AS & A Level Chemistry - 9701 Paper 2 2010 Winter Zone 3

The element magnesium, Mg, proton number 12, is a metal which is used in many alloys which are strong and light.
Magnesium has several naturally occurring isotopes.
(a) What is meant by the term isotope?
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(b) Complete the table below for two of the isotopes of magnesium.
[Table_1: isoptope table]
A sample of magnesium had the following isotopic composition:
$^{24}\text{Mg, 78.60%}; \; ^{25}\text{Mg, 10.11%}; \; ^{26}\text{Mg, 11.29%}$.
(c) Calculate the relative atomic mass, $A_r$, of magnesium in the sample. Express your answer to an appropriate number of significant figures. [2]

Antimony, Sb, proton number 51, is another element which is used in alloys.
Magnesium and antimony each react when heated separately in chlorine.
(d) Construct a balanced equation for the reaction between magnesium and chlorine.
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When a 2.45g sample of antimony was heated in chlorine under suitable conditions, 4.57g of a chloride A were formed.
(e) (i) Calculate the amount, in moles, of antimony atoms that reacted.

(ii) Calculate the amount, in moles, of chlorine atoms that reacted.

(iii) Use your answers to (i) and (ii) to determine the empirical formula of A.

(iv) The empirical and molecular formulae of A are the same. Construct a balanced equation for the reaction between antimony and chlorine.
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(f) The chloride A melts at 73.4°C while magnesium chloride melts at 714°C.
(i) What type of bonding is present in magnesium chloride? .........................................................................................................................
(ii) Suggest what type of bonding is present in A. .........................................................................................................................[2]

(a) Write a balanced equation for each of these reactions in the correct sequence. Where appropriate, use \( \rightleftharpoons \) to indicate that the reaction is an equilibrium.

first reaction .....................................................................................................................

second reaction .................................................................................................................

third reaction ......................................................................................................................[4]

(b) Give three different operating conditions that are used in the second stage.

condition 1 ......................................................................................................................

condition 2 ......................................................................................................................

condition 3 ......................................................................................................................[3]

(c) State one large scale use of sulfuric acid.

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(d) Most of the sulfur that is used in the Contact process is recovered from sulfur compounds present in crude oil and natural gas by using the Claus process.

(i) In this process, about one third of the hydrogen sulfide, \( H_2S \), present in the oil or gas, is converted into sulfur dioxide, \( SO_2 \).

Balance the equation for this reaction.

.....\( H_2S \) + .....\( O_2 \) \( \rightarrow \) .....\( SO_2 \) + .....\( H_2O \)

(ii) The \( SO_2 \) formed is then reacted catalytically with the remaining \( H_2S \), producing sulfur and water.

\( 2H_2S + SO_2 \rightarrow 3S + 2H_2O \)

What are the oxidation numbers of each of the sulfur-containing substances in this reaction?

\( H_2S \).......... \( SO_2 \)......... \( S \) .......

Which substance is reduced? Explain your answer.

substance ...........

explanation ..........................................................................................................................[3]

The sulfur present in crude oil is removed in order to prevent the formation of sulfur dioxide when fuels such as petrol (gasoline) or diesel fuel are burned in internal combustion engines.
Other substances that may be present in the exhaust gases of motor vehicles include CO, \( CO_2 \), \( NO/NO_2 \), and unburnt hydrocarbons.
The emission of sulfur dioxide can produce ‘acid rain’.

(e) (i) Outline, with the aid of equations, how acid rain is formed from the exhaust gases of motor vehicles.

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(ii) State one environmental effect of acid rain.

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(f) Sulfur dioxide is used to preserve dried fruits and vegetables.

What chemical property of \( SO_2 \) enables it to be used as a food preservative?

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(a) (i) Dicarbon monoxide can be produced in a laboratory and analysis of it shows that the sequence of atoms in this molecule is carbon-carbon-oxygen and there are no unpaired electrons, but one of the atoms is only surrounded by six electrons.
Draw a 'dot-and-cross' diagram of $C_2O$ and suggest the shape of the molecule.

shape .................................

(ii) What is meant by the term free radical?

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(iii) Explain why $NH_2$ is described as a 'free radical'.

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(b) Like ethene, acrylonitrile can be polymerised. The resulting polymer can be used to make carbon fibres.

(i) Draw the structural formula of the polymer made from acrylonitrile, showing two repeat units.

(ii) What type of polymerisation is this reaction?

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(c) (i) Suggest the structural formula of the organic compound $Z$.

(ii) Suggest the structural formula of another isomer of vinyl alcohol which has a cyclic (ring) structure.

(d) Give the structural formulae of the organic compounds formed when acrolein is reacted separately with each of the following reagents.

[Table]

reagent    product

$Br_2$ in an inert solvent

$NaCN$ + dilute $H_2SO_4$

Tollens' reagent

$NaBH_4$

Although few halogenoalkanes exist naturally, such compounds are important as intermediates in organic reactions and as solvents.

The bromoalkane B has the following composition by mass: C, 29.3%; H, 5.7%; Br, 65.0%. The relative molecular mass of B is 123.

(a) Calculate the molecular formula of B. [3]

Halogenoalkanes such as bromoethane, $\text{C}_2\text{H}_5\text{Br}$, have two different reactions with sodium hydroxide, \text{NaOH}, depending on the conditions used.

(b) (i) When hot aqueous \text{NaOH} is used, the \text{C}_2\text{H}_5\text{Br} is hydrolysed to ethanol, \text{C}_2\text{H}_5\text{OH}.

Describe the mechanism of this reaction. In your answer, show any relevant charges, dipoles, lone pairs of electrons and movement of electron pairs by curly arrows.

(ii) What will be formed when \text{C}_2\text{H}_5\text{Br} is reacted with \text{NaOH} under different conditions?

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(iii) What are the conditions used?

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(iv) What type of reaction is this?

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When 1,4-dichlorobutane, \text{ClCH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl}, is reacted with \text{NaOH}, two different reactions can occur, depending on the conditions used.

(c) (i) Draw the displayed formula of the product formed when 1,4-dichlorobutane is reacted with hot aqueous \text{NaOH} as in (b)(i).

(ii) Draw the skeletal formula of the product formed when 1,4-dichlorobutane is reacted with \text{NaOH} in the way you have described in (b)(ii) and (b)(iii). [2]

A student placed separate small samples of 1-chlorobutane, 1-bromobutane, and, 1-iodobutane, in three separate test-tubes. To each test-tube, 1 cm$^3$ of ethanol was added, followed by 1 cm$^3$ of aqueous silver nitrate, AgNO$_3$. The tubes were then carefully shaken, placed in a test-tube rack and observed for 30 minutes.
A precipitate was formed in each test-tube but not at the same time; the fastest taking about two minutes to become opaque and the slowest about 20 minutes.

(a) What is the identity of the precipitate formed when 1-chlorobutane is used?
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(b) What will be the colour of this precipitate?
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(c) Which of the three halogenoalkanes will produce a precipitate in about two minutes?
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(d) Use appropriate data from the Data Booklet to explain why this reaction takes place most quickly of the three.
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