Cambridge IGCSE Chemistry - 0620 - Supplement Paper 4 2014 Winter Zone 1

(a) Match the following pH values to the solutions given below.

1   3   7   10   13
The solutions all have the same concentration.

Two macromolecular forms of carbon are graphite and diamond. The structures of graphite and diamond are given below.
Graphite
Diamond
(a) Explain in terms of its structure why graphite is soft and is a good conductor of electricity.
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(b) State two uses of graphite which depend on the above properties.
It is soft ......................................................................................................
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It is a good conductor of electricity .........................................................
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(c) Silicon(IV) oxide also has a macromolecular structure.
(i) Describe the macromolecular structure of silicon(IV) oxide.
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(ii) Predict two physical properties which diamond and silicon(IV) oxide have in common.
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The main use of sulfur dioxide is the manufacture of sulfuric acid.
(a) State two other uses of sulfur dioxide.
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(b) One source of sulfur dioxide is burning sulfur in air.
Describe how sulfur dioxide can be made from the ore zinc sulfide.
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(c) The Contact process changes sulfur dioxide into sulfur trioxide.
$$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$$
the forward reaction is exothermic
temperature 400 to 450°C
low pressure 1 to 10 atmospheres
catalyst vanadium(V) oxide
(i) What is the formula of vanadium(V) oxide?
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(ii) Vanadium(V) oxide is an efficient catalyst at any temperature in the range 400 to 450°C.
Scientists are looking for an alternative catalyst which is efficient at 300°C.
What would be the advantage of using a lower temperature?
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(iii) The process does not use a high pressure because of the extra expense.
Suggest two advantages of using a high pressure?
Explain your suggestions.
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(d) Sulfuric acid is made by dissolving sulfur trioxide in concentrated sulfuric acid to form oleum.
Water is reacted with oleum to form more sulfuric acid.
Why is sulfur trioxide not reacted directly with water?
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Iron is extracted from the ore hematite in the Blast Furnace.



(a) The coke reacts with the oxygen in the air to form carbon dioxide.

$$C + O_2 \rightarrow CO_2$$

(i) Explain why carbon monoxide is formed higher in the Blast Furnace.
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(ii) Write an equation for the reduction of hematite, $Fe_2O_3$, by carbon monoxide.
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(b) (i) Limestone decomposes to form two products, one of which is calcium oxide. Name the other product.
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(ii) Calcium oxide reacts with silicon(IV) oxide, an acidic impurity in the iron ore, to form slag. Write an equation for this reaction.
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(iii) Explain why the molten iron and the molten slag form two layers and why molten iron is the lower layer.
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(iv) Suggest why the molten iron does not react with the air.
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(c) Iron and steel rust. Iron is oxidised to hydrated iron(III) oxide, $Fe_2O_3.2H_2O$, which is rust.
(i) Name the two substances which cause iron to rust.
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(ii) Explain why an aluminium article coated with aluminium oxide is protected from further corrosion but a steel article coated with rust continues to corrode.
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(d) There are two electrochemical methods of rust prevention.
(i) The first method is sacrificial protection.
Explain why the steel article does not rust.



connected
electrically to
steel pipe

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[4]

The second method is to make the steel article the cathode in a circuit for electrolysis.



(ii) Mark on the diagram the direction of the electron flow.
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(iii) The steel girder does not rust because it is the cathode. Reduction takes place at the cathode. Give the equation for the reduction of hydrogen ions.
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Three common pollutants in the air are carbon monoxide, the oxides of nitrogen, NO and NO₂, and unburnt hydrocarbons. They are all emitted by motor vehicles.
(a) Describe how the oxides of nitrogen are formed.
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(b) Describe how a catalytic converter reduces the emission of these three pollutants.
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(c) Other atmospheric pollutants are lead compounds from leaded petrol. Explain why lead compounds are harmful.
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(a) Esters can be made from alcohols and carboxylic acids. For example, the ester ethyl ethanoate can be made by the following reaction.
$\text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}$
(i) Name the carboxylic acid and the alcohol from which the following ester could be made.
!(CH_3-CH_2-CH_2-C=O-O-CH_3..
name of carboxylic acid ............................................................
name of alcohol ..................................................................
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(ii) 6.0 g of ethanoic acid, $M_r = 60$, was reacted with 5.5 g of ethanol, $M_r = 46$. Determine which is the limiting reagent and the maximum yield of ethyl ethanoate, $M_r = 88$.
number of moles of ethanoic acid = .......................... [1]
number of moles of ethanol = ................................... [1]
the limiting reagent is ........................................ [1]
number of moles of ethyl ethanoate formed = ... [1]
maximum yield of ethyl ethanoate = ................. [1]

(b) The following two monomers can form a polyester.
!(HOOC-[block]-COOH-HO-[block]-OH)
Draw the structural formula of this polyester. Include two ester linkages.

Fats and vegetable oils are esters. The formulae of two examples of natural esters are given below.
!(CH_2-CO_2-C_{17}H_{33}-ester_1 CH_2-CO_2-C_{17}H_{35}-ester_2)
(i) One ester is saturated; the other is unsaturated. Describe a test to distinguish between them.
test .........................................................................................
result with unsaturated ester ................................................
result with saturated ester ..................................................
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(ii) Deduce which one of the above esters is unsaturated. Give a reason for your choice.
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(iii) Both esters are hydrolysed by boiling with aqueous sodium hydroxide. What types of compound are formed?
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Nitrogen can form ionic compounds with reactive metals and covalent compounds with non-metals.

(a) Nitrogen reacts with lithium to form the ionic compound lithium nitride, $\text{Li}_3\text{N}$.
(i) Write the equation for the reaction between lithium and nitrogen.
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(ii) Lithium nitride is an ionic compound. Draw a diagram which shows its formula, the charges on the ions and the arrangement of the valency electrons around the negative ion.
Use x for an electron from a lithium atom.
Use o for an electron from a nitrogen atom.
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(b) Nitrogen fluoride is a covalent compound.
(i) Draw a diagram showing the arrangement of the valency electrons in one molecule of the covalent compound nitrogen trifluoride, $\text{NF}_3$.
Use x for an electron from a nitrogen atom.
Use o for an electron from a fluorine atom.
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(ii) Lithium nitride has a high melting point, 813°C. Nitrogen trifluoride has a low melting point, –207°C.
Explain why the melting points are different.
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