CCC HEEP WOH COLLEGE 2019 2020 FINAL EXAMINATION F.5 …aerodrive.ccchwc.edu.hk/~lck/class/19202ndf5examMS.pdf · Iron is extracted from haematite in a blast furnace. (a) What is

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2019 - 2020 / Form Five Chemistry / Marking Scheme / Final Examination / C.K.Lau

CCC HEEP WOH COLLEGE

2019 – 2020 FINAL EXAMINATION

F.5 Chemistry

Marking Scheme

Section A:

1. C 6. D 11. B 16. D 21. B 26. B 31. A 36. C

2. A 7. D 12. D 17. C 22. D 27. C 32. A

3. A 8. A 13. C 18. B 23. B 28. A 33. D

4. D 9. D 14. X 19. D 24. B 29. B 34. A

5. C 10. B 15. B 20. B 25. A 30. C 35. D

Section B:

1. When ammonia dissolves in water, it ionizes to give ammonium ions and

hydroxide ions.

(a) Write an equation for the ionization of ammonia in water. [1]

NH3(aq) + H2O(l) NH4+(aq) + OH−(aq) 1

(b) Ammonium ion is an example for illustrating the formation of dative

covalent bond.

(i) What is the meaning of the term ‘dative covalent bond’? [1]

A dative covalent bond is a covalent bond formed between two atoms

where both electrons of the shared pair are contributed by the same

atom. 1

(ii) Draw the electron diagram for ammonium ion, showing electrons in

the outermost shells only. [1]

1

(c) Ammonium salts react with alkalis to form ammonia. In an experiment, a

mixture of ammonium chloride solution and sodium hydroxide solution is

heated.

(i) Write a chemical equation for the reaction involved. [1]

NH4Cl(aq) + NaOH(aq) ⎯→ NH3(g) + H2O(l) + NaCl(aq) 1

(ii) Suggest a test to show that ammonia is an alkaline gas. [1]

Ammonia turns moist red litmus paper blue. 1

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2. Answer the following questions:

(a) Explain how ammonia solution and sodium hydroxide solution of the same

concentration can be distinguished using aluminium nitrate solution. [2]

When adding a few drops of NH3(aq) and NaOH(aq) to Al(NO3)3(aq)

separately, a white precipitate forms in both solutions. 1

The white precipitate dissolves in excess NaOH(aq) but not in excess

NH3(aq). 1

(b) How does tin-plating prevent iron from rusting? [1]

The tin layer prevents iron from contacting air and water. 1

(c) Explain why mercury can conduct electricity in liquid state. [1]

There are delocalized electrons in the structure of liquid mercury. 1

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3. Iron is extracted from haematite in a blast furnace.

(a) What is raw material A? [1]

Limestone / CaCO3 1

(b) Name the major component in haematite. [1]

Iron(III) oxide 1

(c) Write a chemical equation for the extraction of iron in the blast furnace.

[1]

Fe2O3 + 3CO ⎯→ 2Fe + 3CO2 1

Fe2O3 + 3C ⎯→ 2Fe + 3CO / 2Fe2O3 + 3C ⎯→ 4Fe + 3CO2 (1)

(d) Is this method suitable to extract sodium from its ore? Explain your answer.

[1]

No. Sodium compounds are more stable and so sodium cannot be

extracted by carbon reduction. 1

(e) Suggest one common use of iron. [1]

Making cars/ship bodies (accept other reasonable answers) 1

hot air

iron ore + coke + raw material A

hot air

hot waste gas hot waste gas

molten slag molten iron

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4. In traditional gold extraction process, gold-containing ores are ground into

smaller pieces before mixing with calcium cyanide solution. Air or pure oxygen

will then be bubbled into the mixture. ‘Gold cyanidation’ occurs accordingly:

4Au(s) + 8CN−(aq) + O2(g) + 2H2O(l) ⎯→ 4[Au(CN)2]−(aq) + 4OH−(aq)

Insoluble solids is then removed and the resultant solution is collected. Finally,

zinc powder is added to the resultant solution to recover gold.

