TEMASEK JUNIOR COLLEGE

2021 JC1 Promotional Examination

Higher 2

NAME:_________________________________________________________________

CG: ___________________ Tutor: _______________________

CHEMISTRY 9729
28 September 2021
Section C: Free Response (1 hour)
Candidates answer on the Question Paper.
Additional materials: Data Booklet

READ THESE INSTRUCTIONS FIRST

Write your Name, Civics Group in all the work you hand in.
Write in dark blue or black pen.
You may use a HB pencil for any diagrams or graphs.
Do not use staples, paper clips, glue or correction fluid.

Section C (40 marks)

Answer all questions.

The use of an approved scientific calculator is expected, where appropriate.

The number of marks is given in brackets [ ] at the end of each question or part question.

For Examiner’s Use

Section A /20
Section B /40
Section C 1 /20
2 /20
Total /100

This document consists of 15 printed pages and 1 blank page

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Section C Free Response

Answer all the questions.

1 When in danger, bombardier beetles can fire a hot, toxic mixture of chemicals at their attacker.
This mixture contains quinone, a compound that is formed by the reaction of 1,4-benzenediol,
C6H4(OH)2, with hydrogen peroxide, H2O2.

C6H4(OH)2(aq) + H2O2(aq) → C6H4O2(aq) + 2H2O(l)

Quinone

(a) The structures of 1,4-benzenediol and quinone are shown below.

1,4-benzenediol Quinone

(i) 1,2-benzenediol is an isomer of 1,4-benzenediol.

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1,2-benzenediol

The boiling point for 1,4-benzenediol is found to be higher than 1,2-benzenediol.

By reference to their structure and bonding, explain the difference in their boiling
points. [3]

(ii) Quinone is found to be almost insoluble in water but highly soluble in CCl4.
Explain the solubility of quinone in these two solvents.
[3]

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(b) Use of Data Booklet is relevant to this question.

To determine the enthalpy change of reaction between 1,4-benzenediol and hydrogen
peroxide, a student first carried out an experiment to determine the enthalpy change of
vapourisation of water, Hvap.
H2O(l) → H2O(g) Hvap
Fig. 1.1 shows the experimental set-up.

Thermometer

Retort Copper
calorimeter

Clamp

Water

Butane
burner

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Fig. 1.1
Experimental procedures:
Steps
1. The copper calorimeter and butane burner were weighed separately.
2. Water was placed in the copper calorimeter using a measuring cylinder and the
copper calorimeter together with the water was weighed.
3. The initial temperature of the water was measured using the thermometer.
4. The butane burner was lit.
5. The butane burner was allowed to burn for 10 minutes before the flame was
extinguished.
6. The maximum temperature was recorded.
7. The copper calorimeter and butane burner were weighed separately.

The data obtained from the experiment was recorded in the following tables:

Table 1.1
Mass of calorimeter / g 10.40
Mass of calorimeter and water before heating / g 110.40
Mass of calorimeter and water after heating / g 88.29

Table 1.2
Mass of butane burner before heating / g 135.50
Mass of butane burner after heating / g 133.90

Table 1.3
o
Initial temperature of water / C 30.0
Maximum temperature of water / oC 100.0

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You may assume the heat capacity for the copper calorimeter is negligible.

(i) Describe the structure and bonding in copper with the aid of a labelled diagram.
[2]

(ii) Write an equation to define the standard enthalpy change of combustion of

butane. [1]

(iii) Using data from Table 1.2, calculate the amount of butane used.
Given that the enthalpy change of combustion of butane is -2877 kJ mol-1,
calculate the heat energy evolved from the combustion of the butane in the
experiment. [2]

The heat energy evolved from the combustion of butane was used to heat the water in
the calorimeter to the maximum temperature and to vapourise the water.

(iv) Calculate the mass of water heated. Hence, determine the heat energy needed
to raise the temperature of the water from its initial temperature to the maximum
temperature. [2]

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(v) Calculate the amount of water that has been vapourised. [1]

(vi) The enthalpy change of vapourisation of water is the heat energy absorbed by
1 mole of water for the conversion from liquid state to gaseous state

H2O(l) → H2O(g) Hvap

Using your answers in (b)(iii) to (b)(v), determine the enthalpy change of

vapourisation of water, Hvap. [2]

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(c) 1,2-benzenediol can be produced synthetically using benzene as the starting material.

Fig. 1.2 shows the synthesis of various organic compounds starting from benzene.

Reagents and
Conditions:

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Reagents and
Conditions:

Fig. 1.2

(i) Name the type of reactions for Reactions 1 to 4. Label your answers clearly. [2]

(ii) Using the boxes provided in Fig. 1.2. State the reagents and conditions
necessary for Reaction 1 and Reaction 4. [2]
[Total: 20]

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2 (a) Chlorine is a highly reactive element, which forms ionic chlorides with metals. Some of
these chlorides, such as sodium chloride and magnesium chloride, are soluble in
water.

(i) Write a balanced equation to define the enthalpy change of solution for
magnesium chloride. [1]

(ii) Table 2.1 shows some thermodynamic data obtained when magnesium
chloride dissolves in water at 25 C.

Table 2.1
Enthalpy change of solution, Hsol −155.4 kJ mol−1
Gibbs free energy of solution, Gsol –126.3 kJ mol–1

Using data provided, calculate the entropy change of solution of magnesium

chloride, Ssol, and explain the significance of its sign. [2]

(iii) The magnitude of lattice energy for magnesium chloride is three times larger
than that for sodium chloride.

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By considering the standard enthalpy change for hydration for the ions, explain
why the standard enthalpy change of solution, Hsol for magnesium chloride
is significantly more exothermic than that for sodium chloride. [3]

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(b) (i) State the properties of a gas necessary for it to approach ideal gas behavior.
[2]

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( pV
(ii) Sketch on the same axes given in Fig. 2.1, the variation of against p at
RT
400 K for 1 mole of

I an ideal gas
II chlorine gas.

Label your graphs clearly.

[2]

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Fig. 2.1

( pV
(iii) Using the same axes in Fig. 2.1, sketch the variation of against p for
RT
one mole of chlorine gas at 800 K.
Explain the difference when compared to the graph drawn for one mole of
chlorine at 400 K. [2]

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(c) Chlorine reacts readily with propene in the dark to form 1,2-dichloropropane.
(i) Name and outline the mechanism of the reaction between
propene and chlorine to form 1,2-dichloropropane.

Show all charges, relevant lone pairs and the movement of electron pairs using
curly arrows. [3]

A sample of pure 1,2-dichloropropane is found to rotate the plane of polarised light.

(ii) What features of 1,2-dichloropropane cause the rotation of the plane polarised
light? [1]

(iii) When a student tried to synthesise 1,2-dichloropropane in the laboratory using

the same reaction, a racemic mixture was obtained.

By considering the mechanism drawn in (c)(i), suggest a reason for this. [2]

(iv) Draw the structures of the stereoisomers present in the racemic mixture. [2]

[Total: 20]

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Additional answer space

If you use the following pages to complete the answer to any question, the question number
must be clearly shown.

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BLANK PAGE

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