2016 Examination Briefing - Chemistry and Combined … · - Gave only the molecular formula without showing ... and ‘structural ... 但不能正確書寫「丙稀酸」”propenoic

HKDSE 2016

7, 17 November 2016

Candidates’ Performance in Paper I (Q1 - 4, 10 - 14 )

General and Common Weaknesses

General Comments

• Weak in calculations

• Unable to give the appropriate units for numerical answers

• Failed to describe correct procedures / select appropriate apparatus and instruments for practical tasks


General Comments

• Failed to describe chemical concepts / explaining chemical phenomenon clearly

• Unable to use the appropriate technical terminology

• Weak in planning synthetic schemes

• Did not read the questions carefully

• Gave ambiguous answers / incomplete answers

• Questions 1 – 4 Chemistry (Part I) / CS (Chem)

Q1 – Overall Good (CS: Fair)

• Part (a) (correct: a very high proportion)

- some candidates gave the electron diagram of a phosphorus atom (instead of electronic arrangement).

Chemistry (Part I) / CS (Chem)

Q1 – Overall Good (CS: Fair)

• Part (b) (correct: a very high proportion)

- Some candidates wrongly stated that some chlorine atoms have different numbers of protons or electrons.


Q1 – Overall Good (CS: Fair) • Part (c)(i) (correct: just less than half)

- Gave only the molecular formula without showing any deduction, or incomplete deduction.


Empirical formula ↓

Molecular formula

Q1 – Overall Good (CS: Fair) • Part (c)(ii) (correct: about three quarters )

- Some candidates gave electron diagrams with an incorrect number of outermost electrons, or without showing the lone pairs in the diagrams.


Q2 – Overall Satisfactory (CS: Poor) • Part (a) (correct: about two thirds)

- Some candidates wrongly stated that K2SO4(aq) can provide mobile electrons to conduct electricity, or it allows mobile electrons / atoms to pass through,



Q2 – Overall Satisfactory (CS: Poor) • Part (a) (correct: about two thirds)

“ambiguous / incomplete answer”

Q2 – Overall Satisfactory (CS: Poor) • Part (b) (correct: a very high percentage)

- Some candidates wrongly stated that the colour of FeSO4(aq) is dark green, dirty green, or bluish green.


Q2 – Overall Satisfactory (CS: Poor) • Part (c)

- about one third of the candidates were able to state the correct observation

- about a quarter were able to also give the correct explanation.


“correct observation but incorrect explanation”

Q2 – Overall Satisfactory (CS: Poor) • Part (c)


Q2 – Overall Satisfactory (CS: Poor) • Part (d)

- about one third of the candidates were able to state that there would be no observable change.

- a quarter were able to fully state the correct explanation.


Q2 – Overall Satisfactory (CS: Poor) • Part (d)


“correct observation but incorrect/incomplete explanation”

Q3 – Overall Fair (CS: Very Poor) • Part (a)

- nearly all candidates can give the correct reaction type.

- about two thirds of the candidates were able to suggest a correct importance of this type of reaction in industry.

- A few candidates were confused about catalytic cracking and fractional distillation.


Q3 – Overall Fair (CS: Very Poor) • Part (b) (correct: about two thirds)

- Some candidates wrongly stated that smaller pieces of porcelain can be heated more efficiently.


“incomplete answer”

Q3 – Overall Fair (CS: Very Poor) • Part (c)(i) (correct: about three quarters)

- some candidates failed to give the correct molecular formula of octane

- and some failed to give the correct stoichiometric coefficients for the equation


Q3 – Overall Fair (CS: Very Poor) • Part (c)(ii)(1) (correct: about three quarters)

- Some candidates failed to give the correct colour of bromine solution,

- and a few candidates wrongly stated the colour change is from colourless to brown.


Q3 – Overall Fair (CS: Very Poor) • Part (c)(ii)(2) (correct: about three quarters)

- Some common mistakes:


Q3 – Overall Fair (CS: Very Poor) • Part (d)

- 2/3 of candidates able to state that something had to be done to prevent sucking back of water into the hot boiling tube,

- 1/3 of candidates able to give the correct action to prevent sucking back of water.

- Common incorrect answers: (1) the hot boiling tube should be cooled down slowly; (2) the gaseous products should be taken into a fume cupboard.






Q4 – Overall Fair (Chemistry) • Part (a)(i) (correct: about half)

- Some wrongly gave a bent structure for this molecule, or gave an electron diagram in the answer.

Chemistry (Part I)

Q4 – Overall Fair (Chemistry) • Part (a)(ii) (correct: a very high percentage )

- Common incorrect answers:

Chemistry (Part I)

Q4 – Overall Fair (Chemistry) • Part (b)

- 2/3 of candidates were able to correctly state the polar bonds in a CH2Br2 molecule.

