MINISTRY OF EDUCATION, SINGAPORE in collaboration with UNIVERSITY OF CAMBRIDGE LOCAL EXAMINATIONS SYNDICATE General Certificate of Education Advanced Level

CANDIDATE NAME

CENTRE NUMBER

Higher 2

I I I I I I CHEMISTRY Paper 2 Structured Questions

Candidates answer on the Question Paper.

Additional Materials: Data Booklet.

READ THESE INSTRUCTIONS FIRST

INDEX NUMBER I I I I I

9647/02 OctoberlNovember 2010

2 hours

Write your Centre number, index number and name on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid.

Answer all questions. A Data Booklet is provided.

At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question.

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This document consists of 16 printed pages and 4 blank pages . . ,~""

'MB~ ~ CiJl1 Singapore Examinations and Assessment Board .~

© UCLES & MOE 2010 DC (NF/CGW) 25667/2

~ UNIVERSITY of CAMBRIDGE ~ International Examinations

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1 Planning (P)

We usually think of iron occurring in compounds in the +2 or +3 oxidation states. However, other oxidation states of iron are possible; for example, potassium ferrate(VI) , K2Fe04,

contains iron in the +6 oxidation state.

Solutions containing the FeO 4 2- ion are dark red in colour. The FeO 4 2- ion is a stronger oxidising agent than the Mn04- ion. In acidic conditions, FeOl- ions are reduced to Fe3+

ions.

A student was given a solution, FA1 , containing potassium ferrate(V I) . She was asked to find the concentration of FeO 4 2- ions in FA 1 by titrating it against a standard solution of sodium ethanedioate, Na2C20 4.

Ethanedioate ions are oxidised as shown in the following equation.

C20 l- ~ 2C02 + 2e-

She tested the 0.100 mol dm-3 solution of Na2C20 4 provided. She found that 10 drops of FA 1, when suitably acidified, required 32 drops of this Na2C20 4 solution for complete reaction. She correctly decided that the concentration of this solution was too low and prepared a more concentrated solution of Na2C20 4, FA2, for titration against the FA 1 solution.

(a) Construct a balanced equation for the reaction between FeOl - ions and C20 l- ions.

[1 ]

(b) Explain why the 0.100 mol dm-3 solution of Na2C20 4 first provided would have been unsuitable for use in the student's titration .

.. ... .. .......... ........... ... .. ..... .. ................ .. .... ... ... ..... ............... ........ .. .... .... ... .......... .... ...... .. . [1]

(c) Write a plan for the volumetric analysis of FA 1. You may use the reagents and apparatus normally found in a school or college laboratory.

In your plan you should give:

details, including quantities, for the preparation of FA2 from solid anhydrous Na2C20 4;

the essential details of the titration procedure.

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For •••••••••• ••••••••••.••••••.•••.•••.•••• •.••• •••• ••• .••.•...••••.•. .•••• ••••••.••••.••.••••• •••• .• ••••• .• ••• ••• ••• ••••• •••••••••• •••• ••••• Examiner's

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............ ........... .............. .... .. .... .. ... ... ... ... .... ..... .. ....... .. .. ..... ..... .. ... ......... .............. ........... [7]

(d) In the student's titration, she found that xcm3 of FA1 reacted with exactly ycm3 of FA2 of concentration M mol dm-3.

Outline how you would use her results to determine the concentration of FA 1.

. [2]

(e) Identify one potential safety hazard in this experiment and state how you would minimise this risk .

.. ... .................. .............. ............. ....................... ............ ............................... ........... .. ... [1]

[Total: 12]

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2 1-Bromobutane m·ay be made by reacting together butan-1 -01 , sodium bromide and concentrated sulfuric acid in the presence of water, as described below. Data about these four compounds and 1-bromobutane are given in the table. These data are repeated on page 8.

compound boiling density Mr

solubility in point l oe Igcm-3 water

1-bromobutane 102 1.35 137 insoluble

butan-1-01 118 0.81 74 moderate

sodium bromide 1390 3.20 103 soluble

concentrated sulfuric acid 330 1.84 98 soluble

water 100 1.00 18 -

Preparation of impure 1-bromobutane

Place 35 g of powdered sodium bromide, 30 cm3 of water and 25 cm3 of butan-1-01 in a 250cm3 two-necked flask. In one neck is placed a tap funnel and in the other neck is placed a reflux condenser.

2 Place 25 cm3 of concentrated sulfuric acid in the tap funnel and then add the acid drop by drop to the reagents in the flask. Keep the contents well shaken and cool occasionally in an ice-water bath.

3 . When all of the acid has been added, gently boil the mixture for about 45 minutes, shaking the flask gently from time to time.

