2017 Examination Briefing - Chemistry and Combined Science ...€¦ · Chemistry (Part II) Correct Answer . 10 (a) Some failed to recognise the stoichiometric relationship of the

HKDSE 2017

9, 23 November 2017

Candidates’ Performance in Paper I (Q1 - 5, 10 - 14 )

General and Common Weaknesses

General Comments

• Weak in calculations

• Weak in conversion of units in calculations (e.g. cm3 to dm3)

• Weak in writing / balancing chemical equations.

General and Common Weaknesses

General Comments

• Failed to describe chemical concepts / explaining chemical phenomenon clearly and precisely

• 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 – 5 Chemistry (Part I) / CS (Chem)

Q1 – Overall Good (CS: Poor)

1 (a) correct: a very high proportion

Chemistry (Part I) / CS (Chem)

1 (a) - some candidates gave inappropriate terminology in the answer.

Chemistry (Part I) / CS (Chem)

Q1 – Overall Good (CS: Poor)

1 (b) correct: about 50 – 70% candidates

1 (b)(i) - some candidates wrongly stated that the apparatus is not covered.

1(c)(i) - Generally well-answered

• Some incomplete / incorrect answer

White precipitate

1(c)(ii) - Generally well-answered

Q2 – Overall Satisfactory (CS: Fair) 2(a) correct and complete: about 1/3 candidates

- Some incomplete answer

- Some wrongly apply “rusting” to copper

2(b)(i) - Only a few can give correct answer

- Many candidates wrongly stated that lead can inhibit corrosion by sacrificial protection

- Most students unfamiliar with the properties of alloy

2(c) - about 40% candidates gave correct answer

- some candidates were confused about the conversion of unit from cm–3 to dm–3

Q3 – Overall Poor (CS: Very Poor)

3(b) - Only a small number of candidates able to give correct and complete answer

- Incomplete / unclear answer

- Wrongly stated HO2(CH2)4CO2H has two – OH group

- Wrongly compared CH3(CH2)4CO2H with H2N(CH2)6NH2, and not with HO2(CH2)4CO2H.

3(c) - Only very few candidates could give correct and complete answer

- Incomplete / unclear answer

- Incorrect terminology

- Mistakes in chemical concepts

3(c) – Good Answer

3(c) – Incomplete Answer

Q4 – Overall Very Poor (CS: Very Poor) 4(a) – Incomplete answer 4(a)(i)/(ii) – About 1/3 of candidates only stated the

expected observation without giving an explanation

4(a)(ii) – Very few candidates gave correct and complete answer

– Only mentioned either one of the expected observations (colourless bubble formation / colour change of the phenolphthalein indicator)

4(a) – Incomplete Answer (No explanation)

4(a) – Another Incomplete Answer Colour change missing in 4(a)(ii)

4(b) – Equation not in appropriate form

– Incorrect equation

4(c) – Incomplete answer (No explanation)

Q5 – (CHEM) Overall Satisfactory – About 1/3 of candidates gave correct and

complete answer

– Inaccurate chemical concepts

– Compare the molecular masses of F2 and H2 instead of the molecular sizes

– Hydrogen bond exists between H atom and F atom in a H–F molecule.

Q5 – CS(CHEM) Overall Satisfactory

5(a) – About 1/2 of candidates gave correct answer

– Incorrect shape of the curve

5(a) – Incorrect shape of the curve

5(b) – Imprecise description Amount = volume? mass? no. of mole?

• Questions 10 – 14 Chemistry (Part II)

Q10 – Overall Satisfactory

10 (a) correct: 40 – 50% of candidates

Chemistry (Part II)

Correct Answer

10 (a) Some failed to recognise the stoichiometric relationship of the reaction

2H2O2 2H2O + O2

10(b) – Correct: about 75% of candidates

– Incorrect shape of the curve

10(c) – Correct: about 40-50% of candidates

10(c) – Some candidates wrongly considered the reaction is reversible

10(c) – Imprecise description Amount = volume? mass? no. of mole?

10(d) – Correct: about 50% of candidates

– Some candidates not familiar with practical skills

Q11 – Overall Satisfactory 11(a) – Correct: About 70-80% of candidates

Correct Answer

Incorrect Answer

11(b) – 2/3 candidates gave correct relationship between pH and [H+]

– Some candidates carelessly swapped [HA] and [A–] in the answer

Correct Answer

11(c) – Correct: about 50% of candidates

– Some stated the correct expected colour change without giving an explanation

11(d) – Correct: about 50% of candidates

– Some wrongly stated that 4-nitrophenol can be used to measure the pH of a solution.