(a) State the importance of grinding the ores into smaller pieces before ‘gold

cyanidation’. [1]

Grinding the ore can increase the surface area for the reaction to occur.

1

(b) The calcium cyanide solution used in ‘gold cyanidation’ is produced by

the reaction between hydrocyanic acid (HCN(aq)) and calcium

hydroxide solution.

(i) Draw the electron diagram for hydrogen cyanide molecule, showing

electrons in the outermost shells only. [1]

1

(ii) Write a chemical equation for the reaction between hydrocyanic acid

and calcium hydroxide solution. [1]

Ca(OH)2(aq) + 2HCN(aq) ⎯→ Ca(CN)2(aq) + 2H2O(l) 1

(c) Draw a labelled diagram to show how to remove the insoluble solids in

‘gold cyanidation’. [2]

Correct drawing (1) Correct labels (1) 2

(d) With reference to ‘gold cyanidation’, identify the species undergoing

oxidation and reduction respectively. Explain your answers in terms of

changes in oxidation numbers. [2]

O2 undergoes reduction as the oxidation number of O decreases from 0 to

−2. 1

Au undergoes oxidation as the oxidation number of Au increases from 0 to

+1. 1

(e) Explain why zinc can recover gold from the resultant solution. [1]

Zinc is a reducing agent that can reduce [Au(CN)2]− to gold. 1

filtrate

reaction mixture

insoluble solids

glass rod

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5. The following set-up shows the electrolysis of 0.01 M copper(II) nitrate solution

with graphite electrodes (A and B) and copper electrodes (C and D) in two

electrolytic cells (P and Q) connected in series.

(a) What is the colour of copper(II) nitrate solution? [1]

Blue 1

(b) After passing electricity for some time, state and explain the change of

(i) the pH of copper(II) sulphate solution in cell P. [2]

Cu2+(aq) ions are discharged at electrode B while OH−(aq) ions are

discharged at electrode A. 1

As the concentration of OH−(aq) ions decreases / H+(aq) ions increases,

the pH of the solution decreases. 1

(ii) the colour of copper(II) sulphate solution in cell Q. [2]

Cu2+(aq) ions form at electrode C while Cu2+(aq) ions are discharged

at electrode D. 1

As the concentration of Cu2+(aq) ions remains unchanged, the colour

of the solution remains unchanged. 1

(c) After some time, the polarity of the batteries is reversed.

(i) What would be observed at graphite electrode B? [1]

Colourless gas bubbles form. 1

(ii) Write a half equation for the reaction taking place at electrode C.

[1]

Cu2+(aq) + 2e− ⎯→ Cu(s) 1

P Q

A B C D

dilute

CuSO4(aq)

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6. When lead(II) nitrate is heated strongly, lead(II) oxide, nitrogen dioxide and

oxygen are produced.

(a) Write a chemical equation for the thermal decomposition of lead(II) nitrate.

[1]

2Pb(NO3)2(s) ⎯→ 2PbO(s) + 4NO2(g) + O2(g) 1

(b) State the changes in oxidation number of nitrogen and oxygen

respectively. [2]

The oxidation number of nitrogen decreases from +5 to +4. 1

The oxidation number of oxygen increases from −2 to 0. 1

(c) State and explain an observation in this experiment. [2]

Brown fumes are observed. 1

This is because brown nitrogen dioxide is produced. 1

OR The solid changes from white to orange. The solid turns yellow on cooling.

(1)

This is because lead(II) oxide is produced. (1)

(d) Explain why the experiment should be performed in a fume cupboard.

