Cambridge IGCSE Chemistry - 0620 - Supplement Paper 4 2010 Summer Zone 2

For each of the following unfamiliar elements predict one physical and one chemical property.
(a) caesium (Cs)
physical property .........................................................................................
chemical property .........................................................................................
................................................................................................. [2]
(b) vanadium (V)
physical property .........................................................................................
chemical property .........................................................................................
................................................................................................. [2]
(c) fluorine (F)
physical property .........................................................................................
chemical property .........................................................................................
................................................................................................. [2]

The hydrolysis of complex carbohydrates to simple sugars is catalysed by enzymes called carbohydrases and also by dilute acids.
(a) (i) They are both catalysts. How do enzymes differ from catalysts such as dilute acids?
.................................................................................................................................................. [1]
(ii) Explain why ethanol, C₂H₆O, is not a carbohydrate but glucose, C₆H₁₂O₆, is a carbohydrate.
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.................................................................................................................................................. [2]

(b) Draw the structure of a complex carbohydrate, such as starch. The formula of a simple sugar can be represented by HO🔲OH.

(c) Iodine reacts with starch to form a deep blue colour.
(i) In the experiment illustrated below, samples are removed at intervals and tested with iodine in potassium iodide solution.

Typical results of this experiment are shown in the table.
[Table_1]
Explain these results.
..................................................................................................................................................
..................................................................................................................................................
.................................................................................................................................................. [3]
(ii) If the experiment was repeated at a higher temperature, 60°C, all the samples stayed blue. Suggest an explanation.
.................................................................................................................................................. [1]

The following are examples of redox reactions.

(a) Bromine water was added to aqueous sodium sulfide.

$Br_2(aq) + S^{2-}(aq) \rightarrow 2Br^{-}(aq) + S(s)$

(i) Describe what you would observe when this reaction occurs.
.........................................................
......................................................... [2]

(ii) Write a symbol equation for this reaction.
......................................................... [1]

(iii) Explain, in terms of electron transfer, why bromine is the oxidant (oxidising agent) in this reaction.
.........................................................
......................................................... [2]

(b) Iron and steel in the presence of water and oxygen form rust.



The reactions involved are:

reaction 1
$Fe \rightarrow Fe^{2+} + 2e^{-}$
The electrons move through the iron on to the surface where a colourless gas forms.

reaction 2
$Fe^{2+} + 2OH^{-} \rightarrow Fe(OH)_2$

reaction 3
..........$Fe(OH)_2 + O_2 + ..........H_2O \rightarrow ..........Fe(OH)_3$
The water evaporates to leave rust.

(i) What type of reaction is reaction 1? ........................................ [1]

(ii) Deduce the name of the colourless gas mentioned in reaction 1.
......................................................... [1]

(iii) What is the name of the iron compound formed in reaction 2?
......................................................... [1]

(iv) Balance the equation for reaction 3.
..........$Fe(OH)_2 + O_2 + ..........H_2O \rightarrow ..........Fe(OH)_3$ [1]

(v) Explain why the change $Fe(OH)_2$ to $Fe(OH)_3$ is oxidation.
.........................................................
......................................................... [1]

(vi) Explain why iron in electrical contact with a piece of zinc does not rust.
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.........................................................
......................................................... [3]

But-1-ene is a typical alkene. It has the structural formula shown below.

$$\text{CH}_3\text{CH}_2\text{CH} = \text{CH}_2$$

The structural formula of cyclobutane is given below.



(a) These two hydrocarbons are isomers.

(i) Define the term isomer.
........................................................................................................................
........................................................................................................................ [2]

(ii) Draw the structural formula of another isomer of but-1-ene.
........................................................................................................................ [1]

(iii) Describe a test which would distinguish between but-1-ene and cyclobutane.
reagent ..................................................................................................................
result with but-1-ene ..................................................................................................
........................................................................................................................
result with cyclobutane ..............................................................................................
........................................................................................................................ [3]

(b) Describe how alkenes, such as but-1-ene, can be made from alkanes.
........................................................................................................................
........................................................................................................................ [2]

(c) Name the product formed when but-1-ene reacts with:
bromine, ........................................................................................................................ [1]
hydrogen, ........................................................................................................................ [1]
steam. ........................................................................................................................ [1]

Fuel cells are used in spacecraft to produce electrical energy.



