Cambridge IGCSE Chemistry - 0620 - Supplement Paper 4 2011 Summer Zone 1

The following techniques are used to separate mixtures.

A simple distillation   B fractional distillation   C evaporation
D chromatography   E filtration   F diffusion

From this list, choose the most suitable technique to separate the following.

(a) methane from a mixture of the gases, methane and ethane .................. [1]
(b) water from aqueous magnesium sulfate .................. [1]
(c) glycine from a mixture of the amino acids, glycine and lysine .................. [1]
(d) iron filings from a mixture of iron filings and water .................. [1]
(e) zinc sulfate crystals from aqueous zinc sulfate .................. [1]
(f) hexane from a mixture of the liquids, hexane and octane .................. [1]

Selenium and sulfur are in Group VI. They have similar properties.
(a) One of the main uses of selenium is in photoelectric cells. These cells can change light into electrical energy.
(i) Name a process which can change light into chemical energy.
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(ii) Name a device which can change chemical energy into electrical energy.
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(b) The electron distribution of a selenium atom is 2 + 8 + 18 + 6.
(i) Selenium forms an ionic compound with potassium. Draw a diagram which shows the formula of this ionic compound, the charges on the ions and the arrangement of the valency electrons around the negative ion. Use $o$ to represent an electron from an atom of potassium. Use $x$ to represent an electron from an atom of selenium. [3]
(ii) Draw a diagram showing the arrangement of the valency electrons in one molecule of the covalent compound selenium chloride. Use $x$ to represent an electron from an atom of selenium. Use $o$ to represent an electron from an atom of chlorine. [3]
(iii) Predict two differences in the physical properties of these two compounds.
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(c) The selenide ion reacts with water.
$ ext{Se}^{2-} + ext{H}_2 ext{O} ightarrow ext{HSe}^- + ext{OH}^-$
What type of reagent is the selenide ion in this reaction? Give a reason for your choice.
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........................................................................................................................................ [3]

Iron from the blast furnace is impure. It contains about 4 % carbon and 0.5 % silicon. Most of this impure iron is used to make mild steel, an alloy of iron containing less than 0.25 % carbon.

(a) A jet of oxygen is blown through the molten iron in the presence of a base, usually calcium oxide. Explain how the percentage of carbon is reduced and how the silicon is removed. ..............................................................................................................................
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(b) (i) Why are steel alloys used in preference to iron? .............................................................................................................................. [1]

(ii) State a use of the following alloys.
mild steel ..............................................................................................................................
stainless steel .............................................................................................................................. [2]

(c) Both iron and steel have typical metallic structures - a lattice of positive ions and a sea of electrons.

(i) Suggest an explanation for why they have high melting points. ..............................................................................................................................
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(ii) Explain why, when a force is applied to a piece of steel, it does not break but just changes its shape. ..............................................................................................................................
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A major ore of zinc is zinc blende, ZnS. A by-product of the extraction of zinc from this ore is sulfur dioxide which is used to make sulfuric acid.

(a) (i) Zinc blende is heated in air. Zinc oxide and sulfur dioxide are formed. Write the balanced equation for this reaction.
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(ii) Zinc oxide is reduced to zinc by heating with carbon. Name two other reagents which could reduce zinc oxide.
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(iii) The zinc obtained is impure. It is a mixture of metals. Explain how fractional distillation could separate this mixture.
zinc bp = 908°C, cadmium bp = 765°C, lead bp = 1751°C
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................................................................................................. [2]

(b) Sulfur dioxide is used to make sulfur trioxide in the Contact Process.
$$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$$
The forward reaction is exothermic. The conditions used are:
temperature: 450°C
pressure: 2 atmospheres
catalyst: vanadium(V) oxide

Explain, mentioning both position of equilibrium and rate, why these conditions give the most economic yield.
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Hydriodic acid, HI(aq), is a strong acid. Its salts are iodides.

(a) It has the reactions of a typical strong acid. Complete the following equations.
(i) ...........Li + ..........HI → .................... + ................. [1]
(ii) zinc carbonate + hydriodic acid → ........................... ...........................
........................... + ........................... + ........................... [1]
(iii) MgO + ..........HI → .................. + ................. [1]

(b) Two of the reactions in (a) are acid/base and one is redox. Which one is redox? Explain your choice.
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(c) Describe how you could distinguish between hydriodic, HI(aq), and hydrobromic, HBr(aq) acids, by bubbling chlorine through these two acids.
result with hydriodic acid ..............................................................................................................
result with hydrobromic acid .......................................................................................................... [2]

(d) 20.0 cm³ of aqueous sodium hydroxide, 2.00 mol / dm³, was placed in a beaker. The temperature of the alkali was measured and 1.0 cm³ portions of hydriodic acid were added. After each addition, the temperature of the mixture was measured. Typical results are shown on the graph.



NaOH(aq) + HI(aq) → NaI(aq) + H₂O(l)
(i) Explain why the temperature increases rapidly at first then stops increasing.
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(ii) Suggest why the temperature drops after the addition of 18.0 cm³ of acid.
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(iii) In another experiment, it was shown that 15.0 cm³ of the acid neutralised 20.0 cm³ of aqueous sodium hydroxide, 1.00 mol/dm³. Calculate the concentration of the acid.
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The structural formula of a butanol is given below.

CH$_3$—CH$_2$—CH$_2$—CH$_2$—OH

(a) Butanol can be made from petroleum and also by fermentation.

(i) Describe the chemistry of making butanol from petroleum by the following route.

petroleum → butene → butanol
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[3]

(ii) Explain, in general terms, what is meant by fermentation.
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[3]

(b) Butanol can be oxidised to a carboxylic acid by heating with acidified potassium manganate(VII). Give the name and structural formula of the carboxylic acid.

name .............................................................................................................................
[1]
structural formula

[1]

(c) Butanol reacts with ethanoic acid to form a liquid, X, which has the sweet smell of bananas. Its empirical formula is C$_3$H$_6$O and its M$_r$ is 116.

(i) What type of compound is liquid X?
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[1]

(ii) Give the molecular formula of liquid X.
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[1]

(iii) Draw the structural formula of X. Show all the individual bonds.

[2]

Excess hydrochloric acid was added to powdered zinc. The hydrogen evolved was collected and its volume measured every 20 seconds.
The experiments were repeated at the same temperature using the same number of moles of powdered magnesium and aluminium.

(a) Identify metals A, B and C by choosing from zinc, magnesium and aluminium. Give a reason for each choice.
metal A ............................................................................................................................
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metal B ............................................................................................................................
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metal C ............................................................................................................................
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(b) Using ‘moles’, explain why two of the metals form the same volume of hydrogen but the third metal forms a larger volume.
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There are two types of polymerisation - addition and condensation.

(a) Explain the difference between them.
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(b) Poly(dichloroethene) is used to package food. Draw its structure. The structural formula of dichloroethene is shown below.



H     Cl
|                        |
C=C
|                       |
H     Cl

[2]

(c) The polymer known as PVA is used in paints and adhesives. Its structural formula is shown below.



—CH₂—CH—CH₂—CH—
|                      |
OOCCH₃     OOCCH₃

Deduce the structural formula of its monomer.

[1]

(d) A condensation polymer can be made from the following monomers.

HOOC(CH₂)₄COOH and H₂N(CH₂)₆NH₂

Draw the structural formula of this polymer.

[3]