mock paper i suggestedanswer

St. Paul’s Secondary School

F.7 Mock Examination (2003-2004)

Chemistry Paper I Suggested Answer

Time allowed: 3 hours

Section A (60 marks)

1. Phenylboronic acid, C6H5B(OH)2, and benzoic acid, C6H5COOH, are both monobasic acids,

with pKa values of 8.86 and 4.20 respectively.

(a) State the relationship between the pKa of an acid and its acid dissociation constant, Ka.

pKa =- log Ka (1)

(1 mark)

(b) Give the equation which represents the dissociation of phenylboronic acid.

C6H5B(OH)2(aq)≒ C6H5BO2H-(aq) + H+

(aq) (1)

(1 mark)

(c) Calculate the hydrogen ion concentration and the pH of 0.010M phenylboronic acid.

Let x M be the [H+] at eqm

Ka = [H+][A-]/[HA]～x2/0.01 = 10-8.86

∴x = 3.72 x 10-6 M (1)

pH =5.43 (1)

(2 marks)

(d) Which is the stronger of the two acids? Explain briefly.

Benzoic acid (1)

The smaller the pKa=> the larger the Ka,

∴the eqm shifts to the right=> more acidic (1)

(2 marks)

(e) A 10.0 cm3 sample of X, a solution containing both of these acids, was titrated against

0.050M NaOH using a mixture of two indicators, bromothymol blue and

phenolphthalein.

It was found that 8.6cm3 of NaOH(aq) were needed to change the colour of the first

indicator and a further 7.1cm3 were needed to change the colour of the second indicator.

(i) Sketch the shape of the pH curve during this titration.

Correct graph shape (1)

Correct end points with volume shown (1)

2004-SUGGESTED ANSWER-AL-SPSS-CHEM-I-1 -1-

(ii) Use the data to calculate the concentration of each of the two acids in X.

Let x M be the conc. of benzoic acid.

(8.6)(0.05) =10 x .

∴ x = 4.3 x 10-2 M (1)

Let y M be the concentration of phenylboronic acid

(7.1)(0.05) = 10 y

∴ y = 3.55 x 10-2 M (1)

(4 marks)

2. (a) How and why do CCl4 and SiCl4 differ in their reactions with water? Include in your

answer equations for any reactions that occur.

SiCl4 is hydrolyzed in water but CCl4 does not. (0.5)

It is because Si has low energy lying vacant d- orbital which can accept e-

from water molecule

(0.5)

SiCl4(l) +4H2O(l) Si(OH)4(s) +4HCl(g) (1)

(2 marks)

(b) Predict the shapes of BF3, CF4 and NF3 molecules.

BF3: trigonal planar (0.5)

CF4: tetrahedral (0.5)

NF3: trigonal pyramidal (0.5)

(1.5 marks)

(c) NF3 is inert to hydrolysis, whereas BF3 readily reacts with water. Suggest a reason for

this difference, and give and equation for the reaction that occurs.

BF3 has a vacant p-orbital to accept e- from water molecule. (1)

BF3(aq) + 3H2O(l) H3BO3(aq) +3HF(aq) (1)

(2 marks)

(d) When tin(IV) chloride reacts with potassium hydroxide in a small amount of water,

potassium chloride is produced, together with an anhydrous salt having the following

composition by mass: K,22.05%; Sn, 33.55%; Cl, 30.03%; O, 13.53%;H, 0.85%.

(i) Calculate the empirical formula of the salt.

K Sn Cl O H

% 22.05 33.55 30.03 13.53 0.85

r.a.m. 39.1 118.7 35.45 16 1.008

Relative no. of moles 0.564 0.283 0.847 0.846 0.843

Simplest ratio 2 1 3 3 3


K2[Sn(OH)3Cl3] (@row=0.5; total=1.5)

(ii) Deduce the structural formula of the anion and name its shape.

[Sn(OH)3Cl3]2- (1)

octahedral (1)

(iii) Write an equation for the reaction.

