未能 提出為什麽天然橡膠與硫共熱時會變硬 (a) (i) (80%) 寫出鋅裝填的名稱 (b) (ii) (1) 及展示對不同金屬晶體裝填的認識 (b) (ii) (2) (3) (33%)

78 基本化學知識 (2) 約 75% 未能 / 錯誤推算在銅晶胞內原子配位數, 提供如 13 等錯誤答案 (b) (i) (1) 應與數學能力無關, 相信是與化學計量學的基本知識有關

79 傳意 : 考生在繪畫有機玻璃單體的結構表現欠佳 (c) (i) 甚多考生不能寫出這個單體的天然來源 (c) (i) 考生往往不能準確全面解說 熱塑性 一詞的意思 (c) (iii) (1) (90%)

80 理解關係 : 某物質的性質和其用途的關係 部分考生不知道有機玻璃是可用來製造隱形眼鏡 (c) (ii) 考生往往不能使用簡短文句解釋往往 PHB 的生產和棄置可被視為對環境友善 (c) (iii) (2)

81 理解關係 : 酯鍵合的水解性質 部分考生誤以為 PHB 的酯鍵合經分解後能生成天然物料, 故是對環境友善 (c) (iv) (2)