EdExcel a Level Chemistry Unit 7 Paper Jan 2000

8/17/2019 EdExcel a Level Chemistry Unit 7 Paper Jan 2000

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1. (a) Ammonia is manufactured by passing hot nitrogen and hydrogen at high pressure over acatalyst containing iron.

In an experiment, 9.0 moles of nitrogen and 27 moles of hydrogen were put into an iron

vessel of volume l0 dm3. This was then heated to 250 °C and allowed to reachequilibrium. It was found that two thirds of the nitrogen and hydrogen were converted into

ammonia.

N2(g) + 3H2(g) 2NH3(g) ❉ H = –92.4 kJ mol–l

(i) Give the expression for K c.

(1)

(ii) Complete the table below.

N2 H2 NH3

moles at start 9.0 27 0

moles at equilibrium

concentration at equilibrium /mol dm–3

(2)

(iii) Calculate the value of K c at 250 °C, stating its units.

(2)

(iv) State the effect, if any, of an increase in temperature on the value of theequilibrium constant, giving a reason.

…..……………………………………………………………………………..

…..……………………………………………………………………………..

…..……………………………………………………………………………..(2)

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(v) State the effect, if any, of an increase in pressure on the value of the equilibrium

constant, giving a reason.

…..……………………………………………………………………………..

…..……………………………………………………………………………..

…..……………………………………………………………………………..

(2)

(b) Some of the ammonia manufactured in this way is converted into nitric acid, HNO3. The

first step in this process is the oxidation of ammonia.

4NH3(g)+ 5O2(g) ➤ 4NO(g) + 6H2O(g)

(i) Calculate the standard enthalpy change for this reaction given the followingstandard enthalpies of formation in kJ mol–1:

NH3(g) – 46.2 NO(g) + 90.4 H2O(g) – 242

(3)

(ii) Draw an enthalpy level diagram for this reaction.

Enthalpy

(2)

(c) A mixture of concentrated nitric and sulphuric acids is used in the nitration of benzene to

form nitrobenzene.

(i) The first stage of the reaction is the production of the NO ✰

2 ion

Write an equation to show its formation.

…..……………………………………………………………………………..(1)

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(ii) The formation of the NO ✰

2 ion occurs in two steps, the first of which is:

H2SO4 + HNO3 ➤ H2NO ✰

3 + HSO ✲

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This is an acid/base reaction. Give the formulae of the acid/base conjugate pairs

involved.

acid …………….………… its conjugate base ………….……………

base …………….………… its conjugate acid ………….……………(2)

(iii) Hence comment on the relative strengths of nitric and sulphuric acid.

…..……………………………………………………………………………..(1)

(iv) Give the mechanism for the reaction of the NO ✰

2 ion with benzene.

(3)

(v) What do curly arrows show when used in a mechanism?

(1)

(Total 22 marks)

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2. (a) Benzene diazonium chloride, C6H5N2Cl, in acidic solution is stable below 5 °C but

decomposes rapidly in a first order reaction when heated to 50 °C.

C6H5N2Cl(aq) + H2O(1) ➤ C6H5OH(aq) + HCl(aq) + N2(g)

Describe an experiment that could be used to follow this reaction. Your answer should

include a diagram of the apparatus that you would use and a statement of themeasurements that you would make.

………………………………………………………………………………………..

………………………………………………………………………………………..

………………………………………………………………………………………..

………………………………………………………………………………………..(5)

(b) The decomposition of hydrogen peroxide is also a first order reaction

2H2O2(aq) ➤ 2H2O(1) + O2(g)

At 50 °C it is very much slower than the decomposition of benzene diazonium chloride ofthe same concentration.

(i) Write the rate equation for the decomposition of hydrogen peroxide.

…..……………………………………………………………………………..(1)

(ii) Comment on the relative values of the activation energies and the rate constants forthe decomposition reactions above.

Activation energies ...........................................………………………………

…..……………………………………………………………………………..

Rate constants ..............................................………………………………….

…..……………………………………………………………………………..(2)

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(iii) Explain, in terms of collision theory, the effect on reaction rates of increasing the

temperature to 60 ºC.

…..……………………………………………………………………………..

…..……………………………………………………………………………..

…..……………………………………………………………………………..

…..……………………………………………………………………………..

…..……………………………………………………………………………..(3)

(iv) The decomposition of hydrogen peroxide is catalysed by iodide ions.

Explain why reactions are speeded up by a catalyst.

…..……………………………………………………………………………..

…..……………………………………………………………………………..

…..……………………………………………………………………………..(2)

(c) The lodination of propanone

CH3COCH3 + I2 acid catalyst

CH2ICOCH3 + HI

was studied with an excess of propanone and an acid catalyst. The variation of

concentration of iodine as a function of time is shown below.

[I ]2

time

What is the order of this reaction with respect to iodine? Give your reasoning.

………………………………………………………………………………………..

………………………………………………………………………………………..

………………………………………………………………………………………..

………………………………………………………………………………………..(3)

(Total 16 marks)

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3. Citral is a colourless natural product, which gives lemons their characteristic flavour and smell.

Its structural formula is:

C C

CH CH

CH CH CHO

3 3

CH 3 CH CH2 2

(a) (i) How would you show that citral has a carbonyl group, C==O?

Reagent .....…………………………………………………………….………

Observation ………………………………………..…………………………..(2)

(ii) How would you show that citral is an aldehyde?

Reagent .....…………………………………………………………….………

Observation ………………………………………..…………………………..(2)

(b) Citral has geometric isomers. Draw them and explain why they are not easilyinterconvertable.

Structures:

Reason

………………………………………………………………………………………..(3)

(c) Bromine, dissolved in tetrachloromethane, was added slowly to a solution of citral.

(i) Describe what you would observe when this was done.

…..……………………………………………………………………………..

…..……………………………………………………………………………..(1)

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(ii) Draw the structural formula of the product of this reaction when excess bromine

is added.

(1)

(iii) This product is chiral. Mark all the atoms which cause chirality with a✯

on thestructural formula that you have given in (ii) above.

(2)

(d) Citral can be reduced to an alcohol

(i) State a reagent and the conditions that could be used for this reaction.

Reagent .....…………………………………………………………….………

Conditions ………………………………………..……………………………(2)

(ii) How would you test the product to show that it has an ––OH group?

Reagent .....…………………………………………………………….………

Observation ………………………………………..…………………………..(2)

(e) Citral can be oxidised to an acid C9H

15COOH ( M r = 168) which ionises in water

C9H15COOH + H2O H3O+ + C9H15COO–

4.62 g of this acid was dissolved in water to give a solution of volume 250 cm3.

This solution had a pH of 2.91.

(i) Write the expression for K a for this acid.

(1)

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(ii) Calculate the concentration of the acid in mol dm–3.

(1)

(iii) Calculate the value of K a of the acid.

(3)

(Total 20 marks)

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EdExcel a Level Chemistry Unit 7 Paper Jan 2000