2024-DSE

CHEM
PAPER 2 HONG KONG EXAMINATIONS AND ASSESSMENT AUTHORITY
HONG KONG DIPLOMA OF SECONDARY EDUCATION EXAMINATION 2024

CHEMISTRY PAPER 2

11:45 am — 12:45 pm (1 hour)

This paper must be answered in English

INSTRUCTIONS

(1) This paper consists of THREE sections, Section A, Section B and Section C. Attempt ALL
questions in any TWO sections.

(2) Write your answers in the DSE(D) Answer Book provided. Start each question (not part of a
question) on a new page.

(3) A Periodic Table is printed on page 8 of this Question Paper. Atomic numbers and relative
atomic masses of elements can be obtained from the Periodic Table.

OF BHEH KT/ TR RRE Not to be taken away before the

Hong Kong Examinations and Assessment Authority end of the examination session
All Rights Reserved 2024

2024-DSE-CHEM 2-1 1
Section A Industrial Chemistry

Answer ALL parts of the question.

L@ O Ammonium sulphate is a compound derived from ammonia.

1) Name the industrial process to produce ammonia. (1 mark)

[#))] Explain how ammonium sulphate can alleviate the problem of inadequate food supply
in the world. (1 mark)

(i) Membrane electrolytic cells and flowing mercury cells are used to produce chlorine in the
chloroalkali industry.

) What is the raw material used in flowing mercury cells ? (1 mark)

@) Give one advantage of using membrane electrolytic cells over flowing mercury cells
in the chloroalkali industry. (1 mark)

(i) Alcoholic drinks can be produced by fermentation of sugars in the presence of yeast. State the
function of yeast in the fermentation of sugars. (1 mark)

(b) Ethanoic acid can be manufactured by the following two methods :

Method 1 Method 2
Reaction 2CH3(CH2)2CH3 + 50, —> 4CH3;COOH + 2H,0 | Three consecutive steps are involved :
in the Step (1): CHsOH + HI—> CH; + H;,0
reaction The reaction is exothermic.
Step (2): CHsl + CO —> CH;COI
chamber
Step (3): CH;COI + H,0—> CH;COOH + HI

The overall reaction is exothermic.

Reaction | * 110
- 120 °C and 60 atm N 150 —200 °C and 30 — 60 atm
conditions | ¢ Cobalt(II) ethanoate acts as the catalyst. . Iridium compound acts as the catalyst.

Draw an energy profile for the catalysed reaction involved in Method 1. Label the axes.
(2 marks)
Based on the equation shown, calculate the atom economy of Method 1.
(Relative molecular masses :
CH;3(CH,),CH; = 58.0, O, = 32.0, CH;COOH = 60.0, H,0 = 18.0) (1 mark)

Write the chemical equation for the overall reaction involved in Method 2. (1 mark)

Based on the given information, suggest a reason why HI can also be considered as a catalyst
in Method 2. (1 mark)

Based on the given information and the principles of green chemistry, suggest a reason why
some people have the view that

[0)] Method 1 can be considered as ‘greener’ than Method 2. (1 mark)

@) Method 2 can be considered as ‘greener’ than Method 1. (1 mark)

2024-DSE-CHEM 2-2 2
 1. (c) The equation for the decomposition of N2Os(g) is given below :

2N;05(g) - 2N204(g) + Oa(g)

@) Suggest how the progress of this decomposition can be followed. (1 mark)

(ii) The rate equation for this decomposition is shown below :

Rate = k[N,0s(g)]*

where £ is the rate constant and a is the order of reaction with respect to N2Os(g).

It is given that the rate constant of this decomposition at 318 K is 4.29 x 104571,

m ‘What is the numerical value of @ ? (1 mark)

) The activation energy of this decomposition is 108.5 kJ mol™. Calculate the rate
constant of this decomposition at 338 K.
(Gas constant R = 8.31 ] K™! mol™'; Arrhenius equation : log k = constant — £y )
2.3RT
(2 marks)

(iii) The graph below shows the Maxwell-Boltzmann distribution curve for a sample of N,Os(g) at
temperature T).

