HKDSE Chem FX Mock Exam Paper 1 2012 Set 1 Eng Ans

HONG KONG DIPLOMA OF SECONDARY EDUCATION EXAMINATION

CHEMISTRY PAPER 1

(MOCK EXAMINATION)

Suggested Answers

Section A

Question No. Key Question No. Key

1. D 19. C

2. C 20. B

3. C 21. A

4. C 22. D

5. A 23. C

6. A 24. B

7. D 25. D

8. D 26. B

9. C 27. B

10. A 28. A

11. B 29. B

12. D 30. D

13. D 31. B

14. C 32. A

15. C 33. C

16. B 34. A

17. D 35. C

18. A 36. D

Explanation of selected difficult problems

4. As the oxide of Z cannot be reduced by carbon reduction or direct heating, it is the most reactive metal among the three, and is high in the reactivity series. Y can be extracted by heating alone, so it must be the least reactive metal among the three. As Y is the least reactive, it should be extracted and discovered easier and earlier than Z. Calcium oxide cannot be reduced by carbon reduction.

HKDSE CHEMISTRY Mock Exam Paper 1 (Set 1) Suggested Answers CHEMISTRY© Aristo Educational Press Ltd. 2011 A1

Set 1

6. Number of moles of NaCl = mol = 0.040 mol

As 1 mole of NaCl contains 2 moles of ions, the number of moles of ions in 2.340 g of NaCl = 0.040 2 = 0.080 mol

Formula Number of moles of ions

KCl 2 = 0.080

ZnCl2 3 = 0.069

CaSO4 2 = 0.053

MgBr2 3 = 0.090

7. Assume that the relative atomic mass of X is x.

Percentage by mass of X =

1 − 0.25 =

x = 159.8

9. The graph shows a loss in mass. Most likely, a gas was formed and escaped from the beaker. K2CO3(aq) + 2HCl(aq) 2KCl(aq) + H2O(l) + CO2(g)

10. Ammonia solution is an alkali that does not contain metal ions.

11. From the results of flame test, we predict that X and Q are sodium ion and calcium ion respectively. So the answer should be B or D. When we look at the reactions between XZ and acidified barium chloride solution, the products are barium hydrogencarbonate and sodium chloride. Both of them are colourless and soluble in water.

14. There is a total of 4 carbon atoms and 8 hydrogen atoms in C4H8, so 4 CO2 and 4 H2O are produced. Thus 6 O2 are needed.

17. Chlorine behaves as both an oxidizing agent and reducing agent in the reaction. It is oxidized to OCl(aq) and reduced to Cl(aq) at the same time.

18. For (2), the porous nickel electrodes also act as catalysts. Graphite rods do not have any catalytic property. For (3), hydrogen is fed into the anode compartment and oxygen is fed into the cathode compartment.

19. At the anode, Br(aq) ions are preferentially discharged to form Br2(g) due to the concentration


effect. As electrolysis goes on, Br(aq) ions and H+(aq) ions discharge continuously, OH(aq) ions and Na+(aq) ions accumulate. Therefore, the solution gradually changes to sodium hydroxide solution.

20. The change in internal energy of the reaction is the same in both systems. Since the reaction in the open system involves work done on the surroundings, the heat change of the reaction in the open system is smaller than that in the closed system.

21.

By applying Hess’s Law,

ΔH + ΔH 3 = ΔH 1 + ΔH 2

∴ ΔH = ΔH 1 + ΔH 2 ΔH 3

= x + 2y z

23. Ammonium chloride consists of ammonium ion (NH4+) and chloride ion (Cl). Solid

ammonium chloride does not conduct electricity. It conducts only when molten or in aqueous states. Thus it is an electrolyte.

26. During the reaction, the mass of marble chips decreases and carbon dioxide gas is given off. The carbon dioxide gas cannot escape from the flask as it is stoppered. Therefore, the mass of the reaction mixture remains unchanged.