[1]

Nitrogen dioxide is toxic. 1

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7. Account for each of the following:

(a) HF has a higher boiling point than HCl. [2]

HF molecules are held together mainly by strong hydrogen bonds while

HCl molecules are held together by weak van der Waals’ forces. 1

As hydrogen bonds are stronger than van der Waals’ forces, HF has a

higher boiling point. 1

(b) Metals are malleable. [2]

The electrostatic attractions between metal ions and delocalized electrons

are non-directional. 1

If a force is applied, the layers of metal ions will slide over one another

without breaking the metallic bonds. 1

(c) Carbon can form a large number of compounds. [2]

Each carbon atom has four outermost shell electrons, 1

which can be shared with atoms of other elements (or carbon) to form

strong single, double and triple covalent bonds. Carbon atoms are able to

join up with one another to form chains or rings. The carbon chains may be

straight or branched. 1

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8. Hydrogen peroxide is an oxidizing agent in acidic medium. It can be used as

an antiseptic.

(a) Draw the electron diagram for a hydrogen peroxide molecule, showing

electrons in the outermost shells only. [1]

1

(b) Hydrogen peroxide can oxidize iodide ions in acidic medium. State the

expected observable change and write an ionic equation for the reaction

involved. [2]

The solution changes from colourless to brown. 1

H2O2(l) + 2I−(aq) + 2H+(aq) ⎯→ I2(aq) + 2H2O(l) 1

(c) With the aid of a chemical equation, suggest why the concentration of

hydrogen peroxide in a bottle of antiseptic decreases after prolonged

storage. [2]

Hydrogen peroxide decomposes to give water and oxygen gradually. 1

2H2O2(l) ⎯→ 2H2O(l) + O2(g) 1

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9. Sugar is composed of sucrose (C12H22O11). A can of 330 cm3 soft drink contains

11.0 g of sucrose per 100 cm3.

(a) Calculate the number of moles of sucrose in the can of soft drink. [2]

(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0)

Number of moles of sucrose

= (11.0 x 330 / 100) / (12.0 x 12 + 1.0 x 22 + 16.0 x 11) 1

= 0.106 mol 1

(b) Sucrose is a source of energy in the body. The following equation

summarizes the reaction involved when sucrose releases energy.

C12H22O11(s) + 12O2(g) ⎯→ 12CO2(g) + 11H2O(l)

The standard enthalpy changes of formation of some compounds are

given below.

Compound ΔHf (kJ mol−1)

C12H22O11(s) −2226

CO2(g) −393.5

H2O(l) −285.8

Calculate the standard enthalpy change for the above reaction. [2]

ΔH = 12 × (−393.5) + 11 × (−285.8) − (−2226) 1

= −5640 kJ mol−1 1

(c) Nutrient Reference Value (NRV) can be found in the nutrient labels. It is

based on a 2000-kcal diet and referring to the recommended intake levels of

various nutrients. Calculate the percentage of NRV for the can of soft drink.

(1 kcal = 4.2 kJ) [2]

Energy content for the can of soft drinks

= 5640 x 0.106 / 4.2 kcal

= 142.3 kcal 1

Percentage of NRV = (142.3 / 2000) × 100% = 7.12% 1

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*10. You are provided with common laboratory apparatus, a magnesium ribbon

and dilute hydrochloric acid. Outline how you would perform an experiment to

determine the molar volume of hydrogen gas under the experimental

conditions.

[5]

Weigh the magnesium ribbon accurately. 1

Add the magnesium ribbon to excess dilute hydrochloric acid. 1

Measure the volume of hydrogen gas produced by a syringe. 1

Molar volume of hydrogen gas

= Volume of H2 produced / (Mass of Mg(s) / 24.3) 1

Communication mark 1

11. Outline the synthetic route, with no more than three steps, to accomplish the

following conversion. For each step, give the reagent(s), reaction conditions

(as appropriate) and structure of the organic product. [3]

Step 1: OH−(aq), heat 1

Intermediate:

1

Step 2: Cr2O72−(aq)/H+(aq), heat 1

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*12. Aluminium, silicon and phosphorus are three consecutive elements in Period 3

of the Periodic Table. With reference to the bonding and structure of the three

elements, explain the difference in electrical conductivities between the three

elements. [5]

Aluminium has a giant metallic structure. Since it has a lot of delocalized

electrons in its structure, it is a good conductor of electricity. 1

Silicon has a giant covalent structure. There are very few delocalized electrons

in its structure. Hence, silicon is not a good conductor of electricity at room

temperature. 1

At higher temperatures, more electrons gain energy and become delocalized.

