Chemistry 2002 Paper 1+ans

2002-CE CHEM

HONG KONG EXAMINATIONS AUTHORITY

PA P E R 1

HONG KONG CERTIFICATE OF EDUCATION EXAMINATION 2002

CHEMISTRY PAPER 1 8.30 am – 10.00 am (1½ hours) This paper must be answered in English

1.

There are TWO sections in this paper, Section A and Section B.

2.

Answer ALL questions in Section A and any THREE questions in Section B.

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2002-CE-CHEM 1–1

Section A (Answer ALL questions.) 1.

Both ammonium dihydrogenphosphate and ammonium sulphate are nitrogenous fertilizers. (a)

Why is nitrogen essential for plant growth ?

(b)

List all the elements in ammonium dihydrogenphosphate.

(c)

(i)

Calculate the percentage by mass of nitrogen in ammonium sulphate.

(ii)

The use of ammonium sulphate as a fertilizer adds acidity to the soil. If the soil is too acidic, it is not suitable for plant growth. Suggest ONE substance that is commonly used by farmers to reduce soil acidity. Explain your answer.

(Relative atomic masses : H = 1.0, N = 14.0, O = 16.0, S = 32.0) (6 marks)

2.

For each of the following experiments, state an expected observation and write a chemical equation for the reaction involved. (a)

A magnesium ribbon is placed in a Bunsen flame.

(b)

Excess iron(II) sulphate solution is added to an acidified potassium permanganate solution.

(c)

Chlorine gas is bubbled into a sodium bromide solution. (6 marks)

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3.

Consider the substances listed below: ammonia, manganese(IV) oxide, potassium hydroxide, sodium benzoate, sodium dichromate, sodium nitrite (a)

Which substance is used to preserve bacon ? State how it works.

(b)

Which substance is used in zinc-carbon cells ? State its function.

(c)

Which substance is used in breathalysers to detect the presence of ethanol in the breath of suspected drunk drivers ? State the expected observation in the breathalyser if a positive result is obtained. (6 marks)

For questions 4 and 5, candidates are required to give paragraphlength answers. For each of these two questions, 3 of the marks will be awarded for effective communication of knowledge in chemistry. 4.

Using the electrolysis of copper(II) chloride solution as an example, briefly discuss the factors affecting the discharge of ions in electrolysis. (9 marks)

5.

Using alkenes as an example, describe the characteristics of members of a homologous series. (9 marks)

End of Section A

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Section B (Answer any THREE questions.) 6.

(a)

Magnesium can be extracted from sea water which contains magnesium ions. The extraction of magnesium from sea water involves three stages: Stage 1 : Add slaked lime to sea water to precipitate magnesium ions as magnesium hydroxide. Stage 2 : Heat the magnesium hydroxide obtained in a stream of hydrogen chloride gas to give magnesium chloride. Stage 3 : Extract magnesium by electrolysis of the molten magnesium chloride.

(b)

(i)

What substance is mainly present in slaked lime ?

(ii)

Write a chemical equation, with state symbols, for the reaction in Stage 2.

(iii)

Explain why molten magnesium chloride can conduct electricity.

(iv)

One major use of magnesium is to make aluminium alloys. Name ONE such alloy which is used to make aircraft. (5 marks)

Magnesium occurs naturally in three isotopic forms. The relative abundance of each isotope is shown in the table below: Isotope

24

25

26

Relative abundance (%)

78.6

10.1

11.3

Mg

Mg

Mg

(i)

State the meaning of the term ‘isotopes of an element’.

(ii)

Calculate the relative atomic mass of magnesium.

(iii)

Is it possible to separate the isotopes of magnesium by chemical means ? Explain your answer. (4 marks)

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6.

(c)

Ethyl ethanoate is an ester. It can be prepared by heating a mixture of ethanoic acid and ethanol under reflux in the presence of a catalyst. (i)

What is the catalyst used in the preparation ?

(ii)

Draw a labelled diagram of the set-up used for heating the mixture under reflux.

(iii)

Ethyl ethanoate is commonly used as a solvent. Explain why ethyl ethanoate can dissolve iodine but cannot dissolve sodium iodide.

(iv)

Which ONE of following hazard warning labels should be displayed on a bottle of ethyl ethanoate ?

A (v)

B

C

D

Draw the structure of another ester which has the same molecular formula as ethyl ethanoate, and give its systematic name. (9 marks)

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7.

(a)

Calcite is a mineral which contains mainly calcium carbonate. An experiment, consisting of the following five stages, was conducted to determine the percentage by mass of calcium carbonate in a sample of calcite.

Stage 1 : Weigh the sample. Add dilute nitric acid to it until the acid is in excess. Stage 2 : Filter the mixture obtained in Stage 1 to remove any undissolved solid. Stage 3 : Add excess sodium sulphate solution to the filtrate to precipitate out calcium sulphate. Stage 4 : Collect the calcium sulphate precipitate and wash it with distilled water. Stage 5 : Allow the calcium sulphate to dry and weigh it.

(i)

Write a chemical equation for the reaction of calcium carbonate with dilute nitric acid. Suggest how one can know that excess acid has been added in Stage 1.

(ii)

Draw a labelled diagram of the set-up used in the filtration process in Stage 2.

(iii)

Write the ionic equation for the reaction in Stage 3.

(iv)

Explain why it is necessary to wash the precipitate with distilled water in Stage 4.

