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Example of an Old H2 Chemistry Paper 3 Free-Response Question from a School Preliminary Examination

(The question stem is highlighted in green.) 2007 Anglo-Chinese Junior College / Paper 3 / Question 1 The following experimental results were obtained from the hydrolysis of two bromoalkanes at temperature T1, where the concentration of the solution is measured in mol dm–3 and time taken for the experiment in seconds.

Order of reaction with respect to Reaction Reactants bromoalkane OH– ion

A CH3CH2CH2Br + OH– 1 1 B (CH3)3CBr + OH– 1 0

(a) Using reaction A, (i) write down the rate equation, giving the units of the rate constant.

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Proposed Solution Rate equation : Rate = k[CH3CH2CH2Br][OH–] Units for k : mol–1 dm3 s–1 (ii) describe, and explain in molecular terms, how the rate of this reaction is

affected when it is repeated at a higher temperature T2. You should include a reference to the Maxwell Boltzmann distribution in your answer. [6]

Proposed Solution When temperature of a reaction increases, average kinetic energy of reacting molecules increases. Number of reactant particles, per unit volume, with energy at least equal to the

activation energy, Ea, increases. This increases the frequency of effective collisions. Since rate of reaction is proportional to the frequency of effective collisions, rate of

reaction increases.

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(b) Using reaction B, (i) calculate the time taken for the bromoalkane to decrease to 6.25 % of its

original concentration, given that the half-life for this reaction is approximately 10.6 minutes.

Proposed Solution Let n be the number of half-lives.

∴ Time taken for the bromoalkane to decrease to 6.25 % of its original concentration = 4 × 10.6 = 42.4 min

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(ii) state and explain the effect on the rate of this reaction when the

concentration of the hydroxide ion is doubled and the concentration of the bromoalkane is halved simultaneously.

Proposed Solution Reaction is zero-order with respect to [OH–]. ⇒ Rate of reaction is unaffected by changes to [OH–].

Rate equation: Rate = k[(CH3)3CBr]

∴ Rate of reaction is halved.

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(iii) The following mechanism is proposed for this reaction: Slow : (CH3)3CBr → (CH3)3C+ + Br–

Fast : (CH3)3C+ + OH– → (CH3)3COH Draw a labelled enthalpy profile diagram for this proposed mechanism. [5]

Ent

halp

y, H

Reaction coordinate

(CH3)3CBr + OH–

(CH3)3COH+ Br–

(CH3)3C+ + Br – + OH–

Ea,2Ea,1

!Hr

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(c) The following observations were obtained when two halogenoalkanes react

separately with aqueous silver nitrate. Halogenalkane Observations after adding AgNO3 (aq)

CH3CH2CH2Br cream ppt forms after 2 minutes CH3CH2CH2I yellow ppt forms almost immediately

Explain these observations as fully as you can. [3] Proposed Solution Reactions: i CH3CH2CH2X + H2O → CH3CH2CH2OH + H+ + X– (where X = halogen) ii Ag+(aq) + X–(aq) → AgX(s) In the nucleophilic substitution of halogenoalkanes, the slow step involves the

breaking of C–X. ⇒ Rate of this reaction depends on the strength of the C–X bond.

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Since the C–Br bond is stronger than the C–I bond, the rate at which Br– (Reaction i) is released into solution to form cream precipitate of AgBr with Ag+ (Reaction ii) ions is slower than the rate at which I– is released into solution to form yellow precipitate of AgI.

(d) Describe a simple chemical test that you could use to distinguish the following

compounds. You are to include reagents and conditions, observations and a balanced equation(s) in each case.

(i)

and

I II

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Proposed Solution Test : To separate samples of compounds I and II, add Br2(aq). I II Observation Reddish brown bromine is

decolourised, and a white precipitate is formed.

Reddish brown bromine is not decolourised, and a white precipitate is not observed.

Equation:

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(ii)

and

III IV

[6]

[Total: 20] Proposed Solution Test : To separate samples of compounds III and IV, add KMnO4(aq) and H2SO4(aq)

and heat. I II Observation Purple solution of KMnO4 is

decolourised. Purple solution of KMnO4 is not decolourised.

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Equations: Step 1: Hydrolysis of ester

!

Step 2: Oxidation of primary alcohol