HKDSE Chem FX ExamS5 2011 Set1 Eng

HKDSE CHEMISTRY S5 Exam Paper (Set 1) CHEMISTRY © Aristo Educational Press Ltd. 2011 1

HONG KONG DIPLOMA OF SECONDARY EDUCATION EXAMINATION

CHEMISTRY

SECONDARY FIVE

EXAMINATION PAPER (SET 1)

Time allowed: 2 hours 30 minutes This paper must be answered in English.

INSTRUCTIONS 1. There are TWO sections, A and B, in this Paper.

Section A carries 36 marks and Section B carries 84 marks.

2. Section A consists of multiple-choice questions

while Section B contains conventional questions. 3. Answer ALL questions. 4. All multiple-choice questions carry equal marks. 5. You should mark only ONE answer for each

multiple-choice question. If you mark more than one answer, you will receive NO MARKS for that question. No marks will be deducted for wrong answers.

6. A Periodic Table is printed on the back of this

question paper. Atomic numbers and relative atomic masses of elements can be obtained from the Periodic Table.

COMMON PHYSICAL CONSTANTS

Avogadro constant (L) = 6.0 × 1023 mol–1 Molar volume of gas at s.t.p. = 22.4 dm3 mol–1

Molar volume of gas at room conditions = 24.0 dm3 mol–1

Name

Class

Class no.

Section B Question

No. Marks

Full Marks

1 − 8

2 − 7

3 − 7

4 − 6

5 − 7

6 − 5

7 − 7

8 − 9

9 − 7

10 − 8

11 − 5

12 − 8

Total 8 4


Section A (Multiple-choice Questions) Answer ALL questions. Choose the best answer for each question. PART I 1. The percentage by mass of the three most abundant elements on the Earth’s crust

increases in the ascending order: A. Aluminium, Silicon, Oxygen B. Oxygen, Silicon, Aluminium C. Oxygen, Aluminium, Silicon D. Silicon, Oxygen, Aluminium 2. Which of the following diagrams represents a mixture of elements? A. B. C. D.

3. Which of the following statements concerning the formation of magnesium ions from

magnesium atoms is correct? A. The atomic number of magnesium decreases by 1. B. The mass number of magnesium decreases by 1. C. The charge of magnesium increases by 1. D. The number of shells occupied by electrons in a magnesium atom decreases by 1. 4. Silicon carbide has a structure similar to that of diamond. Which of the following

statements concerning silicon carbide is correct? A. It can be used as a cutting tool. B. It can be used as a lubricant. C. It can be used as jewellery. D. It is a good conductor of electricity. 5. An oxide of iodine, X, reacts with carbon monoxide as shown in the equation below: X + 5CO → I2 + 5CO2

Which of the following is the correct formula of X? A. I2O B. IO2

C. IO5

D. I2O5 6. Which of the following processes would NOT produce hydrogen gas? A. Adding calcium to water B. Adding copper to dilute sulphuric acid C. Adding magnesium to dilute hydrochloric acid D. Passing steam over red hot iron


7. Consider the following reaction: Ba(NO3)2(aq) + H2SO4(aq) → 2HNO3(aq) + BaSO4(s) Which of the following ions are the spectator ions? (1) Ba2+

(2) H+

(3) SO42–

(4) NO3–

A. (1) and (3) only B. (1) and (4) only C. (2) and (3) only D. (2) and (4) only 8. What is the percentage by mass of carbon in glucose, C6H12O6? A. 6.7% B. 25% C. 40% D. 60% 9. A white solid is found near the rim of a reagent bottle containing sodium hydroxide

solution. The white solid is likely to be A. sodium oxide. B. sodium carbonate. C. sodium hydrogencarbonate. D. sodium hydroxide. 10. A sample of concentrated hydrochloric acid has a density of 1.21 g cm−3 and contains

32.3% of HCl by mass. What is the concentration (correct to one decimal place) of hydrochloric acid in the sample?

