DP Chemistry (last assessment 2024)

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Question 21N.2.sl.TZ0.4

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Date November 2021 Marks available [Maximum mark: 8] Reference code 21N.2.sl.TZ0.4
Level sl Paper 2 Time zone TZ0
Command term Explain, Identify, Justify, Label, Outline, Sketch, Suggest Question number 4 Adapted from N/A
4.
[Maximum mark: 8]
21N.2.sl.TZ0.4

1-chloropentane reacts with aqueous sodium hydroxide.

(a(i))

Identify the type of reaction.

[1]

Markscheme

«nucleophilic» substitution/SN2 ✔


Do not accept if “electrophilic” or “free radical” substitution is stated.

(a(ii))

Outline the role of the hydroxide ion in this reaction.

[1]

Markscheme

«acts as a» nucleophile/Lewis base
OR
donates/provides lone pair «of electrons»
OR
attacks the «partially» positive carbon ✔

(a(iii))

Suggest, with a reason, why 1-iodopentane reacts faster than 1-chloropentane under the same conditions. Use section 11 of the data booklet for consistency.

[2]

Markscheme

bond enthalpy C–I lower than C–Cl
OR
C–I bond weaker than C–Cl ✔


«weaker bond» broken more easily/with less energy
OR
lower Ea «for weaker bonds» ✔


Accept the bond enthalpy values for C–I and C–Cl for M1.

The reaction was repeated at a lower temperature.

(b(i))

Sketch labelled Maxwell–Boltzmann energy distribution curves at the original temperature (T1) and the new lower temperature (T2).

[2]

Markscheme

peak at T1 to right of AND lower than T2 ✔

lines begin at origin AND T1 must finish above T2 ✔

(b(ii))

Explain the effect of lowering the temperature on the rate of the reaction.

[2]

Markscheme

«rate is» lower AND «average» kinetic energy of molecules is lower
OR
«rate is» lower AND less frequent collisions
OR
«rate is» lower AND fewer collisions per unit time ✔

«rate is» lower AND fewer/smaller fraction of molecules/collisions have the E ≥ Ea ✔


Lower «rate» needs to be mentioned once only.

Do not accept “fewer collisions” without reference to time/frequency/probability for M1.

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