Cambridge IGCSE Chemistry - 0620 - Supplement Paper 4 2013 Summer Zone 2

Air is a mixture of gases. The main constituents are the elements oxygen and nitrogen.
(a) (i) Name another element in air.
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(ii) Give the formula of a compound in unpolluted air.
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(b) Common pollutants present in air are the oxides of nitrogen and sulfur dioxide.
(i) How are the oxides of nitrogen formed?
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(ii) How is sulfur dioxide formed?
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(iii) These oxides are largely responsible for acid rain. State \textit{two} harmful effects of acid rain.
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(c) The percentage of oxygen in air can be determined by the following experiment.

The gas syringe contains 50 \text{cm}^3 of air. The large pile of copper is heated and the air is passed from one gas syringe to the other over the hot copper. The large pile of copper turns black. The gas is allowed to cool and its volume measured.
The small pile of copper is heated and the remaining gas passed over the hot copper. The copper does not turn black. The final volume of gas left in the apparatus is less than 50 \text{cm}^3.
(i) Explain why the copper in the large pile turns black.
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(ii) Why must the gas be allowed to cool before its volume is measured?
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(iii) Explain why the copper in the small pile did not turn black.
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(iv) What is the approximate volume of the gas left in the apparatus?
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(a) The table below gives the number of protons, neutrons and electrons in atoms or ions. Complete the table. The first line is given as an example. You will need to use the Periodic Table.

\begin{array}{cccc} \text{particle} & \text{number of} & \text{number of} & \text{number of} & \text{symbol or formula} \\ & \text{protons} & \text{electrons} & \text{neutrons} \\ \hline A & 4 & 4 & 5 & _{4}^{9}\text{Be} \\ B & 19 & 18 & 20 & \text{..................} \\ C & 30 & 30 & 35 & \text{..................} \\ D & 8 & 10 & 8 & \text{..................} \\ E & 31 & 31 & 39 & \text{..................} \\ \end{array} \hspace{8em} [6]
(b) Using the data in the table, explain how you can determine whether a particle is an atom, a negative ion or a positive ion.
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The diagram shows some of the processes which determine the percentage of carbon dioxide in the atmosphere.

(a) Explain how the following two processes alter the percentage of carbon dioxide in the atmosphere.
(i) combustion
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(ii) respiration
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(b) Photosynthesis reduces the percentage of carbon dioxide in the atmosphere.
(i) Complete the word equation for photosynthesis.
carbon dioxide + water \rightarrow ........................... + ........................... [2]
(ii) State two essential conditions for the above reaction to occur.
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At present the most important method of manufacturing hydrogen is steam reforming of methane.
(a) In the first stage of the process, methane reacts with steam at 800°C.
$$\text{CH}_4(g) + \text{H}_2\text{O}(g) \rightleftharpoons 3\text{H}_2(g) + \text{CO}(g)$$
In the second stage of the process, carbon monoxide reacts with steam at 200°C.
$$\text{CO}(g) + \text{H}_2\text{O}(g) \rightleftharpoons \text{CO}_2(g) + \text{H}_2(g)$$

(i) Explain why the position of equilibrium in the first reaction is affected by pressure but the position of equilibrium in the second reaction is not.
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(ii) Suggest why a high temperature is needed in the first reaction to get a high yield of products but in the second reaction a high yield is obtained at a low temperature.
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(b) Two other ways of producing hydrogen are cracking and electrolysis.

(i) Hydrogen can be a product of the cracking of long chain alkanes.
Complete the equation for the cracking of $\text{C}_8\text{H}_{18}$.
$$\text{C}_8\text{H}_{18} \rightarrow 2\text{................} + \text{H}_2$$ [1]

(ii) There are three products of the electrolysis of concentrated aqueous sodium chloride. Hydrogen is one of them.
Write an equation for the electrode reaction which forms hydrogen.
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(iii) Name the other two products of the electrolysis of concentrated aqueous sodium chloride and give a use of each one.
product ................................................ use ................................................
product ................................................ use ................................................ [4]

Many monomer molecules react together to form one molecule of a polymer. This reaction is called polymerisation.

