Cambridge IGCSE Chemistry - 0620 - Supplement Paper 4 2010 Summer Zone 1

Choose an element which fits each of the following descriptions.
(i) It is a yellow solid which burns to form an acidic oxide. .............................................................................................................................. [1]
(ii) This element is a black solid which, when heated, forms a purple vapour. .............................................................................................................................. [1]
(iii) Most of its soluble salts are blue. .............................................................................................................................. [1]
(iv) It has a basic oxide of the type MO which is used to treat acidic soils. .............................................................................................................................. [1]
(v) It is an unreactive gas used to fill balloons. .............................................................................................................................. [1]

Ozone is a form of oxygen. Ozone is present in the upper atmosphere and it prevents dangerous solar radiation from reaching the Earth's surface. Some of the chemicals that diffuse into the upper atmosphere decompose ozone. Chemicals that have this effect are methane ($CH_4$), chloromethane ($CH_3Cl$) and an oxide of nitrogen ($NO_2$).

(i) Which of these three chemicals diffuses the most slowly? Give a reason for your choice.
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(ii) Chloromethane is formed when seaweed decomposes. Name the compounds in the environment from which seaweed might have obtained the following elements:

carbon; ...............................................................................................................
hydrogen; ...........................................................................................................
chlorine. ............................................................................................................. [3]

(iii) How can chloromethane be made from methane?

reagent ...................................................................................................................
condition .................................................................................................................. [2]

(iv) The oxides of nitrogen are atmospheric pollutants. Describe how they are formed.
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........................................................................................................................ [2]

(v) Complete the equation for the decomposition of ozone.
............$O_3$ $\rightarrow$ ............ [2]

A diagram of the apparatus which could be used to investigate the rate of reaction between magnesium and an excess of an acid is drawn below.

(a) The magnesium kept rising to the surface. In one experiment, this was prevented by twisting the magnesium around a piece of copper. In a second experiment, the magnesium was held down by a plastic net fastened to the beaker.
(i) Suggest a reason why magnesium, which is denser than water, floated to the surface. [1]
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(ii) Iron, zinc and copper have similar densities. Why was copper a better choice than iron or zinc to weigh down the magnesium? [1]
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(b) The only difference in the two experiments was the method used to hold down the magnesium. The results are shown below.

(i) In which experiment did the magnesium react faster? [1]
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(ii) Suggest a reason why the experiment chosen in (i) had the faster rate. [1]
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(c) The experiment was repeated using 1.0 mol/dm³ propanoic acid instead of 1.0 mol/dm³ hydrochloric acid. Propanoic acid is a weak acid.
(i) How would the graph for propanoic acid differ from the graph for hydrochloric acid? [1]
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(ii) How would the graph for propanoic acid be the same as the graph for hydrochloric acid? [1]
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(d) Give two factors which would alter the rate of this reaction. For each factor explain why it alters the rate. [4]
factor ..........................................................................................................................
explanation .................................................................................................................
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factor ..........................................................................................................................
explanation .................................................................................................................
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Hydrolysis is used in chemistry to break down complex molecules into simpler ones.
(a) Compounds containing the group $\overset{O}{\parallel}C$ or $\text{--COO--}$ are esters.
(i) Give the names and formulae of the two compounds formed when the ester ethyl propanoate is hydrolysed.

name ....................................................... name .......................................................
formula formula
[4]
(ii) Fats are naturally occurring esters. They can be hydrolysed by boiling with aqueous sodium hydroxide.

What type of compound has the formula $C_{17}H_{35}COONa$ and what is its main use?
type of compound ................................................................................
use ..................................................................................... [1]
(iii) Name a synthetic polyester.
.......................................................................................... [1]

(b) The structure of a typical protein is drawn below.

(i) What is the name of the polymer linkage?
.......................................................................................... [1]
(ii) Draw the structural formula of a man-made polymer with the same linkage.