A. 3.9 M B. 10.3 M C. 10.7 M D. 14.3 M 11. Mosquitoes inject mild acid into the skin when they bite us. Which of the following

substances should NOT be used to treat the affected area? A. Plain soap B. Soda C. Toothpaste D. Black coffee 12. Which of the following pieces of apparatus should be used when a standard solution is

prepared from a pure liquid? (1) Pipette (2) Volumetric flask (3) Electronic balance A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)


13. A compound has the following structure:

Which of the following is the systematic name for this compound? A. 2-bromomethyl-2-chloro-2-methylethanol B. 3-bromo-2-chloro-2-methylpropan-1-ol C. 1-bromo-2-chloro-2-methylpropan-3-ol D. 1-bromo-2-chlorobutan-2-ol 14. Which of the following molecular formulae represents an alkanoic acid? A. CH2O B. CH2O2

C. C2H2O2

D. C2H6O2 15. Which of the following bonds is the least polar? A. P–F B. O–F C. Si–N D. B–Cl 16. Which of the following species is isoelectronic with that of carbonate ion? A. NO3

–

B. CH3+

C. SO32–

D. PO43–

17. Consider the following chemical equation: x CH4(g) + y NO3

–(aq) + 8H+(aq) → x CO2(g) + z N2(g) + 14H2O(l) Which of the following combinations is correct? x y z A. 2 6 3 B. 2 4 2 C. 5 8 4 D. 8 10 5 18. Which of the following chemical reactions involves the greatest change in the

oxidation number of nitrogen? A. Mg + 2NO3

– + 4H+ → Mg2+ + 2NO2 + 2H2O B. 2NO + O2 → 2NO2

C. N2 + O2 → 2NO D. N2 + 3H2 → 2NH3


19. Which of the following equations shows the reaction occurred at the positive electrode

of a silver oxide cell? A. Zn(s) → Zn2+(aq) + 2e– B. Zn(s) + 2OH–(aq) → ZnO(s) + H2O(l) + 2e–

C. Ag2O(s) + H2O(l) + 2e– → 2Ag(s) + 2OH–(aq) D. Ag+(aq) + e– → Ag(s) 20. Which of the following conditions is NOT required in the definition of ‘standard

conditions’? A. Temperature of 25°C B. Pressure of 101.3 kPa C. Concentrations of all solutions are 1.0 mol dm–3. D. All reactants and products are in their gaseous state. 21. Which of the following equations represents the standard enthalpy of formation for

hydrogen iodide? A. H2(g) + I2(g) → 2HI(g)

B. 2

1 H2(g) + 2

1 I2(g) → HI(g)

C. H(g) + I(g) → HI(g) D. H+(g) + I–(g) → HI(g) 22. Consider the following reaction: 2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(l) Given the following data: Compound Standard enthalpy change of formation NaHCO3(s) –948 kJ mol–1

Na2CO3(s) –1131 kJ mol–1

CO2(g) –395 kJ mol–1

H2O(l) –286 kJ mol–1

What is the enthalpy change for the reaction? A. +84 kJ mol–1

B. –84 kJ mol–1

C. –864 kJ mol–1

D. +864 kJ mol–1 Directions: Each question below (Questions 23 to 24) consists of two separate statements.

Decide whether each of the two statements is true or false; if both are true, then decide whether or not the second statement is a correct explanation of the first statement. Then select one option from A to D according to the following table:

A. Both statements are true and the 2nd statement is a correct explanation of the 1st

statement. B. Both statements are true and the 2nd statement is NOT a correct explanation of the 1st

statement. C. The 1st statement is false but the 2nd statement is true. D. Both statements are false.


1st statement 2nd statement

23. Concentrated sulphuric acid can be used to dry any gas.

24. Thermal decomposition of calcium

carbonate is a redox reaction.

Concentrated sulphuric acid has a high affinity for water. Calcium carbonate is converted into calcium oxide in the decomposition.