4 Rearrange the apparatus for distillation. Distil off the impure 1-bromobutane (about 30cm3) .

(a) The overall reaction may be considered to take place in two stages, the first between inorganic reagents only and the second involving the organic reagent. Write an equation for each of these stages.

stage I .......... .. , ...... ............ ....... .. .. ........... ... ..... .. .. ........ ... .. ...... .. ...... .... .. ............... .. ...... ... ..

stage 11 ...... .. .. .. .. .. ... .. ...... .. .. .......... ..... .. ... ... .. ........... .......................... .. .. .... .... .. ............ [2]

(b) By using the amounts given above, one of the reagents , butan-1-01 or sodium bromide, will be present in an excess in this preparation. Use the data above to determine, by calculation, which reagent is in an excess.

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[3]

(c) When the concentrated sulfuric acid is added to the reaction mixture (step 2) , cooling is necessary. Two by-products, one inorganic and one organic, may be produced if the temperature is not controlled carefully.

(i) Suggest the main cause of heat being produced at this stage.

(ii) What by-products may be formed? In each case, identify the by-product and write an equation showing its formation .

inorganic by-product .. .... .. .... .... .......... .

equation ........ ... .. .... ... .. ... ... ...... ... .... .. .... .. .... .... ..... .......... .. .. .... .. .... ........ ....... .... ........ . .

organic by-product .... .. .. .... ...... .......... ..

equation [5]

(d) The reaction mixture is heated for 45 minutes (step 3). Why is this process necessary for the preparation of many covalent organic compounds? Explain your answer .

...... .......... .. .. ..... ... .. .. ... .. ... ... ...... .. .. .... ... ..... ..... ............. .. .. .... ... .... ... .... .... ...... .... ... ...... .. .. [2]

(e) When the mixture is distilled (step 4), the main product is 1-bromobutane. Two other compounds are also present as impurities in the distillate. Use the data on page 6 to suggest the identities of these two compounds and a reason for their presence.

compounds are .......... .. .... ...... .. ........ .. ... ..... ... .. . and .. .. ..... .. ........ .......... .. .. ... .. .. .... .... .. .... .

reason ................ .. .... .. ... .. ... ...... .... .. ... .... .. .. ............ .. .. ..... .. ...... ...... ....................... .. .. ... [2]

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The preparation of organic compounds usually produces a mixture of the required compound and other impurities. Obtaining the required compound in a pure state involves the application of chemical knowledge and principles. The purification of crude 1-bromobutane is described below. These data are repeated from page 6.

compound boiling density Mr

solubility in point 1°C Igcm-3 water

1-bromobutane 102 1.35 137 insoluble

butan-1-01 118 0.81 74 moderate

sodium bromide 1390 3.20 103 soluble

concentrated sulfuric acid 330 1.84 98 soluble

water 100 1.00 18

Purification of impure 1-bromobutane

5 Shake the distillate with water in a separating funnel and separate the aqueous layer from the 1-bromobutane. Reject the aqueous layer.

6 Return the 1-bromobutane to the funnel , add about half its volume of concentrated hydrochloric acid, and shake. Separate and discard the layer of acid.

7 Shake the 1-bromobutane cautiously with dilute aqueous sodium carbonate in the separating funnel, releasing the pressure at intervals.

8 Transfer the 1-bromobutane into a conical flask and add some granular anhydrous calcium chloride. Swirl the mixture until the liquid is clear.

9 Filter the 1-bromobutane into a clean , dry round-bottomed flask and distil it. Collect the fraction by boiling over a suitable range.

(1) After distillation from the reaction mixture, the impure 1-bromobutane is shaken with water (step 5) and the two layers are allowed to separate.

Will the 1-bromobutane be the upper or lower layer? ........... .. .. .. ........... .

Explain your answer.

..... ................. ... ...... ....... ...... ....... ..................... .. ......... .. .. .. ..... .... ... ......................... ...... [1 ]

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(9) After separating the 1-bromobutane from the water there will still be a very small amount of one of the original reactants present as an impurity. To remove this, the 1-bromobutane is shaken (step 6) with concentrated hydrochloric acid - a strong acid. A reaction occurs with the impurity present in the 1-bromobutane, making it more soluble in water.

(i) Suggest with which of the original reactants the concentrated hydrochloric acid reacts.

(ii) By using structural formulae , construct an equation for this reaction.

(iii) Suggest why the product in (ii) is more soluble in water than the original reactant.