(a) How is oxygen obtained from liquid air?
......................................................................................................................................................................
............................................................................................................................................................................ [2]

(b) Hydrogen and oxygen react to form water.
\[2H_2 + O_2 \rightarrow 2H_2O\]

(i) Give an example of bond breaking in the above reaction.
...................................................................................................................................................................... [1]

(ii) Give an example of bond forming in the above reaction.
...................................................................................................................................................................... [1]

(iii) Is the change given in (i) exothermic or endothermic?
...................................................................................................................................................................... [1]

(c) (i) Give \textbf{two} reasons why hydrogen may be considered to be the ideal fuel for the future.
......................................................................................................................................................................
......................................................................................................................................................................
...................................................................................................................................................................... [2]

(ii) Suggest a reason why hydrogen is not widely used at the moment.
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...................................................................................................................................................................... [1]

Thallium is a metal in Group III. It has oxidation states of +1 and +3.
(a) Give the formula for the following thallium compounds.
(i) thallium(I) sulfide ........................................................... [1]
(ii) thallium(III) chloride ......................................................... [1]
(b) Thallium(I) chloride is insoluble in water. Complete the description of the preparation of a pure sample of this salt.
Step 1
Mix a solution of sodium chloride with thallium(I) sulfate solution. A white precipitate forms.
Step 2
............................................................................................ [1]
Step 3
............................................................................................ [1]
Step 4
............................................................................................ [1]
(c) When thallium(I) chloride is exposed to light, a photochemical reaction occurs. It changes from a white solid to a violet solid.
(i) Name another metal halide which changes colour when exposed to light. Give the major use of this metal halide.
name .................................................................
use ................................................................. [2]
(ii) A piece of paper coated with thallium(I) chloride is exposed to a bright light.

Suggest two ways of increasing the time it takes for the violet colour to appear.
............................................................................................
............................................................................................
............................................................................................ [2]
(d) Thallium(I) hydroxide is an alkali. It has similar properties to sodium hydroxide.
(i) Complete the following word equation.
thallium(I) hydroxide + ammonium sulfate $ ightarrow$ .............. + .............. + .............. [1]
(ii) Complete the equation.
.......... TlOH + H$_2$SO$_4$ $ ightarrow$ .............. + .............. [2]
(iii) Aqueous thallium(I) hydroxide was added to aqueous iron(II) sulfate. Describe what you would see and complete the ionic equation for the reaction.
observation ........................................................ [1]
equation Fe$^{2+}$ + ........ OH$^-$ $ ightarrow$ .............. [1]

Aluminium was first isolated in 1827 using sodium.
$AlCl_3 + 3Na \rightarrow Al + 3NaCl$
Aluminium, obtained by this method, was more expensive than gold.
(a) Suggest an explanation why aluminium was so expensive.
...............................................................................................................................
............................................................................................................................ [1]
(b) The modern method for extracting aluminium is the electrolysis of a molten electrolyte, aluminium oxide dissolved in cryolite. The aluminium oxide decomposes.
$2Al_2O_3 \rightarrow 4Al + 3O_2$
Both electrodes are made of carbon.
(i) Give two reasons why the oxide is dissolved in cryolite.
.................................................................................................................................
.................................................................................................................................
........................................................................................................................... [2]
(ii) Complete the ionic equation for the reaction at the anode.
......... $O^{2-} \rightarrow O_2 + ......... e^{-}$
[2]
(iii) Why do the carbon anodes need to be replaced frequently?
...............................................................................................................................
........................................................................................................................... [1]
(c) The electrolysis of a molten electrolyte is one method of extracting a metal from its ore. Other methods are the electrolysis of an aqueous solution and the reduction of the oxide by carbon. Explain why these last two methods cannot be used to extract aluminium.
electrolysis of an aqueous solution ........................................................................
.................................................................................................................................
using carbon ...........................................................................................................
........................................................................................................................... [2]

Nitrogen dioxide is a brown gas. It can be made by heating certain metal nitrates. $$2Pb(NO_3)_2 \rightarrow 2PbO + 4NO_2 + O_2$$ (a) (i) Name another metal whose nitrate decomposes to give the metal oxide, nitrogen dioxide and oxygen. ............................................................................................................ [1] (ii) Complete the word equation for a metal whose nitrate does not give nitrogen dioxide on decomposition. metal nitrate $\rightarrow$ ............................................. + oxygen [1] (b) At most temperatures, samples of nitrogen dioxide are equilibrium mixtures. $$2NO_2(g) \rightleftharpoons N_2O_4(g)$$ dark brown                 pale yellow (i) At 25°C, the mixture contains 20% of nitrogen dioxide. At 100°C this has risen to 90%. Is the forward reaction exothermic or endothermic? Give a reason for your choice. ...................................................................................................... ...................................................................................................... ...................................................................................................... [2] (ii) Explain why the colour of the equilibrium mixture becomes lighter when the pressure on the mixture is increased. ...................................................................................................... ...................................................................................................... ...................................................................................................... [2]

(c) A 5.00g sample of impure lead(II) nitrate was heated. The volume of oxygen formed was 0.16 dm³ measured at r.t.p. The impurities did not decompose. Calculate the percentage of lead(II) nitrate in the sample. $$2Pb(NO_3)_2 \rightarrow 2PbO + 4NO_2 + O_2$$ Number of moles of $O_2$ formed = ....................................... Number of moles of $Pb(NO_3)_2$ in the sample = ....................................... Mass of one mole of $Pb(NO_3)_2$ = 331 g Mass of lead(II) nitrate in the sample = ....................................... g Percentage of lead(II) nitrate in sample = ....................................... [4]