END OF PART I


PART II 25. Consider the following compounds: (1) CH3CH2CH2SH (2) CH3CH2CH2OH (3) CH3CH2CH2CH3

What is the order of increasing boiling point of the compounds? A. (3) < (2) < (1) B. (1) < (2) < (3) C. (2) < (3) < (1) D. (3) < (1) < (2) 26. Which of the following compounds is the most soluble in water? A. Propane B. Propene C. Propan-1-ol D. Hexan-1-ol 27. Which of the following tests can be used to distinguish between CH3CH2CHO and

CH3CH(OH)CH3? A. Heating with acidified potassium dichromate solution B. Adding phosphorus pentachloride C. Adding bromine (dissolved in tetrachloromethane) in darkness D. Adding cold acidified potassium permanganate solution 28. Consider the following equilibrium: CH4(g) + H2O(g) CO(g) + 3H2(g) ∆H = +206 kJ mol–1

Which of the following statements are correct when the temperature increases? (1) The equilibrium position shifts to the right. (2) The rates for both forward and backward reactions increase. (3) The forward reaction becomes exothermic. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) 29. Consider the following two equilibria and their equilibrium constants: Equilibrium constant 2SO2(g) + O2(g) 2SO3(g) K1

2SO3(g) 2SO2(g) + O2(g) K2

Which of the following expressions correctly relates K1 and K2? A. K2 = K1

2

B. K22 = K1

C. K2 = K1

D. K2 = 1K

1


Directions: Questions 30 to 31 refer to the following diagrams.

The above four diagrams represent a mixture of gases at different times at the same temperature. The equation of the reaction is shown below:

A2(g) + B2(g) 2AB(g) where Kc = eqm2eqm2

2eqm

(g)][B(g)][A

[AB(g)]

30. Which of the above diagrams has the highest value of reaction quotient? 31. If diagram B represents the system which is already at equilibrium at that temperature,

which of the above diagrams represents the equilibrium position when a positive catalyst is added?

32. Which of the following changes is correct when a positive catalyst is added to an

equilibrium system with an endothermic forward reaction? A. More products are formed within the same period of time of reaction. B. The equilibrium position of the reaction shifts to the left. C. The equilibrium position of the reaction shifts to the right. D. No change in the concentrations of the reactants and products. 33. The rate of reaction of magnesium with acid is studied at 25°C. Which of the following

cases will have the highest initial reaction rate? A. 0.5 g magnesium powder reacts with 100 cm3 of 1.0 mol dm–3 HCl. B. 0.5 g magnesium powder reacts with 50 cm3 of 2.0 mol dm–3 HCl. C. 0.5 g magnesium powder reacts with 100 cm3 of 1.0 mol dm–3 CH3COOH. D. 0.5 g magnesium powder reacts with 50 cm3 of 2.0 mol dm–3 CH3COOH. 34. Which of the following properties is/are common for iron and calcium? (1) Existence of variable oxidation states (2) Reaction with dilute hydrochloric acid to form hydrogen (3) Corrosion upon exposure to moist air A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only


Directions: Each question below (Questions 35 to 36) consists of two separate statements. Decide whether each of the two statements is true or false; if both are true, then decide whether or not the second statement is a correct explanation of the first statement. Then select one option from A to D according to the following table:

A. Both statements are true and the 2nd statement is a correct explanation of the 1st

statement. B. Both statements are true and the 2nd statement is NOT a correct explanation of the 1st

statement. C. The 1st statement is false but the 2nd statement is true. D. Both statements are false.

1st statement 2nd statement 35. One mole of methane occupies the

same volume as one mole of carbon dioxide at the same temperature and pressure.

36. The best method to purify ethanol

contaminated with ethanoic acid is to mix the liquid with sodium hydroxide solution.

The molar volume of any gas is 24.0 dm3

mol–1 at room temperature and pressure. Ethanoic acid reacts with sodium hydroxide to form sodium ethanoate and water.

END OF PART II

END OF SECTION A


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Section B PART I Answer ALL questions. Write your answers in the spaces provided. 1. Consider the following situation and answer the questions that follow.

Imagine that your elder sister, Angel, is a scientist. After 20 years of hard work, she finally confirmed the existence of a new element, with atomic number 119, following the last element (with atomic number 118) in the Periodic Table. This element was artificially made in the laboratory and finally approved to name after your elder sister. It is called ‘Angelium’. After seeking opinions, the chemical symbol of ‘Angelium’ was designated to be ‘Ae’. Your elder sister concluded that the chemical properties of ‘Angelium’ are very similar to elements such as potassium and caesium.