28. Initial number of moles of SO2Cl2(g) =

= 0.10 mol

Initial concentration of SO2Cl2(g) = = 0.025 mol dm3

Equilibrium concentration of Cl2(g) = = 0.01725 mol dm3


+O2(g)

C2H2(g) + 2H2(g)

+O2(g)

ΔH

ΔH 2 ΔH 1 ΔH 3

+O2(g)

2CO2(g) + 3H2O(l)

C2H6(g)

Concentration (mol dm3) SO2Cl2(g) SO2(g) + Cl2(g)Initial 0.025 0 0

Change 0.01725 +0.01725 +0.01725Equilibrium 0.025 0.01725

= 7.75 × 1030.01725 0.01725

Kc =

Kc = = 0.0384 mol dm3

29. As the forward reaction is exothermic, increasing the temperature shifts the equilibrium position to the left producing more N2(g) and H2(g), thus the value of Kc decreases.

31. (2) is incorrect. The ketone group should also be reduced to a secondary alcohol,

.

33. Both aluminium oxide and silicon dioxide do not react with water.

34. MnO4 ions are reduced by chloride ions to form MnO2, which appears as brown precipitate.

35. The first statement is incorrect but the second statement is correct. Only alkanols of short hydrocarbon chain (e.g. the first four members of alkanols) are soluble in water.

36. For the first statement, a catalyst is neither a reactant nor a product in a reaction. For the second statement, some catalysts can slow down a reaction.

Section B

PART IMarks

1. (a) Calcium carbonate 1

(b) Calcium carbonate + dilute hydrochloric acid

calcium chloride + carbon dioxide + water 1

(c) Carbon dioxide. It turns limewater milky. 2

(d) His statement is correct. Calcium carbonate is an ore of calcium metal. We can obtain calcium metal by electrolysis of its molten ores. 2

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Marks

2. (a) To increase electrical conductivity. 1

(b) Blue 1

Copper(II) ions 1

(c) Orange 1

Dichromate ions 1

(d) Irritant 1

(e) The bulb does not light up. 1

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3. (a) Methyl orange 1

(b) From yellow to orange 1

(c) (i) Ignore the first titration data.

Average volume of sulphuric acid used

= = 15.20 cm3 1

(ii) 2NH3(aq) + H2SO4(aq) (NH4)2SO4(aq)

Number of moles of sulphuric acid used

= 0.1 × mol = 0.00152 mol 1

From the equation, mole ratio of NH3 : H2SO4 = 2 : 1

Number of moles of ammonia in 25.0 cm3 ammonia solution

= 0.00152 2 mol = 0.00304 mol 1

Molarity of ammonia solution = M = 0.122 M 1

(d) Ammonium sulphate 1

It is used to make fertilizer. 1

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4. (a) Formula mass of H2O = 1.0 × 2 + 16.0 = 18.0 g

Mass of H in 9.0 g of water = 9.0 × g = 1.0 g 1


(b) Mass of C in 5.8 g of X = (5.8 − 1.0) g = 4.8 g 1

(c) Since all the C in CO2 and H in H2O came from the same compound,

MarksC H

Masses (in g) 4.8 1.0

Number of moles (mol) = 0.4 = 1.0

Relative number of moles = 1 = 2.5

Simplest whole number ratio 1 × 2 = 2 2.5 × 2 = 52

∴ The empirical formula of compound X is C2H5. 1

5

5. (a) Chloroethene 1

(b) A and C 1, 1

(c) A:

1

C:

1

(d) Polyvinyl chloride 1

It can be used to make pipes/ shower curtains/ tablecloths/ raincoats/ water hoses/ electrical wire insulation. 1

7

6. (a) Since electrons flow from metal Y to metal X, indicating that metal Y is more reactive. ` 1

(b) Metal X. 1

(c) (i) It can be prepared by soaking a piece of filter paper in sodium nitrate solution/sodium chloride solution. 1

(ii) To complete the circuit by allowing ions to move from one half-cell to another half-cell. 1

It provides ions to balance the charges in the solutions of the two half-cells. 1


5

Marks

7. (a) 2HCl(aq) + Na2CO3(aq) 2NaCl(aq) + CO2(g) + H2O(l) 1

(b) The temperature of the solution rises. 1

Gas bubbles are evolved. 1

(c) Mass of the reaction mixture = (20.0 + 10.0) cm3 × 1.0 g cm3 = 30.0 g 1

Heat released = m × c × ΔT

= 30.0 g × 4.2 J g1 K1 × 15.0 K

= 1890 J 1

Number of moles of HCl = 1.0 mol dm3 × 0.02 dm3 = 0.02 mol

Number of moles of Na2CO3 = 1.0 mol dm3 × 0.01 dm3 = 0.01 mol

∴ number of moles of H2O formed = 0.01 mol 1

Heat released per mole of H2O formed = = 189.0 kJ mol1

∴ the enthalpy change of the reaction is 189.0 kJ mol1. 1

(No mark if negative sign is omitted.)