(v)

The results obtained in the experiment are listed below: mass of the calcite sample = 7.98 g mass of the calcium sulphate obtained = 10.52 g (1)

Calculate the percentage by mass of calcium carbonate in the sample of calcite.

(2)

State ONE assumption in the calculation.

(Relative atomic masses: C = 12.0, O = 16.0, S = 32.0, Ca = 40.0) (10 marks) 2002-CE-CHEM 1-6 –5– 保留版權 All Rights Reserved 2002

7.

(b)

(c)

Chlorine bleach can be made by reacting chlorine with sodium hydroxide solution. (i)

Write a chemical equation for the reaction.

(ii)

The reaction is known to be a redox. State whether chlorine acts as an oxidizing agent, a reducing agent or both. Explain your answer in terms of the change in oxidation number.

(iii)

Apart from being used as a bleach, suggest ONE other use of chlorine. (5 marks)

Ammonia was once used to detect the leakage of chlorine in chemical plants. If there was a leakage, white fumes would be observed. The word equation below represents the reaction of chlorine with ammonia : chlorine + ammonia → ammonium chloride + nitrogen (i)

Transcribe the word equation into a chemical equation.

(ii)

Suggest what the white fumes might have been.

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(3 marks)

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8.

(a)

Sulphur dioxide is formed when coal is burnt in a power station. (i)

The coal used in the power station contains 1.5% of sulphur by mass. Calculate the volume of sulphur dioxide released, measured at room temperature and pressure, when 1.0 kg of the coal is burnt. (You may assume that all the sulphur in coal is converted to sulphur dioxide upon burning.)

(ii)

State ONE environmental problem associated with the emission of sulphur dioxide into the atmosphere.

(iii)

Suggest ONE measure to reduce the emission of sulphur dioxide from the power station.

(iv)

Particulates are also present in the flue gas generated in the power station. (1)

State ONE environmental problem associated with the discharge of particulates into the atmosphere.

(2)

Suggest ONE way to remove particulates from the flue gas.

(Relative atomic masses : O = 16.0, S = 32.0; molar volume of gas at room temperature and pressure = 24 dm3) (7 marks)

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8.

(b)

(c)

Both carbon and silicon are Group IV elements in the Periodic Table. (i)

Draw the electronic diagram of a carbon dioxide molecule, showing electrons in the outermost shells only.

(ii)

Explain why carbon dioxide can be used in fire fighting.

(iii)

Explain why carbon dioxide is a gas, whereas silicon dioxide is a solid at room temperature and pressure.

(iv)

Silicon can be obtained by heating silicon dioxide with carbon strongly. (1)

Write a chemical equation for the reaction involved.

(2)

Suggest ONE use of silicon.

(8 marks)

Polyethylene terephthalate (PET) is a polymer which is commonly used in making plastic bottles for fizzy drinks. PET has the following repeating unit : O C

O C OCH2CH2O

(i)

PET is formed by condensation polymerization. What is meant by the term ‘condensation polymerization’ ?

(ii)

PET is a polyester formed from two monomers. Draw the structures of the monomers. (3 marks)

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9.

(a)

Ammonia is a weak alkali. It is used as an active ingredient in domestic glass cleaners. (i)

(ii)

(b)

(1)

Write a chemical equation to represent the ionization of ammonia in water.

(2)

Explain why an alkaline solution can help remove oily dirt on glass.

Suggest, with explanation, a precaution necessary when using such glass cleaners. (4 marks)

In an experiment to determine the concentration of ammonia in a sample of glass cleaner, 25.0 cm3 of the sample was diluted to 250.0 cm3 in a volumetric flask. 25.0 cm3 of the diluted sample was transferred to a conical flask and was then titrated against 0.23 M hydrochloric acid. 28.7 cm3 of the acid was required to reach the end-point. (i)

State the liquid that should be used to rinse the following pieces of apparatus used in this experiment. (1)

volumetric flask

(2)

conical flask

(ii)

Name the apparatus that should be used to transfer 25.0 cm3 of the diluted sample to the conical flask.

(iii)

Calculate the concentration, in mol dm–3, of ammonia in the sample of glass cleaner. (You may assume that ammonia is the only substance in the sample that reacts with hydrochloric acid.) (6 marks)

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9.

(c)

A student used the set-up shown below to conduct a microscale experiment on electrolysis. d.c. supply – +

carbon rod B

carbon rod A microscope slide

(i)

a drop of aqueous sodium sulphate solution with some universal indicator

(1)

The initial colour of the drop shown above was green. State the colour change of the liquid around carbon rod A after a current was passed through the circuit for some time. Explain your answer with the help of a half equation.

(2)

A gas was liberated at carbon rod B. What was the gas ? Explain its formation.

(ii)

Some objects readily available in daily life contain carbon rods which can be used in this experiment. Suggest ONE such object.

(iii)

The use of microscale experiments in studying chemistry is becoming more popular nowadays. Suggest TWO advantages of carrying out experiments in microscale. (8 marks)

END OF PAPER

2002-CE-CHEM 1-11 – 10 – 保留版權 All Rights Reserved 2002

2002

Chemistry 1 Section A 1.

(c)

(i)

21.2

Section B 6.

(b)

(ii)

24.3

7.

(a)

(v)

(1)

8.

(a)

(i)

11.25 dm3

9.

(b)

(iii)

2.64

97.0

2002-CE-CHEM 1-12 – 11 – 保留版權 All Rights Reserved 2002