(a) (i) In which group and period of the Periodic Table should ‘Angelium’ be

placed? (ii) What is the name commonly given to the group of elements, in which

‘Angelium’ belongs to? (b) The following three options of the suggested symbols of ‘Angelium’ were

considered to be inappropriate. Give ONE mistake for each of the suggested symbols.

Suggested Symbol Mistake Option 1 Ang Option 2 Ag Option 3 AE

(c) (i) Suggest ONE reason why the chemical properties of ‘Angelium’ are very

similar to that of elements such as potassium and caesium. (ii) Potassium can form potassium oxide (K2O) when exposed to air. Write the

chemical equation for the reaction between ‘Angelium’ and oxygen to form ‘Angelium’ oxide.

(d) Draw an electron diagram, showing electrons in the outermost shell only, for the

compound formed in (c)(ii).

(8 marks)


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2. Aluminium chloride adopts different structures depending on the temperature and its

state. In aqueous state, aluminium chloride is hydrated by the surrounding water molecules to form [Al(H2O)6]

3+ ion and Cl– ion, in which six water molecules directly attach to the central aluminium ion. In liquid and gaseous states, aluminium chloride exists as dimers. At temperatures above 200°C, the dimers dissociate into a gaseous form, which is analogous to BF3 in terms of structure.

(a) (i) Draw the three-dimensional structure of [Al(H2O)6]

3+. (ii) Give the name of the shape of this cation. (iii) State the type of bond that exists between aluminium and oxygen in water. (b) Draw the three-dimensional structure of gaseous aluminium chloride dimer. You

should show clearly the type of bonds that exists between aluminium and chlorine.

(c) (i) Draw the structure of the gaseous form at temperatures above 200°C. (ii) Give the name of the shape of this gaseous form and state its bond angles.

(7 marks)


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3. Explain the following phenomena. (a) Explain why bromine can dissolve in 1,1,1-trichloroethane in terms of the specific

types of intermolecular forces of attraction present. (b) Explain why SbCl3 has a net dipole moment, whereas SbCl5 does not. Draw

suitable diagrams to illustrate your answer.

(7 marks)


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4. A type of portable fuel cell operates using a liquid fuel, such as petrol and methanol, in

which chemical energy is converted into heat energy, and finally into electrical energy.

Let us look at the working principle of a portable hair curler. Liquid fuel is put in a cartridge and the vaporized fuel is mixed with air and passed into a combustion chamber coated with platinum. When the fuel gas/air mixture is ignited, an exothermic reaction heats up the curler and will continue until the supply of fuel is turned off.

(a) Write a chemical equation for the reaction that occurs during the operation of a

portable hair curler, using methanol as liquid fuel.

(b) (i) Suggest ONE function of platinum inside the combustion chamber.

(ii) Suggest ONE method to ignite the fuel gas/air mixture in the combustion chamber.

(c) Suggest TWO criteria for a portable energy supply.

(d) Justify whether petrol or methanol is more suitable to be used for fuel in a portable hair curler.

(6 marks)


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5. Hydrogen peroxide can act as a powerful oxidizing agent as well as a reducing agent

under certain conditions.

(a) Draw the three-dimensional structure of a hydrogen peroxide molecule.

(b) Write the overall equations for the reactions below.

In each case,

(1) state whether H2O2 acts as an oxidizing agent or a reducing agent; and

(2) state the change in oxidation number in oxygen.

(i) Fe2+(aq) reacts with H2O2(aq) to form Fe3+(aq)

(ii) MnO4–(aq) reacts with H2O2(aq) to form Mn2+(aq)

(7 marks)


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6. The standard enthalpy change for the combustion of phenol, C6H5OH(l) is –3050 kJ

mol–1. (a) Write an equation for the combustion of phenol. (b) By drawing an enthalpy change cycle, calculate the standard enthalpy change of

formation of phenol. Given that the standard enthalpy changes of formation of carbon dioxide and

water are –394 kJ mol–1 and –286 kJ mol–1 respectively.