Q12 – Overall Poor 12(a) – Correct: About 80% of candidates

Correct Answer

Correct Answer

12(b)(i) – Correct structure: about 50% of candidates

– Very small number of candidates gave the appropriate deduction

No deductions were given

12(b)(ii) – Correct: about 50% of candidates

– Common mistakes: ‘subsituation’, ‘halogenation’, ‘bromination’ , ‘hydrobromination’

「鹵化作用」、「溴化作用」 、 「溴氫化作用」

12(c)(i) – Correct structure: about 1/4 of candidates

– Very small number of candidates gave the appropriate deduction

- Some missed the “*” to indicate the chiral center.

12(c)(ii) – Correct answer: about 1/3 of candidates

– Some omitted the required catalyst

– Some confused about “hydrogenation” and “reduction”, and wrongly suggested “LiAlH4” or “NaBH4” for the answer.

Incorrect Answer

Q13 – Overall Fair

1/4 of candidates gave correct and complete answer

Common mistakes: – Some failed to recognise that the cyclic ester can

be hydrolysed to open the ring

– About 1/3 of candidates unable to give an appropriate intermediate

– Some candidates wrongly used NaBH4 to reduce a carboxylic acid group into a hydroxyl group

– Some omitted the reaction conditions (e.g. “heating” in the hydrolysis of cyclic ester)

– Stated inappropriate reagent (e.g. using concentrated H2SO4 for the hydrolysis of ester)

– Stated inappropriate reaction conditions (e.g. using LiAlH4 in aqueous acid)

– Included inappropriate steps (e.g. oxidise the –OH group of the intermediate with K2Cr2O7/H+(aq))

Q13 – Frequently encountered mistake

– Reaction conditions and intermediate not matched

Q14 – Overall Poor

Only a small number of candidates gave correct and complete answer

– 1/3 of candidates gave the correct balanced equation

– 1/2 of candidates were able to state at least two correct characteristics of transition metal with reference to Mn

Common mistakes in Q14

– Incorrect oxidation number of Mn in MnO4–(aq)

and Mn2+(aq)

– Failed to deduce the catalytic property of Mn2+(aq) from the information provided

– Wrongly stated that MnO2(s) or KMnO4(aq) is the catalyst in the reaction

Good Answer

General and Common Weaknesses

General Comments

• Weak in calculations

• Weak in conversion of units in calculations (e.g. cm3 to dm3)

• Weak in writing / balancing chemical equations.

General and Common Weaknesses

General Comments

• Failed to describe chemical concepts / explaining chemical phenomenon clearly and precisely

• 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 6至 9 題

2017 卷 I

題 6 表現令人滿意

(a) 約66%的考生能夠正確指出應使用的2個危險警告標籤。

(b) (i) 約有一半的考生能夠說明為什麽不應直接滴定濃硫酸。

(b) (ii) 約有一半的考生能夠指出滴定終點的顏色改變。

(b) (iii) 約有三分之二的考生能夠正確算出的樣本的濃度。

題 6 表現令人满意

(c)

• 約有一半的考生能夠正確寫出反應方程式及預期的觀察。

• 為數不少的考生書寫了以下不正確的反應方程式：

Cu(s) + H2SO4(l) CuSO4(aq) + H2(g)

題 7 表現尚可

(a) 考生的表現欠佳，只有接近四分一考生能夠解釋為什麽不能從實驗直接測定生成焓變。

(b) 約有四分一的考生能夠寫出赫斯定律。

題 7 表現尚可

(c)

(i) 約有30%的考生能夠繪畫要求焓變循環及使用準確標示。

(ii) (1)和(2) ，約有一半的考生能夠提供要求的標準條件及計算出C2H2(g) 的生成焓變。

• 最常見的錯誤是「省略／錯誤物態符號」。

題 8 表現尚可

(a) 約有66%的考生能夠書寫所需的平衡反應方程式。

(b) 約有75%的考生能夠繪畫二氧化碳的電子圖。

(c) 只有極少數的考生能夠說明支持使用電動車輛有助紓緩全球暖化的理由。

題 8 表現尚可

(d) 只有一半的考生能夠如「空氣不足」或「氧不足」作為答案。

(e) (i) 約有三分二的考生能夠寫出「催化轉化器」 。

(e) (ii) 剛超過一半考生能夠說明要求的空氣污染物，然而有不少考生提出如「二氧化碳」等錯誤答案。

題 9 表現尚可

• 約有40%的考生能夠提出合適的化學測試來分辨該4個液體，並正確地說明預測觀察。

• 部分考生能準確描述使用溴來測試含碳碳雙鍵的丙烯酸時宜在暗黑環境中進行測試。

• 以下為一些常見的錯誤： –忘記使用酸化或沒有酸化的高錳酸鉀會有不同的測試觀察。

–錯誤以為CH3CO2CH3含有酮官能基。

題9總結

考生書寫化學式和 化學反應式的表現