SnCl4(l) + 3KOH(aq) K2[Sn(OH)3Cl3](aq) + KCl(aq) (1)

(4.5 marks)

3. (a) Phenol, phenylmethanol and benzoic acid are three compounds containing benzene

rings. None of these compounds is particularly soluble in water. Phenol and benzoic acid

each dissolves in NaOH(aq), but only benzoic acid dissolves in Na2CO3(aq).

C6H5OH C6H5CH2OH C6H5COOH

Phenol Phenylmethanol Benzoic acid

(i) Write equations for the reactions taking place.

C6H5OH(aq) + NaOH(aq) C6H5ONa(aq) +H2O(l) (1)

C6H5COOH(aq) + NaOH(aq) C6H5COONa +H2O(l)

C6H5COOH(aq) + Na2CO3(aq) 2 C6H5COONa(aq) +H2O(l) +CO2(g) (1)

(ii) Arrange the trend in acidity of these compounds. Explain briefly.

C6H5COOH > C6H5OH > C6H5CH2OH (1)

Benzoic acid is the most acidic,

∵ e- is delocalized and –COO- gp is e- withdrawing . (0.5)

C6H5O- got the delocalisation of the –ve charge. (0.5)

(4 marks)

(b) The antiseptic TCP contains 2, 4, 6-trichlorophenol.

(i) Which reagent and conditions are used to convert phenol to 2,4,6-

trichlorophenol?

Cl2(aq)/Fe(s) + reflux (0.5+0.5)

(ii) Suggest whether 2,4, 6-trichlorophenol is more or less acidic than phenol.

Explain briefly.

It is more acidic than phenol (1)

The EN of Cl is high, ∴ they withdraw e- from the benzene ring and

stabilize the phenoxide ion. (1)

(3 marks)

(c) Suggest the structural formulae for the organic products of the reactions of

phenylmethanol with

(i) HBr(aq),

C6H5CH2Br (1)

(ii) Cr2O72-/H+(aq)

C6H5COOH (1)

(2 marks)

(d) Suggest why phenol does not undergo a similar reaction to that in (c)(ii).

The ring has extra-stability due to the delocalization of Π e-s in the benzene ring. (1)

(1 mark)

4. (a) Describe and name the mechanism of the reaction between ethanal and hydrogen


cyanide.

Mechanism

Nucleophilic addition (1)

(4 marks)

(b) The above reaction produces two isomeric products in equal amounts. Name the type of

isomerism involved, and draw their structures.

Enantiomers (1)

(1+1)

(3 marks)

(c) As a consequence of starvation or diabetes, the blood plasma and urine of patients can

contain large amounts of “ketone bodies”. These include propanone, 3-oxobutanoic

acid, and 3-hydroxybutanoic acid.

Describe a separate simple test in each case to distinguish 3-oxobutanoic acid from

(i) propanone, and

3-oxobutanoic acid reacts with sodium carbonate/hydrogencarbonate to give

CO2(g). (1)

Propanone will not react with it. (0.5)

(ii) 3-hydroxybutanoic acid.

3-oxobutanoic acid has a C=O gp which reacts with 2, 4-DNP (1)

to give red/orange ppt. (0.5)

(3 marks)

5. Haloalkanes can be formed by reaction of a halogen with an alkane in the presence of

sunlight e.g.

C6H14 + Br2 C6H13Br + HBr (i)

However, it would seem possible that the reaction could also proceed as follows:

C6H14 + Br2 2C3H7Br (ii)

A student is given the task of discovering which reaction usually takes place or whether a

combination of both reactions occurs. The student decides to adopt two approaches:

(1) a purely theoretical approach involving calculations based on mean bond

enthalpies;


(2) a practical approach using a titration approach.

In method (2) hexane and bromine are mixed in a 1:1 mole ratio in a stoppered container in

the presence of water. The container is placed in direct sunlight and shaken occasionally so

that any HBr produced dissolves in the aqueous layer. When all the bromine has reacted, the

aqueous solution is titrated with 0.1 mol dm-3 sodium hydroxide.

The following values of mean bond enthalpies in kJ mol-1 may be useful:

C-C: 348; C-H: 412; Br-Br: 193; H-Br: 366; C-Br: 276.