(5 marks)


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7. Seawater is a mixture of a large amount of substances. It covers about 70% of the

Earth’s surface. The salinity of seawater is ranging from 3.2% to 4.0%. On average, it is approximately 3.5%. It is equivalent to dissolving 35 g of salt in 1000 cm3 of water (roughly equivalent to 1000 g of water). The following table gives the percentage composition by mass of some elements in seawater.

Name of elements Percentage composition by mass

W 86.0 X 10.7 Y 1.9 Z 1.0

Magnesium and other elements 0.4

It is known that W, X and Y are non-metals, while Z is a metal. (a) Name the elements W and Y. (b) Draw an electron diagram, showing electrons in the outermost shell only, for the

compound formed between (i) W and X; (ii) Y and Z. (c) (i) Low salinity is found near polar regions, particularly during summer seasons.

Explain briefly. (ii) The Red Sea is located in the desert region. It shows the highest salinity

(4.0%) of all the oceans. Suggest TWO reasons for this.

(7 marks)


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For question 8, candidates are required to give answers in paragraph form. For this question, 6 marks will be awarded for chemical knowledge and 3 marks for effective communication. 8. Give detailed experimental procedures to show how you can prepare 200 cm3 of

artificial salt solution of exactly 3.5% salinity. (It is equivalent to dissolving 35 g of table salt in 1000 cm3 of water.)

You can only use the following reagents and apparatus:

Reagents Table salt, distilled water Apparatus Electronic balance, 250 cm3 beaker, weighing bottle,

100 cm3 measuring cylinder, glass rod, spatula

(9 marks)


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END OF PART I


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PART II Answer ALL questions. Write your answers in the spaces provided. 9. Transition metal ions are commonly used for catalysis. For example, the reaction

between aqueous potassium sodium 2,3-dihydroxybutanedioate (C4H4O62–(aq)) and

hydrogen peroxide solution is very slow at room temperature, even when heated. It can be speeded up by using cobalt(II) ions. The equation involved is shown below:

C4H4O6

2–(aq) + 3H2O2(aq) → 2CO2(g) + 2HCO2–(aq) + 4H2O(l)

The catalysed mixture appears pink initially due to the presence of cobalt(II) ions. During the reaction, the mixture turns green. When the reaction is complete, the mixture returns to pink. This explains why the reaction is sometimes called ‘traffic light reaction’.

(a) Suggest how you can follow the progress of the reaction. (b) (i) Explain why the colour of the mixture turns from pink to green and then

returns to pink as the reaction proceeds. (ii) Suggest how you can keep the green colour for several minutes without

returning to pink during the progress of the ‘traffic light reaction’. (c) Explain why cobalt(II) ions can be used as a catalyst in this reaction.

(7 marks)


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10. The inter-conversion between the isomers of but-2-ene is represented by the equation

below.

ISOMER A ISOMER B The value of the equilibrium constant, Kc, for this reaction is 3.2 at 25°C and 1.0 atm. (a) (i) Name the type of isomerism that exists between the two isomers. (ii) Give the systematic name of each isomer. (b) At equilibrium at 25°C and 1.0 atm, which isomer is present at a higher

concentration? Justify your answer. (c) If 1.50 mol dm–3 of pure isomer A was introduced into an evacuated container at

25°C and 1.0 atm, calculate the equilibrium concentration, in mol dm–3, of isomer B.

(d) Given that value of Kc for this reaction at 125°C is 1.3, predict whether the

forward reaction is endothermic or exothermic. Justify your answer.

(8 marks)


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11. In each of the following substances, give an appropriate example from Period 3 of the

Periodic Table and write appropriate equation(s) of a chemical reaction, if necessary, to show what is meant by

(a) a basic oxide; (b) an amphoteric oxide; (c) an acidic oxide; and (d) a neutral oxide.

(5 marks)


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12. Esters are compounds widely found in nature. They have pleasant odours. Food and

beverage manufacturers often use them as additives to enhance flavour or odour. Although the fruity tastes and odours of esters are pleasant, they are seldom used in

perfumes or scents that are applied to the body. The ingredients of more expensive perfumes are hydrocarbons (terpenes), ketones and ethers extracted from natural sources. The reason is that ester is not as stable upon sweating as the more expensive ingredients.