[3]

(h) What impurity will be removed when the 1-bromobutane is then treated with dilute aqueous sodium carbonate (step 7)? Write an equation for this reaction.

impurity .. .. .. ..... ... ..... .. .. .. .... .... ...... ... .. ... ... .. . .

equation .. ... .. .. ... .. . .... ... ... ... .... .. . ... .. ...... .. .. ... . ... .... . ... .. . ... .. .. .. .... .. .... .. ... . .... .. .... ... .. ..... . ... [2]

(i) Suggest what is removed when the impure 1-bromobutane is treated with anhydrous calcium chloride (step 8) .

[1 ]

(j) The final step in the process of purification is distillation of the pure product (step 9). Suggest a suitable range of temperature for the collection of the required fraction.

from .... .... ...... ...... .. cc to ........ .. .. ...... .. cc [1 ]

[Total : 22]

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10

Nitrogen monoxide, NO, is a by-product of the combustion of hydrocarbon fuels in internal combustion engines.

(a) NO is considered to be involved in the formation of 'acid rain'. State one other undesirable consequence of the presence of NO in the atmosphere.

........... .... ... .. .. .... ... .. .... .. .. .. ... ... .. .... ..... .. ....... .. ........ ..... ...... .... ...... ... ..... .... ... .. ..... .. .... ..... . [1 ]

Nitrogen monoxide is readily converted into nitrogen dioxide.

In order to determine the rate equation for this reaction, an investigation was carried out at a constant temperature and with the same partial pressure of oxygen , Po ' for each experiment. The following results were obtained. 2

initial rate/N m- 2s-1 16.2 47.2 78.8 124 172

PNo /N m- 2 0.30 0.50 0.66 0 .82 0.95

(PNO)2/ N2 m- 4 0.09 0.25 0.44 0.67 0.90

(b) To determine the order of reaction with respect to PNO' use these data to plot a suitable graph on the grid below.

[3]

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(c) What is the order of reaction with respect to PNO? Explain your answer.

................................................ ............................................................... .. ................... [2]

(d) The partial pressure of oxygen was halved and a new series of experiments carried out at the same temperature. When a similar graph was plotted, the gradient at each point was half that obtained from the first graph. What is the order of reaction with respect to Po

2? Explain your answer.

............................. ....... ......... ...... ..... ........... ........ ......................................................... [2]

(e) (i) Construct the rate equation for the reaction between NO and 02.

(ii) What are the units of the rate constant for this reaction?

[2]

[Total: 10]

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Data concerning the elements of Group 11 of the Periodic Table, at 298 K, are given in the table. Further data may be found in the Data Booklet. No calculation is requi red.

Mg Ca Sr Ba

~Hhydration of M2+ I kJ mol- 1 -1980 -1650 -1480 -1365

~Shydration of M2+ I J mol- 1 K-1 -293 -238 -222 -188

~Ghydration of M2+ IkJ mol-1 -1895 -1582 -1415 -1310

solubility of sulfate I mol per 100 g water 1.8 x 10-1 4.7 x 10-3 7.1 x 10 -5 9.4 x 10-7

(a) When a salt such as a Group 11 sulfate dissolves in water, the lattice energy must be overcome.

(i) How will the magnitude of the lattice energy of Group 11 sulfates change from MgSO 4 to BaSO 4 ?

(ii) Suggest a reason for this trend.

[2]

(b) When the sulfate has dissolved, the anions and cations are each surrounded by a number of water molecules. The formation of these hydrated ions is an exothermic process.

(i) Draw simple diagrams to show how a water molecule can be attached to a magnesium cation, and to a sulfate anion. Label each diagram to show the type of interaction involved.

Mg2+ cation

(ii) Suggest why ~HhYdration of M2+ becomes less negative from Mg2+ to Ba2+.

[4]

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Most of the chemistry of magnesium relates to its ionic compounds. However, magnesium does form · an important group of covalent compounds which are known as Grignard reagents. In 1911 , the French chemist F.A. V. Grignard reacted small pieces of magnesium with a warm solution of bromoethane in a dry, non-polar solvent and obtained a solution containing ethylmagnesium bromide, C2HsMgBr. Many Grignard reagents, with different alkyl or aryl groups, have now been prepared and are widely used in organic syntheses. A typical example of the use of a Grignard reagent is the two-step reaction of C2HsMgBr with propanone, CH3COCH3 , to form 2-methylbutan-2-01.

OMgBr OH step I I step II I

C2HsMgBr + CH3COCH3 .. CH3 - C - CH3 --'--- CH3 - C - CH3 I I

+ Mg(OH)Br

C2HS C2HS

(c) (i) What type of reaction takes place in step I?

(ii) Suggest the type of reaction which occurs in step 11.

[2]

(d) Suggest the structural formula of the final organic product formed when Q-MgBr is reacted with ethanal, CH3CHO, in a similar two-step process.