- below half of the candidates were able to give the correct explanation.

- Some candidates misinterpreted the question, and discussed whether CH2Br2 is a polar molecule.

Chemistry (Part I)

Q4 – Overall Fair (Chemistry) • Part (c) (correct: about one third)

- Common incorrect answers:

- CO2 is a non-polar molecule while CS2 is polar

- CO2 molecules are held by weak van der Waals’ forces, while CS2 has a giant covalent structure.

Chemistry (Part I)

Q4 – Overall Very Poor (CS Chem) • Part (a) (correct: only a small number )

- Unable to draw the ester group:

- Unable to give the correct carbon skeleton

CS (Chemistry)

Q4 – Overall Very Poor (CS Chem) • Part (b)(i)(1) (correct: about one third)

- Some candidates gave a vague answer such as the product is insoluble in water.

• Part (b)(i)(2) (correct: about one third)

- Some candidates merely stated concentrated sulphuric acid was added in trial 2 without giving any explanation for the use of concentrated sulphuric acid.

CS (Chemistry)

Q4 – Overall Very Poor (CS Chem) • Part (b)(ii)(1) (correct: a very small number )

- A number of candidates only stated the merit in the answer without providing explanation.

CS (Chemistry)

√

Q4 – Overall Very Poor (CS Chem) • Part (b)(ii)(2) (correct: a small number )

- Common mistakes:

- gave the drawing of a distillation set-up

- missing labels in the diagrams

- inappropriate positions of water inlet and outlet on the condenser

- drawing the set-up as a closed system.

CS (Chemistry)

• Questions 10 – 14 Chemistry (Part II)

Q10 – Overall Fair • Part (a) (correct: about half)

- Examples of incorrect answers:

Chemistry (Part II)

Q10 – Overall Fair • Part (b) (correct: about 1/3)

- Some failed to correctly solve the corresponding equations for the equilibrium expression.

Chemistry (Part II)

Chemistry (Part II)

Q10 – Overall Fair • Part (c)(i) (correct: less than 1/3)

- Some failed to recognise the effect of changing the temperature on exothermic/endothermic reactions.

Chemistry (Part II)

Q10 – Overall Fair • Part (c)(ii) (correct: less than 1/2)

- Some only stated that adding a suitable catalyst would not increase the number of moles of SO3(g) obtained without giving a proper explanation.

Chemistry (Part II)

“incomplete answer”

Q11 – Overall Poor • Part (a) (correct: less than 1/2)

- less than half of the candidates were able to correctly state keeping the total volume of the reactions mixtures the same for conducting fair tests among the trials, so that the concentration of NaOH(aq) would be the only variable.

Chemistry (Part II)

Q11 – Overall Poor • Part (b) (correct: about 1/4)

- Common mistakes:

- confused about number of moles of NaOH and concentration in the calculation,

- failed to recognise the meaning of [H+(aq)][OH–(aq)] = 1 .0 x 10–14 mol2dm–6

- confused about [H+(aq)] and [OH-(aq)] in the calculation.

Chemistry (Part II)

Q11 – Overall Poor • Part (c)

- about ½ of candidates could recognise the rate of the reaction was affected by the concentration of NaOH(aq)

- Some failed to recognise the focus of this question, and wrongly stated that the rate of the reaction was affected by the volume of NaOH(aq) added.

- Only a very few could give a complete deduction.

Chemistry (Part II)

Q11 – Overall Poor • Part (d) (correct: very high percentage )

- incorrect answer such as ‘calorimetry’

Chemistry (Part II)

Q12 – Overall Poor • (correct: only a small number)

Chemistry (Part II)

√


- Even though about half of the candidates were able to recognize that this synthetic route involves two key steps, some candidates failed to give complete answers.

Chemistry (Part II)


- Common mistakes included:

- omitting the reaction conditions,

- stating the inappropriate reagents for the steps (using dilute H2SO4 for esterification),

- including inappropriate steps (oxidation with acidified K2Cr2O7, reduction with LiAlH4) in the proposed synthetic route.

Chemistry (Part II)

Chemistry (Part II)

Q13 – Overall Fair • Part (a) (correct: about 2/3)

- Common mistakes:

- gave the drawing of a distillation set-up

- missing labels in the diagrams

- inappropriate positions of water inlet and outlet on the condenser

- drawing the set-up as a closed system.