Ethyl butyrate is an ester with a smell like that of pineapple. The structure is shown

below:

(a) Give the systematic name of ethyl butyrate.

(b) (i) Name TWO organic compounds used for the formation of ethyl butyrate by esterification.

(ii) Give the reagent(s) and condition(s) that are necessary for the formation of

ethyl butyrate during esterification. (c) Explain why ester is unstable upon sweating. By using ethyl butyrate, write a

chemical equation to illustrate your answer. (d) Suggest ONE disadvantage of adding esters as additives in food. (e) ‘A good-quality flavouring agent can be made from a single ester.’ Comment on

this statement.

(8 marks) END OF PART II

END OF SECTION B


HKDSE CHEMISTRY S5 Exam Paper (Set 1) Suggested Answers CHEMISTRY © Aristo Educational Press Ltd. 2011 A1

HKDSE CHEMISTRY SECONDARY FIVE

EXAMINATION PAPER (SET 1)

SUBJECT: CHEMISTRY

SUGGESTED ANSWERS for Section A 1. A 2. A 3. D 4. A 5. D 6. B 7. D 8. C 9. B 10. C 11. D 12. A 13. B 14. B 15. B 16. A 17. C 18. D 19. C 20. D 21. B 22. A 23. C 24. C 25. D 26. C 27. B 28. A 29. D 30. C 31. B 32. A 33. B 34. D 35. A 36. C SUGGESTED ANSWERS for Section B 1. (a) (i) Group I, Period 8 [1] (ii) Alkali metals [1] (b)

Suggested symbol Mistake Option 1 Ang Three letters Option 2 Ag Same as existing symbol of silver Option 3 AE Two capital letters

[3] (c) (i) They have the same number of electrons in the outermost shell. [1] (ii) 4Ae(s) + O2(g) → 2Ae2O(s) [1] (d)

[1]

2. (a) (i)

[1] (ii) Octahedral [1] (iii) Dative covalent bond [1] (b)

[1] Correctly shows dative covalent bond [1]


(c) (i)

[1] (ii) Trigonal planar; 120° [1] 3. (a) When bromine and 1,1,1-trichloroethane have not been mixed, van der Waals’ forces

exist between bromine molecules and between 1,1,1-trichloroethane molecules. [2] When bromine and 1,1,1-trichloroethane are mixed, the amount of energy released from

the formation of van der Waals’ forces between bromine molecules and 1,1,1-trichloroethane molecules is large enough to compensate for that required to break the van der Waals’ forces between 1,1,1-trichloroethane molecules. Hence, bromine can dissolve in 1,1,1-trichloroethane. [2]

(b) SbCl3 has a trigonal pyramidal structure.

[1] SbCl5 has a trigonal bipyramidal structure.

[1] The dipole moments of three polar Sb–Cl bonds do not cancel out in SbCl3 and the five

dipole moments cancel out each other in SbCl5. [1]

4. (a) CH3OH(l) + 2

3 O2(g) → CO2(g) + 2H2O(l) [1]

OR 2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(l) (b) (i) Catalyst; to increase the rate of combustion. [1] (ii) Any ONE: [1] - by creating sparks - by electrically heating the platinum (c) Any TWO: [2] Compact/lightweight/constant supply of electricity/long-lasting supply of electricity (d) Methanol is a more suitable fuel. Any ONE explanation: [1] because it burns more completely than petrol. / its combustion produces less black smoke and particulates. / it has a high efficiency of energy conversion.