[1 ]

(e) The Grignard reagent CH3CH2CH2MgBr can be readily converted into a carboxylic acid by using carbon dioxide.

Suggest a two-step reaction sequence for this conversion, giving structural formulae for the compounds involved.

step I step II .. + Mg(OH)Br

[2]

[Total: 11]

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14

Animal skin and bone contains the protein collagen. Gelatin, which is used in the food and pharmaceutical industries, is another protein which may be obtained from collagen.

The structure of a portion of the gelatin molecule is shown below. This portion contains a number of amino acids and imino acids. The latter are related to amino acids but contain the >NH group instead of the -NH2 group. In this question we will refer to both sorts of acid as 'amino acid'.

o 0 0 0 OH

C- N- CH-C-N- CH- C- N-CH- C-N H II H II H IIH 1I y:

o 0 b I I I HI ~~-CH-~-~-CH-~-N Cl H2 H Cl H2 C- N-CH-C-NC2 I I 11 11

CH H CH2 CH2 0 0

3 I I ~ CH2 C=O 11 I I 0 NH OH I C=NH I NH2

Gelatin is prepared by the denaturation of collagen.

(a) (i) What is meant by the term denaturation?

..... ... .. .. ...... ... ...... ....... ... ..... .... ........ .. .. .... ...... ......... ............. .. ... .. .. ... .... ..... ... .... ... ... ......

.. ... ... .... ..... ...... ..... ... ... ......... .. ..... .. ....... ...... ... .... ....... .... ....... ... .. ... .. .... ........ ... ..... .... .. ....

(ii) Suggest a brief outline of one method by which collagen may be denatured to produce gelatin .

.. ... .... ... .. .... ..... ... .... ... ... .. ... ..... ...... .. .. ... .... .... ... .. ..... .. .......... ... ... ........... .. ............... ...... .

.. ..... ... .. .. ....... .... ..... ........... .. .......... .. .. .... ... .. ...... .. ..... ... ... .... .. ... .... .. ... .. ..... .. .... .. ........ .. .. [2]

(b) How many different amino acid residues are present in this portion of the gelatin molecule?

[1 ]

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(c) Two of these amino acid residues are present more than once in this portion.

(i) In the boxes below, draw a displayed formula for each of these amino acids.

(ii) One of the amino acid residues present in this portion of the gelatin molecule contains more than one chiral atom.

Draw the displayed formula of this amino acid and indicate each chiral atom with an asterisk (*).

[5]

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Skin is a very tough , flexible and waterproof substance which is easily pierced by sharp objects.

Kevlar® is an aromatic polyamide which is used in stab-proof vests and also in high quality bicycle tyres.

o 0 11 11 C-Q-C-N----O-N

I~I H H n

Kevlar®

Proteins and Kevlar® may be hydrolysed in a similar manner.

(d) (i) How could proteins be hydrolysed in the laboratory to form a mixture of their constituent amino acids?

.... ........ .. ........... .. ... ......... .. .. .... ........ .. ...... ... .... .. .. ... .. ..... .. .. ......... ..... ...... ..... ... ........ ......

............. .. .... ... ............. .... .. .... .. ....... ... ... ....... ..... .. ............... .. .. .. ..... .. ............................

(ii) What compounds will be produced when Kevlar® is hydrolysed? Give their structural formulae.

(iii) Proteins and Kevlar® are polyamides. What is the essential difference between their monomers?

................. ..... ............... ... ...... ... .... .... .... ... .. ....................... .. ............ ...... ... ..................

.... ............. ........ .................. .. .............. .. .... ............... .. ..... .......... .......... ........ . .. . ......... .. [5]

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Sutures are the stitches that surgeons use to hold skin, internal organs, blood vessels and other tissues of the human body together after they have been severed by injury or surgery.

'Soluble stitches' which are used within the body may be made by reacting together glycolic acid, HOCH2C02H, and lactic acid, CH3CH(OH)C02H.

(e) When one molecule of glycolic acid reacts with one molecule of lactic acid, it is possible to form two esters with different structural formulae. .

Draw the structural formula of each of these esters.

[2]

In the material used for 'soluble stitches', many molecules of each acid have been reacted to form a long chain 'polyester' molecule which contains many ester groups. This polyester material may be used in surgery to sew up wounds inside the body. Over a period of time the polyester undergoes a chemical reaction and breaks up to re-form the two individual hydroxyacids.

(f) (i) Suggest what type of chemical reaction causes the polyester material to break up over a period of time.

(ii) Suggest why the products of this reaction are soluble in water.

[2]

[Total: 17]

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