Ty
number of molecules

kinetic energy

(1) In the following graph, which of the curves A, B and C can represent the Maxwell-
Boltzmann distribution curve of the same sample of N2Os(g) at a temperature higher
than T; ?
number of molecules

>

kinetic energy
(1 mark)

@) With the help of Maxwell-Boltzmann distribution curves, explain why increasing the
temperature will increase the rate of a gaseous reaction. (3 marks)

END OF SECTION A

2024-DSE-CHEM 2-3 3 Go onto the next page

Section B Materials Chemistry

Answer ALL parts of the question.

2. @ O Rubber bands are made of vulcanised rubber. When a rubber band is stretched and then
released, it can return to its criginal shape. In terms of molecular structure, explain the elasticity
of rubber bands. (2 marks)

(ii) Chitin is a component of the exoskeleton of lobsters. A part of the structure of chitin is shown
below :

HyOH H,OH

OH
H
& o)

X CH,OH X

(1) WhatisX? (1 mark)

) In terms of the structure of chitin, explain why chitin is related to the hardness of the
exoskeleton of lobsters. (1 mark)

(i) Name the liquid crystal phase in which the molecules are arranged along the same direction
and positionally aligned with each other along a straight line. (1 mark)

(b) Polylactide (PLA) is a synthetic biomaterial. Consider the following information related to PLA :

Structure
)
0——(|?—-C
H n

Type of polymer condensation polymer

Raw material corn starch
Use surgical threads for stitching wounds

(0] State the functional group of PLA. (1 mark)

(i) Draw the structure of the monomer of PLA. (1 mark)

(iii) Suggest why PLA is classified as a condensation polymer. (1 mark)

(iv) According to the principles of green chemistry, explain why it is environmentally friendly to
use corn starch as the raw material to produce PLA. (1 mark)

W Suggest one property of PLA making it suitable to be used as surgical threads. (1 mark)

(vi) PLA can also be used to make disposable cups. Explain whether PLA disposable cups are
suitable to be made by compression moulding. (1 mark)

(vii) Suggest one reason why PLA dissolves in ethyl ethanoate but not in hexane. (1 mark)

2024-DSE-CHEM 2-4 4
 2. © The crystal structures of iron at atmospheric pressure under different temperatures are shown below :

Temperature Crystal structure

below 912 °C body-centred cubic
912°C-1394°C cubic close-packed

1394°C—-1538°C body-centred cubic

@ ‘What is the coordination number of an iron atom in the crystal structure at 25 °C ? (1 mark)

(i) Iron has a cubic close-packed crystal structure at 1000 °C.

6] Draw a unit cell of iron crystal at 1000 °C. (1 mark)

@ How are the iron atoms arranged in this crystal structure ? (1 mark)

(iii) In view of the same crystal structure of iron at 25 °C and 1450 °C, a student deduced that the
density of iron is the same at these two temperatures. Explain whether this deduction is correct.
(1 mark)

(iv) Some information about iron and Kevlar at room temperature is listed below :

Iron Kevlar
Relative tensile strength 1 6.7
Density / g cm™ 7.87 1.44
Conduction of heat good poor

1) Which one of the above materials is a better choice for making bullet-proof vests ?
Based on the given information, suggest TWO reasons to explain your answer.
(2 marks)

?) Both Kevlar and nylon-6,6 are polyamides. Suggest TWO reasons why Kevlar has a
higher rigidity than nylon-6,6. (2 marks)

END OF SECTION B

2024-DSE-CHEM 2-5 s
Section C Analytical Chemistry

Answer ALL parts of the question.