5. (a)

[1] (b) (i) 2Fe2+(aq) + H2O2(aq) + 2H+(aq) → 2Fe3+(aq) + 2H2O(l) [1] (1) Oxidizing agent [1] (2) Oxidation number of O decreases from –1 to –2. [1] (ii) 2MnO4

–(aq) + 5H2O2(aq) + 6H+(aq) → 2Mn2+(aq) + 8H2O(l) + 5O2(g)[1] (1) Reducing agent [1] (2) Oxidation number of O increases from –1 to 0. [1] 6. (a) C6H5OH(l) + 7O2(g) → 6CO2(g) + 3H2O(l) [1] (b)

6C(s) + 3H2(g) + 2

1 O2(g) → C6H5OH(l)

+6O2(g) +O2(g) +7O2(g)

6CO2(g) + 3H2O(l) [2] By Hess’s Law, 6∆Hf[CO2] + 3∆Hf[H2O] = ∆Hf + ∆Hcom [1] 6(–394) + 3(–286) = ∆Hf + (–3050) ∆Hf = –172 kJ mol–1 [1] 7. (a) W: oxygen Y: chlorine (only if both correct) [1] (b) (i)

[1] (ii)

[1] (c) (i) When ice melts, the volume of water increases. [1] Hence, the concentration of brine drops and the salinity drops. [1] (ii) Desert region has very little rainfall. [1] Desert region has a high rate of evaporation of water. [1] 8. Weigh out 7 g of table salt using an electronic balance. [1] Transfer the table salt to the weighing bottle using a spatula. [1] Pour the weighed table salt to a 250 cm3 beaker. [1] Measure 200 cm3 distilled water using a measuring cylinder. [1] First, pour 100 cm3 distilled water into the beaker. Repeat with another 100 cm3 distilled water. [1] (If only mentioned 200 cm3 water needed, without correct apparatus, award 1 mark only.) Stir the solution with a glass rod until ALL solid dissolves. [1] **3 marks for effective communication [3]

∆Hf


9. (a) Measure the volume of carbon dioxide gas produced [1] at regular time intervals (say 30-second interval) for 10 minutes. [1] Plot the graph of volume of carbon dioxide gas produced against time. [1] (b) (i) Pink cobalt(II) ions are first oxidized to green cobalt(III) ions. [1] Then cobalt(III) ions are reduced back to cobalt(II) ions. [1] (ii) To stabilize the reaction mixture by cooling it in an ice-water bath rapidly. [1] (c) Cobalt(II) ions can be oxidized in the first step and then reduced in the subsequent step. [1] 10. (a) (i) Geometrical isomerism [1] (ii) A: cis-but-2-ene B: trans-but-2-ene [1] (b) B is present at a higher concentration. [1]

Kc = eqm

eqm

][

][

A

B and Kc > 1 [1]

(c) Let the volume of the container be 1 dm3 and x be the change in concentration of isomer A.

A B initial conc. 1.50 0 change –x +x eqm conc. 1.50 – x x

Kc = eqm

eqm

][

][

A

B [1]

3.2 = x.

x

−501

x = 1.14

∴ the equilibrium concentration of isomer B is 1.14 mol dm–3. [1] (d) The reaction is exothermic. [1] At a higher temperature, Kc is smaller and less isomer B is present. Since the equilibrium

position shifts to the left at a higher temperature, it indicates that the forward reaction is exothermic. [1]

11. (a) Na2O(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) [1] (b) Al2O3(s) + 6H+(aq) → 2Al3+(aq) + 3H2O(l) [1] Al2O3(s) + 2OH–(aq) + 3H2O(l) → 2[Al(OH)4]

–(aq) [1] (c) SO2(g) + 2NaOH(aq) → Na2SO3(aq) + H2O(l) [1] (d) There is no neutral oxide in Period 3. [1] 12. (a) Ethyl butanoate [1] (b) (i) Ethanol and butanoic acid [1] (ii) Concentrated sulphuric acid and heat under reflux [1] (c) On contact with sweat, esters undergo hydrolysis, giving organic acids. [1] These acids generally do not have a pleasant odour. [1] CH3CH2CH2COOCH2CH3 + H2O → CH3CH2CH2COOH + CH3CH2OH [1] (d) Esters would possibly attract fruit flies and insects. [1] (e) No. This is because the perception of flavour requires combined responses (bitter, sour,

sweet and salty) from taste buds on the tongue. A complex mixture (in which a single ester predominates) is needed. [1]

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HKDSE Chem FX ExamS5 2011 Set1 Eng