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9701/22

Chemistry 9701/22 — May/June 2011

Cambridge AS Level · AS Level Structured Questions · worked solutions for every part, with the mark scheme

5
questions
60
marks
75
minutes

Topics Atoms, Molecules and Stoichiometry · Hydrocarbons · Equilibria · Reaction Kinetics · Chemical Energetics · States of Matter · +8 more

Q1EquilibriaAtoms, Molecules and StoichiometryReaction KineticsFree sample

Ethanoic acid can be reacted with alcohols to form esters, an equilibrium mixture being formed.

CH3CO2H+ROH⇌CH3CO2R+H2O\text{CH}_3\text{CO}_2\text{H} + \text{ROH} \rightleftharpoons \text{CH}_3\text{CO}_2\text{R} + \text{H}_2\text{O}

The reaction is usually carried out in the presence of an acid catalyst.

(a)

Write an expression for the equilibrium constant, KcK_c, for this reaction, clearly stating the units.

Kc=K_c =

units .................................................

2M
(b)

In an experiment to determine KcK_c a student placed together in a conical flask 0.10 mol of ethanoic acid, 0.10 mol of an alcohol ROH, and 0.005 mol of hydrogen chloride catalyst.
The flask was sealed and kept at 25 °C for seven days.
After this time, the student titrated all of the contents of the flask with 2.00 mol dm−3^{-3} NaOH using phenolphthalein indicator.
At the end-point, 22.5 cm3^3 of NaOH had been used.

4M
(i)

Calculate the amount, in moles, of NaOH used in the titration.

(ii)

What amount, in moles, of this NaOH reacted with the hydrogen chloride?

(iii)

Write a balanced equation for the reaction between ethanoic acid and NaOH.

(iv)

Hence calculate the amount, in moles, of NaOH that reacted with the ethanoic acid.

(c)
3M
(i)

Use your results from (b) to calculate the amount, in moles, of ethanoic acid present at equilibrium. Hence complete the table below.

CH3CO2H\text{CH}_3\text{CO}_2\text{H}ROH\text{ROH}CH3CO2R\text{CH}_3\text{CO}_2\text{R}H2O\text{H}_2\text{O}
initial amount / mol0.100.1000
equilibrium amount / mol
(ii)

Use your results to calculate a value for KcK_c for this reaction.

(d)

Esters are hydrolysed by sodium hydroxide. During the titration, sodium hydroxide reacts with ethanoic acid and the hydrogen chloride, but not with the ester.

Suggest a reason for this.

1M
(e)

What would be the effect, if any, on the amount of ester present if all of the water were removed from the flask and the flask kept for a further week at 25 °C?

Explain your answer.

2M

The rest of this paper

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