For the theoretical technique (1):

(a) Using any of the mean bond enthalpy values, calculate △H for each of the reactions (i)

and (ii) and hence show which reaction is energetically more favourable.

(i)Δ H = ΔHC-H + ΔHBr-Br - ΔHC-Br - ΔHHBr

= 412+193-276-366

= -37 kJ mol-1 (0.5)

(ii)Δ H = ΔHC-C + ΔHBr-Br – (ΔHC-Br) x 2

= 348+193-(276) x 2

= -11kJ mol-1 (0.5)

∴(i) is more energetically favourable (1)

(2 marks)

(b) Considering only the mean bond enthalpy values for C-C and C-H, predict which of

reactions (i) or (ii) seems more likely to occur. Give a reason for your choice.

(ii) is the most likely because it involves less energy to break the C-C bond. (1)

(1 mark)

(c) What conclusions can you draw from the answers obtained in (a) and (b).

(i) is energetically more favourable. (1)

(ii) is kinetically more favourable. (1)

(2 marks)

For the practical technique (2):

(d) How would you know when the reaction between hexane and bromine was complete?

The bromine will be decolorized. (1)

(1 mark)

(e) Explain concisely why sodium hydroxide can used to show that reaction (i) is occurring

rather than reaction (ii). Name an indicator which could be used for this titration.

If reaction (i) occurs, the HBr produced will neutralize with NaOH. (1)

Any indicators. (1)

(2 marks)

(f) In such an experiment, 0.50g of bromine was added to suitable volumes of hexane and

water. Titration of the aqueous layer required 31.25 cm3 of the sodium hydroxide. Used

these figures to show that reaction is occurring only by equation (i).

No. of moles of Br2 = 0.5/2(79.9) =3.13 x 10-3

No. of moles of NaOH = (0.1)(31.25/1000) = 3.13 x 10-3 (1)

∴ no. of moles of Br2 : No. of moles of NaOH = 1:1

it follows route (i) (1)

(2 marks)

6. Comprehension


(a) Write balanced equations for the two reactions described in lines 2-4. (2 marks)

CClF2CF2Cl(s) +Zn(s) CF2CF2(g) + ZnCl2(s) (1)

CF2CF2(g) +HCl(g) CHF2CF2Cl(l) (1)

(b) From the empirical formula given in line 9, calculate the percentage composition of F.

(R.A.M.: C=12, F=19) (1 mark)

% of F = 38/50 x 100% = 76% (1)

(c) Which two properties of Teflon described in the second paragraph make it particularly

suitable for artifical hip joints? (2 marks)

Unreactivity – will not ract in the body;

Insolubility – will not dissolve in body fluids;

Slipperiness – will not wear as the hip moves. (ANY 2) (1+1)

(d)t is meant by the van der Waals’ radius of an atom (lines 40-41)? How does it differ from

the covalent radius? (3 marks)

Half the distance between two identical atoms which are attracted to each other by van

der Waals ’ foces . (1+1)

It is larger than the covalent radius. (1)

(e) Consider the molecular data given in the diagrams and the last paragraph.

How can this explain the unreactivity of Teflon. (2 marks)

For teflon to react, very strong C-F bonds must be broken, ∴ there is a high activation

energy; (1)

The C-C bond is shielded from attack by the F atoms. (1)

Section B (20 marks)

7. Ethyl benzoate, C6H5COOC2H5, is a colourless liquid immiscible with water. It reacts slowly

with aqueous with aqueous sodium hydroxide to form sodium benzoate solution and

ethanol.

Ethanol has a boiling point of 78℃ and forms an azeotrope with water containing 96%

ethanol which boils at 78.5℃. Benzoic acid has a melting point of 121℃. Its solubility in

water is about 0.3g/100cm-3 at 18℃ and about 3g/100cm-3 at 100℃.

(a) Describe, with full practical details, how to carry out this experiment. State necessary

safety precautions.