3. @ @ Suggest a chemical test to distinguish between compounds A and B shown below :

CH;3;CH,CH,COCH;3 CH;CH,CH,COOH
A B (2 marks)

(i) Paper chromatography can be used to separate the dye components in black ink. Draw a
labelled diagram for the set-up required for this paper chromatography experiment. (2 marks)

(iiiy Some organic solvents can be used to extract organic compounds from their aqueous solutions.
Suggest one property of these solvents rendering them to be used in the liquid-liquid extraction.
(1 mark)

(b) Asolid sample consists of PbCO; and a small amount of CuCOs. The following steps were performed
to estimate the percentage by mass of PbCO; in the sample.

Step (1): Excess dilute HNOs(aq) was added to 2.03 g of the sample to give a mixture X.
Step (2): Excess NHs(aq) was added to the mixture X to form a precipitate.
Step (3): The precipitate was collected by filtration. After washing and drying, the precipitate
was weighed and its mass was 1.47 g.

[0] Explain why excess dilute HNOs(aq) was added in Step (1). (1 mark)

(ii) After the addition of excess NHs(aq) to the mixture X in Step (2), apart from the precipitate
formed, state another expected observation. (1 mark)

i)y (1) Why is it necessary to wash and dry the precipitate before weighing in Step (3) ?
(2 marks)
?) ‘What is the P precipitate ? (1 mark)

A3) Estimate, by calculation, the percentage by mass of PbCOs in the solid sample.
(Relative atomic masses : H=1.0, C =12.0, O = 16.0, Pb =207.2) (2 marks)

(¢) Formaldehyde (systematic name : methanal) is an indoor air pollutant. Instrumental analytical methods
are generally used for analysing formaldehyde in indoor air.

(i) With reference to the information given in the table below, suggest how the presence of the
functional group of formaldehyde can be detected in an indoor air sample by using infra-red
spectroscopy.

Characteristic Infra-red Absorption Wavenumber Ranges (Stretching modes)

Bond Compound type ‘Wavenumber range / cm™
C=C | Alkenes 1610 to 1680
C=0 | Aldehydes, ketones, carboxylic acids and derivatives 1680 to 1800
C=C | Alkynes 2070 to 2250
C=N | Nitriles 2200 to 2280
O-H | Acids (hydrogen-bonded) 2500 to 3300
C-H | Alkanes, alkenes, arenes 2840 to 3095
O-H | Alcohols (hydrogen-bonded) 3230 to 3670
N-H | Amines 3350 to 3500
(1 mark)

2024-DSE-CHEM 2-6 6
3 © G The mass spectrum of formaldehyde shows a strong peak at m/z = 29. Suggest one chemical
species corresponding to this peak. (1 mark)

(iii) The following steps were carried out to determine the amount of formaldehyde in an indoor
air sample.

Step (1) :
Excess reagent R reacts with formaldehyde to give a green colour solution. Using the same
procedure, standard formaldehyde solutions of various concentrations were separately treated
with excess R to give green solutions of different colour intensities. The absorbances of these
green solutions were measured using an instrument Y. The following graph was plotted from
the data obtained :

>
1
0.8
o
8 = /’
g o6 ad
s -~
| 04 //./
1
02
/’.’
0.0 e >
0.0 0.1 02 03 0.4
concentration of formaldehyde / mg dm™

Step (2) :
An indoor air sample with a volume of 0.1 m® was collected inside a room. The sample was
treated with excess reagent R to give 0.03 dm’ of a green solution S. The absorbance of the
green solution S measured with the instrument Y was 0.5.

(1) Name the instrument Y. (1 mark)

(2) State the relationship between absorbance and concentration of formaldehyde in the
above graph. (1 mark)

3) By using the above graph, calculate the mass of formaldehyde in solution S, in mg.
(2 marks)

“@ If the concentration of formaldehyde in indoor air is less than 0.1 mg m™, the quality
of indoor air is considered as good. Based on the given experimental result, explain
whether the quality of the air inside the room is good. (1 mark)

®) Suggest why instrumental analytical methods are generally used for measuring the
formaldehyde levels in indoor air. (1 mark)

END OF SECTION C
END OF PAPER

2024-DSE-CHEM 2-7 7
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