[3]
(iii) A protein can be hydrolysed to a mixture of amino acids which are colourless. Individual amino acids can be identified by chromatography. The $R_f$ value of the amino acid glycine is 0.5. Describe how you could show that glycine was present on a chromatogram.
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Carbon and silicon are elements in Group IV. Both elements have macromolecular structures.
(a) Diamond and graphite are two forms of the element carbon.
(i) Explain why diamond is a very hard substance.
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(ii) Give one use of diamond.
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(iii) Explain why graphite is a soft material.
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(iv) Give one use of graphite.
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(b) Two of the oxides of these elements are carbon dioxide, $CO_2$, and silicon(IV) oxide, $SiO_2$.
(i) Draw a diagram showing the arrangement of the valency electrons in one molecule of the covalent compound carbon dioxide. Use x to represent an electron from a carbon atom. Use o to represent an electron from an oxygen atom.
[3]
(ii) A section of the macromolecular structure of silicon(IV) oxide is given below.

Use this diagram to explain why the formula is $SiO_2$ not $SiO_4$.
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(iii) Predict two differences in the physical properties of these two oxides.
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Iodine reacts with chlorine to form dark brown iodine monochloride.
$ I_2 + Cl_2 \rightarrow 2ICl $
This reacts with more chlorine to give yellow iodine trichloride. There is an equilibrium between these iodine chlorides.
$ ICl(l) + Cl_2(g) \rightleftharpoons ICl_3(s) $
dark brown          yellow

(a) Explain what is meant by equilibrium.
.................................................................................................................................................................................. .................................................................................................................................................................................. ................................................................................................................................................................................. [2]

(b) When the equilibrium mixture is heated it becomes a darker brown colour. Is the reverse reaction endothermic or exothermic? Give a reason for your choice.
.................................................................................................................................................................................. .................................................................................................................................................................................. .................................................................................................................................................................................. [2]

(c) The pressure on the equilibrium mixture is decreased.
(i) How would this affect the position of equilibrium and why?
It would move to the ............................................................................................................................. [1]
reason .................................................................................................................................................. .................................................................................................................................................................................. [1]

(ii) Describe what you would observe.
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Titanium is a transition element. It is isolated by the following reactions.
titanium ore $\rightarrow$ titanium(IV) oxide $\rightarrow$ titanium(IV) chloride $\rightarrow$ titanium
$\text{TiO}_2 \quad \text{TiCl}_4 \quad \text{Ti}$
(a) Why is it usually necessary to include a number in the name of the compounds of transition elements?
.............................................................................................................................................................. [1]
(b) Titanium(IV) chloride is made by heating the oxide with coke and chlorine.
$$\text{TiO}_2 + 2\text{Cl}_2 \rightleftharpoons \text{TiCl}_4 + \text{O}_2$$
$$2\text{C} + \text{O}_2 \rightleftharpoons 2\text{CO}$$
Explain why the presence of coke ensures the maximum yield of the metal chloride.
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(c) Explain why the change, titanium(IV) chloride to titanium, is reduction.
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(d) Complete the table which shows some of the properties of titanium and its uses. The first line has been completed as an example.

Methanoic acid is the first member of the homologous series of carboxylic acids.
(a) Give two general characteristics of a homologous series.
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(b) In some areas when water is boiled, the inside of kettles become coated with a layer of calcium carbonate. This can be removed by adding methanoic acid.
(i) Complete the equation.
............ HCOOH + CaCO$_3$ → Ca(HCOO)$_2$ + ............... + ............... [2]
(ii) Methanoic acid reacts with most metals above hydrogen in the reactivity series. Complete the word equation.
zinc + methanoic acid → ............................................. + ............................................. [2]
(iii) Aluminium is also above hydrogen in the reactivity series. Why does methanoic acid not react with an aluminium kettle?
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.......................................................................................................................................................... [1]
(c) Give the name, molecular formula and empirical formula of the fourth acid in this series.
name ..................................................................................................................................... [1]
molecular formula ...................................................................................................... [1]
empirical formula ........................................................................................................ [1]