Silicon becomes a good conductor of electricity. 1

Phosphorus has a simple molecular structure. It has no delocalized electrons in

its structure and so phosphorus is a non-conductor of electricity. 1

Communication mark 1

13. The structural formula of 2-chlorobutane is shown below:

(a) 2-chlorobutane exists in two isomeric compounds.

(i) Draw the three-dimensional structures for the two compounds. [1]

1

(ii) State one difference in physical property of the two compounds. [1]

The two compounds can rotate the plane polarized light to the same

extent but in opposite directions. 1

(b) Outline a synthetic route, with no more than three steps, to accomplish the

conversion of butanal to 2-chlorobutane. For each step, give the

reagent(s), reaction conditions (as appropriate) and structure of the

intermediate. [3]

1

1

1

HCl

conc. H2SO4

heat

1. LiAlH4, dry ether

2. H+(aq)

butanal

2-chlorobutane

and

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14. When pentan-3-ol is heated with concentrated sulphuric acid, two isomeric

compounds form.

(a) Name the type of reaction involved. [1]

Dehydration / elimination 1

(b) Write the structural formulae and names of the two isomeric compounds.

[2]

Cis-pent-2-ene Trans-pent-2-ene 2

(1 mark for the correct structure and name of an isomer)

(c) Do the two isomeric compounds have similar chemical properties? Explain

briefly. [1]

Yes, this is because they have the same functional group. 1

(d) Suggest another reagent that can be used to replace concentrated

sulphuric acid in the above reaction. [1]

Aluminium oxide / Al2O3 / concentrated phosphoric acid / H3PO4 1

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15. Compound K contains carbon, hydrogen and oxygen only. It contains 66.7%

carbon, 11.1% hydrogen and 22.2% oxygen. The relative molecular mass of K is

72.0.

(a) Determine the molecular formula of K. [3]

(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0)

Mole ratio of C : H : O

= 66.7/12.0 : 11.1/1.0 : 22.2/16.0 = 5.56 : 11.1 : 1.39 = 4 : 8 : 1 1

∴ empirical formula of K is C4H8O. 1

Let the molecular formula of K be (C4H8O)n.

n × (12.0 × 4 + 1.0 × 8 + 16.0) = 72.0

n = 1

∴ molecular formula of K is C4H8O. 1

(b) Given that K is a ketone, draw the skeletal formula of K. [1]

1

(c) When K is mixed with lithium aluminium hydride in dry ether and then the

reaction mixture is treated with dilute hydrochloric acid, alcohol L is

produced.

(i) Name the type of reaction involved. [1]

Reduction 1

(ii) Suggest another reagent that can be used to convert K to L. [1]

Sodium borohydride in water/NaBH4(aq) 1

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16. Two different samples of magnesium carbonates (A and B), each weighed 1.0

g, were allowed to react with 25.0 cm3 of 1.0 M hydrochloric acid under room

conditions. The volumes of carbon dioxide evolved with time are shown in the

graph below:

(a) Write a chemical equation for the reaction between magnesium

carbonate and hydrochloric acid. [1]

MgCO3(s) + 2HCl(aq) ⎯→ MgCl2(aq) + CO2(g) + H2O(l) 1

(b) For sample A, calculate the average rate of the formation of carbon

dioxide for the time interval from the 0th to the 40th second. [2]

Average rate

= (86 – 0) / (40 – 0) 1

= 2.15 cm3 s−1 1

The End

time (s)

vo

lum

e o

f c

arb

on

dio

xid

e (

cm

3)

sample A

sample B

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CCC HEEP WOH COLLEGE 2019 2020 FINAL EXAMINATION F.5 …aerodrive.ccchwc.edu.hk/~lck/class/19202ndf5examMS.pdf · Iron is extracted from haematite in a blast furnace. (a) What is