June 1999 Module 2 Exam

1. This question concerns the compound Q, pent-3-en-2-ol.

CH₃CHCH=CHCH₃
|
OH

(a) Give the structural formula of a compound isomeric with Q which does not contain a carbon-carbon double bond.

(1)

(b) Q shows two types of stereoisomerism.

(i) Draw the geometric isomers of Q, and state why Q shows this type of isomerism.

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(ii) Draw the optical isomers of Q and state what effect such isomers have on the plane of plane-polarised monochromatic light.

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(c) Give the mechanism for the reaction of bromine with Q; you may represent the compound as >C=C< for the purposes of this question.

(3)

(d) (i) Give the structural formula of the product obtained when Q is reacted with potassium dichromate(VI) solution in the presence of dilute sulphuric acid.

(1)

(ii) Describe a test to show that the product formed in (i) is not an aldehyde. Suggest a positive test which would give the identity of the new functional group which is present in the molecule, and state what you would see.

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(e) The iodine number is a measure of the degree of unsaturation of a molecule. It relates to the number of double bonds it contains. It is defined as the number of grams of iodine, I₂, that react with 1g of the compound.

Show that the iodine number of Q is approximately 3 by calculating the number of grams of iodine, I₂, which react with 1g of Q.

(3)

Total 17 marks.

2. Iodine reacts with propanone in the presence of an acid catalyst according to the equation:

H⁺
CH₃COCH₃ + I₂ ----> CH₃COCH₂I + HI

Data concerning an experiment to determine the rate equation for this reaction are given in the following table:

Relative concentrations			Relative rate
[CH₃COCH₃]	[I₂]	[H⁺]
1	1	1	2
1	2	1	2
2	1	1	4
1	1	2	4

(a) (i) State why a rate equation cannot be written from a knowledge only of the chemical equation representing the reaction.

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(ii) Use the data to deduce the order of reaction with respect to

propanone . . . . . . . . . . . iodine . . . . . . . . . . . hydrogen ions . . . . . . . . . . . (3)

(iii) What does the value for iodine tell you about the part that iodine plays in the rate determining step of the reaction?

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(iv) Write the rate equation for the reaction.

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(b) (i) Use the bond enthalpies given, in kJ mol^-1, to calculate the enthalpy change for the iodination of propanone:

C--H	I--I	C--I	H--I
+413	+151	+228	+298

(2)

(ii) Draw an enthalpy level diagram for the reaction, showing on it both catalysed and uncatalysed pathways.

(3)

Total 12 marks

3. The manufacture of sulphuric acid is based on the equilibrium reaction

SO₂(g) + 0₂(g) --> 2SO₃(g)

/\H = -197 kJ mol^-1

(a) The equilibrium is a dynamic equilibrium; explain the nature of such an equilibrium.

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(2) (b) Give the expression for K, for this equilibrium, giving consideration to the units.

(2)

(i) an increase in temperature;

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(2) (ii) the presence of a catalyst.

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(d) State and explain the effects of the following on the rate of attainment of equilibrium:

(i) an increase in temperature;

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(2) (ii) the presence of a catalyst.

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(e) Although the reaction is exothermic in the forward direction there is no appreciable reaction between sulphur dioxide and oxygen at room temperature. Use this reaction to explain the ideas of thermodynamic and kinetic stability.

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Although the reaction is an equilibrium reaction, industrially this and other similar reactions do not usually achieve equilibrium. Suggest why this is so.

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Total 16 marks

4. (a) Use Bronsted-Lowry theory of acid/base behaviour and acid/base conjugate pairs to explain how sulphuric acid shows its acidic nature when added to water.

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(b) If the enthalpy of neutralisation /\H_neut is measured for hydrobromic acid, HBr, or for nitric acid, HN0₃, both with sodium hydroxide solution, the value is -57.6 kJ mol^-1; for the reaction of ethanoic acid, CH₃COOH, with sodium hydroxide solution it is -55.2 kJ mol^-1.

(i) Why is /\H_neut the same for both HBr and HN0₃ with NaOH?

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(ii) Give one reason why /\H_neut for the reaction of ethanoic acid with sodium hydroxide solution is less exothermic than for hydrobromic acid with sodium hydroxide solution.

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(ii) When sodium hydrogensulphate, Na⁺HS0₄^-, is added to a solution of sodium sulphate the mixture behaves as a buffer solution. Show how it functions by writing equations for the reactions which occur on the addition of acid or base:

acid: ...............................................................................................................................

base: ..............................................................................................................................

(d) Sulphuric acid is used in the production of soapless detergents.

(i) Why is sulphuric acid used in the production of these materials?

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(ii) Briefly indicate the advantages of soapless detergents over soap itself.

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Total 15 marks