Chemistry (Part II)

- Closed system

- Missing label for ‘heating’ Chemistry (Part II)

Q13 – Overall Fair • Part (b) (correct: about 1/2)

- Some wrongly stated using LiAlH4 in an acidic aqueous medium (LiAlH4 / H+(aq))

• Part (c) (correct: about 1/2)

- Some gave incorrect answers such as ‘positional isomers’, ‘geometrical isomers’, and ‘structural isomers’

Chemistry (Part II)

Q13 – Overall Fair • Part (d) (correct: only about 1/3)

- Some wrongly stated that the two compounds have different melting points or boiling points.

- The molecules rotate in opposite directions

• Part (e) (correct: about 1/2)

- Common mistakes include the observations did not match with the reagent, or missing “acidified” for K2Cr2O7 or KMnO4

Chemistry (Part II)

Q14 – Overall Fair • About 1/3 of candidates gave a correct and complete

answer.

• About 2/3 of candidates could arrange the four elements in the correct decreasing order of electrical conductivity.

• Some failed to recognise that Na and Al have giant metallic structures or Si has a giant covalent structure.

• Some wrongly stated that Na has a higher electrical conductivity than Al because Na is more reactive, and the Na atom loses the outermost electrons more easily.

Chemistry (Part II)


General Comments

• Weak in calculations

• Unable to give the appropriate units for numerical answers

• Failed to describe correct procedures / select appropriate apparatus and instruments for practical tasks


General Comments

• Failed to describe chemical concepts / explaining chemical phenomenon clearly

• Unable to use the appropriate technical terminology

• Weak in planning synthetic schemes

• Did not read the questions carefully

• Gave ambiguous answers / incomplete answers

HKDSE

第 5 至 9 題

2016 卷 I

題 5 表現令人滿意

(a)

• 約50%的考生能夠正確繪畫化合物A的結構，但不能正確書寫「丙稀酸」”propenoic acid”

• 以下是一些常見不正確的結構：

C C

COOHH

H

H H

H C C

COOHH

H

H H

H C C

CO2HOH

H

H H

H

題 5 表現令人滿意

(b) 約75%的考生能夠指出聚合作用的類別

(c) 只有約40%的考生答對

(d) 約50%的考生未能繪畫C的結構： –鈉和碳中有共價鍵 (Na－C)

–出現羥基團 (－OH)

–省略了n

題 6 表現良好

(a) 約70%的考生能夠正確指出在檸檬酸結構中的三個可電離氫原子 (ionisable hydrogen

atom)

(b) 約70%的考生能夠正確說應使用的儀器，但不少考生提供以下錯誤答案：

「錐形瓶」、「燒杯」、「滴定管」和「移液管」

約2/3考生能夠正確計算所求的質量百分率

題 6 表現良好

(c)

(i) 相當多考生能正確寫出「預期觀察」

(ii) 為數不少的考生書寫了以下不正確的離子方程式：

2H+ + CO32- H2O + CO2

題 7 表現尚可

(a)考生的表現差劣，只有極少數考生能夠寫出所需條件是「恆壓」“Constant Pressure”

(b) 約有33%的考生能夠解釋為什麼要使用「間接方法」

題 7 表現尚可

(c)

(i) 約有70%的考生能夠指出可能出現的誤差

(ii) 只有少過50%的考生能解釋為什麼 CaCO3(s) 與 H2SO4(aq) 的反應焓變未能藉這個步驟來測定

(d)

• 最常見的錯誤是「省略了物態符號」

• 不少考生未能書寫所需的平衡化學式，以致未能完成所需的計算

題 8 表現尚可

(a)

(i) 約有34%的考生能夠寫出「在電極 X 會有紅棕色氣體釋出」

(ii) 約50%考生能夠寫出所需的半方程式

(b) 約有66%的考生能寫出「溴是有毒」，亦有考生錯誤地認為「溴是有害」、「溴具刺激性」

題 8 表現尚可

(c)

(i) 約有50%的考生能夠根據 Mn 氧化數的變化( +4 降至 +3 )，推定 MnO2 為氧化劑

(ii) 大部分考生未能寫出所需的半反應式，其中甚多考生錯誤地在公式中包含了 H+(aq)

註：有考生認為 Zn 是氧化劑，並錯誤地寫了：Zn(s) Zn2+(aq) + 2e-

題 9 表現尚可

• 大部分考生忘記應首先使用水來溶解這些固體

• 約有40%的考生能夠提出正確測試來分辨該等固體，並正確地預測觀察

• 以下為一些常見的錯誤： –以為可用焰色試驗，MgSO4(s)／

MgSO47H2O(s) 會發出明亮的白光 –以為當把 ZnSO4(s) 強熱時會呈現黃色，

冷卻後呈現白色

考生書寫化學式和化學反應式的表現

Documents

2016 Examination Briefing - Chemistry and Combined … · - Gave only the molecular formula without showing ... and ‘structural ... 但不能正確書寫「丙稀酸」”propenoic