(d) The carbon dioxide released brings away some of the heat energy. 1

There is heat loss from the reaction mixture to the surroundings. 1

(Accept any other possible reasons.)

9

8. There are three common methods to extract a metal from its ore:

Heating the metal ore alone:

is used to extract unreactive metals 1

metals at the bottom of the reactivity series / e.g. mercury / silver / is obtained by heating mercury(II) oxide/ silver oxide 1

Heating metal ore with carbon

heating metal ores with carbon is used to extract less reactive metals 1

metals at the middle of the reactivity series / e.g. iron / is obtained by heating iron(III) oxide with carbon / carbon removes the oxygen from iron(III) oxide 1

Electrolysis of molten metal ore

extraction of metals by electrolysis of their molten ores is used to extract very reactive metals 1

metals at the top of the reactivity series / e.g. aluminium / is obtained by electrolysis of its molten oxide 1

Effective communication (3 marks)

9HKDSE CHEMISTRY Mock Exam Paper 1 (Set 1) Suggested Answers CHEMISTRY© Aristo Educational Press Ltd. 2011 A7

Marks

PART II

9. (a) Kc = [H2S(g)]eqm[NH3(g)]eqm 1

(b)

Concentration (mol dm3)

NH4HS(s) H2S(g) + NH3(g)

Initial 0 0.60

Change +0.270.87 0.60

= +0.27

Equilibrium 0.27 0.87

1

Kc = [H2S(g)]eqm[NH3(g)]eqm

= (0.27 mol dm3)(0.87 mol dm3) 1

= 0.2349 mol2 dm6 1 + 1

(c) Let V be the volume of the sealed flask.

Doubling the volume of the sealed flask, the new volume = 2V

The initial number of moles of ammonia gas = molarity × volume = 0.60 × V

The initial concentration of ammonia gas =

= = 0.30 mol dm3 1

Let x mol dm3 be the change in the concentration of H2S(g).

Concentration (mol dm3)

NH4HS(s) H2S(g) + NH3(g)

Initial 0 0.30

Change +x +x

Equilibrium x 0.30 + x

1

0.2349 = [H2S(g)]eqm[NH3(g)]eqm

0.2349 = (x)(0.30 + x) 1

0.2349 = 0.3x + x2

x = 0.357 or x = 0.657 (rejected) 1

∴ [NH3(g)]eqm = 0.30 + 0.357 = 0.657 mol dm3 1

10Marks

10. (a)HKDSE CHEMISTRY Mock Exam Paper 1 (Set 1) Suggested Answers CHEMISTRY© Aristo Educational Press Ltd. 2011 A8

3

(b) (i) Concentrated sulphuric acid and heating under reflux 2

(ii) The ester is ethyl propanoate. 1

(c) The overall percentage yield = 70% × 67% × 42% = 19.7% 1

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11. 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(l)

C2H4(g) + 3O2(g) 2CO2(g) + 2H2O(l)

Let the volume of C2H2(g) be x cm3, then the volume of C2H4(g) is (10.0 x) cm3.

Volume of O2(g) reacted with C2H2(g) = cm3 = 2.5x cm3 1

Volume of O2(g) reacted with C2H4(g) = 3 × (10.0 x) cm3 = 30.0 3x cm3 1

Therefore, 30.0 2.5x (30.0 3x) = 2.0

x = 4.0

Volume of C2H2(g) = 4.0 cm3 1

Volume of C2H4(g) = 6.0 cm3 1

4

12. (a) (i) Sulphur dioxide:

1

Sulphur trioxide:

1

(ii) 2SO2(g) + O2(g) 2SO3(g) 1

Vanadium(V) oxide/V2O5 1

(iii) Colourless 1

(b) (i) 1

(ii) Orange 1

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HKDSE Chem FX Mock Exam Paper 1 2012 Set 1 Eng Ans