(1) Known volumes of water and sodium hydroxide solution are put into a pear-

shape flask. (1)

(2) Add anti-bumping granules to the mixture to ensure smooth boiling. (1)

(3) Fir the water-condenser to the pear shape flask and reflux the mixture by using

a water bath. (1)

(4) Ethanol is flammable, therefore a water bath is used; NaOH is corrosive,

therefore gloves should be worn. (1)

(5) When the reaction is complete, EtOH is collected by distillation and sodium

hydroxide is left in the flask. (1)

(5 marks)

(b) Give the equation and state the type of reaction taking place.

C6H5COOC2H5(aq) + NaOH(aq) C6H5COONa(aq) C2H5OH(l) (1)

Alkaline hydrolysis of ester (Saponification) (1)

(2 marks)


(c) Can sodium hydroxide be replaced by dilute sulphuric acid? Explain briefly.

Yes or No.

But the yield is lower because (1)

(i) the reaction is reversible: R’COOR +H2O ≒R’COOH + ROH (1)

(ii) separation of acid and ethanol is difficult ∵ benzoic acid is quite soluble in hot

water. (1)

(3 marks)

8. (a) Outline laboratory tests to identify the cation and the anions in NaSO3(s) and

NaNO3(s). Give reagents, reaction conditions an expected observations for the tests.

Test for SO32- : room temp. ; Cr2O7

2-/H+ (1)

Gas given out (SO2) which turns Cr2O72-/H+ from orange to green.

(1)

Test for NO3- : conc. H2SO4; FeSO4(aq) solution; room temp. (1)

A brown ring is formed in the interface between the aqueous layer

and conc. H2SO4(l). (1)

Test for Na+ : golden yellow flame under burning in flame test. (1)

(5 marks)

(b) The average oxidation number of the two sulphur atoms in sodium thiosulphate,

Na2S2O3, is +2.

When solid sulphur containing the radioactive isotopes 35S is boiled with sodium

sulphite, Na232SO3, a radioactive sample of sodium thiosulphate is produced. Adding

HCl(aq) to this sample causes all the 35S to precipitate as sulphur. The resulting solution

contains non-radioactive sulphite ions.

(i) Draw the structural formula of the sulphite ion.

(1)

(ii) Use your answer in (i) and the reactions described to suggest a structure for the

thiosulphate ion produced. On your structure, label any radioactive atom.

(1+1)


(iii) Deduce the oxidation number for each sulphur atom in the thiosulphate ion.

(1+1)

(5 marks)

Section C (20 marks)

Answer ONE question in this section.

Marks will be allocated approximately as follows:

Chemical knowledge 50%

Organization 30%

Presentation (including proper use of English) 20%

9.

Discuss the chemical reactions of D in which X = C2H5, Y = H, and also comment on how

the acidic and physical properties of D may be modified by different substituents X and Y.

Several examples of D (with appropriate substituents) are of great benefit to man. Illustrate

this with 2 examples containing nitrogen, one of which occurs in nature, and the other, a

man-made example of great industrial importance for making large molecules. For each of

your specific examples indicate their use, and outline the chemical reactions involved in

obtaining each compound.

10. Give an account of the general features shown by transition metals. Illustrate your answer

with suitable examples.

Guidelines only:

Introduction:

Transition Metals: elements which form compounds in which there is an incomplete

subshell of d-electrons in more than one of its compounds.

d-block Elements: atoms/elements with electron configuration in which the d-orbitals

are being filled.

The general features of transition metals are as follows:

(i) variable O.S (2)

(ii) complex formation (2)

(iii) coloured compounds of transition metals

- they are usually coloured ∵under the influence of a ligand in a complex, the

3d orbitals split into two groups with small energy difference. As sufficient light


energy is absorbed, the 3d e- of a lower energy level to a higher energy level.

This resulting d-d transition causes for light absorption, and the remainder of

the light reflected in the visible region accounts for the specific colour of that

ion. E.g. Copper(II) sulpahte-5-water is blue in colour.

(2)

(iv) catalytic properties (2)

(v) magnetic properties (2)

END OF PAPER


mock paper i suggestedanswer

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