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

This question is concerned with the following oxides.

sulfur dioxide
carbon monoxide
lithium oxide
aluminium oxide
nitrogen dioxide
strontium oxide

(a) (i) Which of the above oxides will react with hydrochloric acid but not with aqueous sodium hydroxide?
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(ii) Which of the above oxides will react with aqueous sodium hydroxide but not with hydrochloric acid?
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(iii) Which of the above oxides will react with both hydrochloric acid and aqueous sodium hydroxide?
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(iv) Which of the above oxides will not react with hydrochloric acid or with aqueous sodium hydroxide?
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(b) Two of the oxides are responsible for acid rain. Identify the two oxides and explain their presence in the atmosphere.
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(c) Lithium oxide is an ionic compound.
(i) Identify another ionic oxide in the list on page 3.
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(ii) Draw a diagram which shows the formula of lithium oxide, the charges on the ions and the arrangement of the valency electrons around the negative ion. Use x to represent an electron from an atom of oxygen. Use o to represent an electron from an atom of lithium.
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Two important greenhouse gases are methane and carbon dioxide.
(a) Methane is twenty times more effective as a greenhouse gas than carbon dioxide. The methane in the atmosphere comes from both natural and industrial sources.
(i) Describe two natural sources of methane.
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(ii) Although methane can persist in the atmosphere for up to 15 years, it is eventually removed by oxidation. What are the products of this oxidation?
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(b) How do the processes of respiration, combustion and photosynthesis determine the percentage of carbon dioxide in the atmosphere?
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Aluminium is extracted by the electrolysis of a molten mixture of alumina, which is aluminium oxide, and cryolite.



(a) (i) Alumina is obtained from the main ore of aluminium. Name this ore.
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(ii) Explain why it is necessary to use a mixture, alumina and cryolite, rather than just alumina.
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(iii) Copper can be extracted by the electrolysis of an aqueous solution. Suggest why the electrolysis of an aqueous solution cannot be used to extract aluminium.
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(b) The ions which are involved in the electrolysis are $\text{Al}^{3+}$ and $\text{O}^{2-}$. The products of this electrolysis are given on the diagram. Explain how they are formed. Use equations where appropriate.
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(c) The uses of a metal are determined by its properties.
(i) Foods which are acidic can be supplied in aluminium containers.



Explain why the acid in the food does not react with the aluminium.
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(ii) Explain why overhead electrical power cables are made from aluminium with a steel core.



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Reversible reactions can come to equilibrium. The following are three examples of types of gaseous equilibria.
A₂(g) + B₂(g) ⇌ 2AB(g) \text{ reaction 1}
A₂(g) + 3B₂(g) ⇌ 2AB₃(g) \text{ reaction 2}
2AB₂(g) ⇌ 2AB(g) + B₂(g) \text{ reaction 3}
(a) Explain the term \textit{equilibrium}.
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(b) The following graphs show how the percentage of products of a reversible reaction at equilibrium could vary with pressure. For each graph, decide whether the percentage of products decreases, increases or stays the same when the pressure is \textit{increased}, then match each graph to one of the above reactions and give a reason for your choice.
(i)
effect on percentage of products ..........................................................
reaction ............................................................................................
reason ............................................................................................
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(ii)
effect on percentage of products ..........................................................
reaction ............................................................................................
reason ............................................................................................
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(iii)
effect on percentage of products ..........................................................
reaction ............................................................................................
reason ............................................................................................
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The rate of the reaction between iron and aqueous bromine can be investigated using the apparatus shown below.
(a) A piece of iron was weighed and placed in the apparatus. It was removed at regular intervals and the clock was paused. The piece of iron was washed, dried, weighed and replaced. The clock was restarted. This was continued until the solution was colourless. The mass of iron was plotted against time. The graph shows the results obtained. [Graph_1]
(i) Suggest an explanation for the shape of the graph.
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(ii) Predict the shape of the graph if a similar piece of iron with a much rougher surface had been used. Explain your answer.
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(iii) Describe how you could find out if the rate of this reaction depended on the speed of stirring.
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(b) Iron has two oxidation states +2 and +3. There are two possible equations for the redox reaction between iron and bromine.
\( ext{Fe} + ext{Br}_2 ightarrow ext{Fe}^{2+} + 2 ext{Br}^- \)
\( 2 ext{Fe} + 3 ext{Br}_2 ightarrow 2 ext{Fe}^{3+} + 6 ext{Br}^- \)
(i) Indicate, on the first equation, the change which is oxidation. Give a reason for your choice.
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(ii) Which substance in the first equation is the reductant (reducing agent)?
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(c) Describe how you could test the solution to find out which ion, \( ext{Fe}^{2+} \) or \( ext{Fe}^{3+} \), is present.
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Structural formulae are an essential part of Organic Chemistry.
(a) Draw the structural formula of each of the following. Show all the bonds in the structure.
(i) ethanoic acid
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(ii) ethanol
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(b) (i) Ethanoic acid and ethanol react to form an ester. What is the name of this ester?
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(ii) The same linkage is found in polyesters. Draw the structure of the polyester which can be formed from the monomers shown below.
HOOC—C$_6$H$_4$—COOH and HO—CH$_2$—CH$_2$—OH
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(iii) Describe the pollution problems caused by non-biodegradable polymers.
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(c) Two macromolecules have the same amide linkage.
Nylon, a synthetic polymer, has the following structure.

Protein, a natural macromolecule, has the following structure.

How are they different?
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Some hydroxides, nitrates and carbonates decompose when heated. (a) (i) Name a metal hydroxide which does not decompose when heated.
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(ii) Write the equation for the thermal decomposition of copper(II) hydroxide.
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(iii) Suggest why these two hydroxides behave differently.
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(b) (i) Metal nitrates, except those of the Group 1 metals, form three products when heated. Name the products formed when zinc nitrate is heated.
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(ii) Write the equation for the thermal decomposition of potassium nitrate.
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(c) There are three possible equations for the thermal decomposition of sodium hydrogencarbonate.

$2NaHCO_3(s) \rightarrow Na_2O(s) + 2CO_2(g) + H_2O(g)$ equation 1
$NaHCO_3(s) \rightarrow NaOH(s) + CO_2(g)$ equation 2
$2NaHCO_3(s) \rightarrow Na_2CO_3(s) + CO_2(g) + H_2O(g)$ equation 3
The following experiment was carried out to determine which one of the above is the correct equation.
A known mass of sodium hydrogencarbonate was heated for ten minutes. It was then allowed to cool and weighed.
Results
Mass of sodium hydrogencarbonate = 3.36g
Mass of the residue = 2.12g
Calculation
$M_r$ for $NaHCO_3 = 84 g$; $M_r$ for $Na_2O = 62 g$; $M_r$ for $NaOH = 40 g$
$M_r$ for $Na_2CO_3 = 106 g$
(i) Number of moles of $NaHCO_3$ used = ..............
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(ii) If residue is $Na_2O$, number of moles of $Na_2O$ = ..............
If residue is $NaOH$, number of moles of $NaOH$ = ..............
If residue is $Na_2CO_3$, number of moles of $Na_2CO_3$ = ..............
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(iii) Use the number of moles calculated in (i) and (ii) to decide which one of the three equations is correct. Explain your choice.
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