(a) The structural formula of the polymer, poly(chloroethene), is given below. This polymer is also known as PVC.

[Image_1: Structural formula of PVC]

(i) A major use of PVC is insulation of electric cables. PVC is a poor conductor of electricity. Suggest another property which makes it suitable for this use.
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(ii) One way of disposing of waste PVC is by burning it. This method has the disadvantage that poisonous gases are formed. Suggest two poisonous gases which could be formed by the combustion of PVC.
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(b) (i) Deduce the structural formula of the monomer from that of the polymer.

[Image_2: Structural formula of polymer]

structural formula of monomer .................................................................................................................................. [1]

(ii) Deduce the structural formula of the polymer, poly(phenylethene), from the formula of its monomer, phenylethene.

[Image_3: Structural formula of phenylethene]

structural formula of polymer ............................................................................................................................ [2]

(c) The carbohydrate, glucose, polymerises to form the more complex carbohydrate starch. If glucose is represented by

[Image_4: Representation of glucose] then the structural formula of starch is as drawn below.

[Image_5: Structural formula of starch]

How does the polymerisation of glucose differ from that of an alkene such as phenylethene?
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Aluminium is an important metal with a wide range of uses.
(a) Aluminium is obtained by the electrolysis of aluminium oxide dissolved in molten cryolite.

(i) Solid aluminium oxide is a poor conductor of electricity. It conducts either when molten or when dissolved in molten cryolite. Explain why.
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(ii) Why is a solution of aluminium oxide in molten cryolite used rather than molten aluminium oxide?
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(iii) Explain why the carbon anodes need to be replaced periodically.
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(iv) One reason why graphite is used for the electrodes is that it is a good conductor of electricity. Give another reason.
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(b) Aluminium is used to make food containers because it resists corrosion. Explain why it is not attacked by the acids in food.
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(c) Aluminium is used for overhead power (electricity) cables which usually have a steel core.

(i) Give two properties of aluminium which make it suitable for this use.
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(ii) Explain why the cables have a steel core.
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The ester linkage showing all the bonds is drawn as or more simply it can be written as $-COO-$. (a) (i) Give the structural formula of the ester ethyl ethanoate. (ii) Deduce the name of the ester formed from methanoic acid and butanol. (b) (i) Which group of naturally occurring compounds contains the ester linkage? (ii) Draw the structural formula of the polyester formed from the following monomers. HOOC C_6H_4COOH and HOCH_2CH_2OH You are advised to use the simpler form of the ester linkage.

(c) Esters can be used as solvents in chromatography. The following shows a chromatogram of plant acids. An ester was used as the solvent and the chromatogram was sprayed with bromothymol blue. (i) Suggest why it was necessary to spray the chromatogram. (ii) Explain what is meant by the $R_f$ value of a sample. (iii) Calculate the $R_f$ values of the two samples and use the data in the table to identify the plant acids. [Table_1] sample 1 $R_f = ..............$ It is ................. acid. sample 2 $R_f = ..............$ It is ................. acid.

(a) Define the following
(i) the mole
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(ii) the Avogadro constant
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(b) Which \textbf{two} of the following contain the same number of molecules? Show how you arrived at your answer.

2.0 g of methane, CH$_4$
8.0 g of oxygen, O$_2$
2.0 g of ozone, O$_3$
8.0 g of sulfur dioxide, SO$_2$
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(c) 4.8 g of calcium is added to 3.6 g of water. The following reaction occurs.

Ca + 2H$_2$O → Ca(OH)$_2$ + H$_2$

(i) the number of moles of Ca = ...................
the number of moles of H$_2$O = ................... [1]

(ii) Which reagent is in excess? Explain your choice.
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(iii) Calculate the mass of the reagent named in (ii) which remained at the end of the experiment.
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