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Date May 2022 Marks available 1 Reference code 22M.2.hl.TZ2.4
Level HL Paper 2 Time zone TZ2
Command term Predict and Justify Question number 4 Adapted from N/A

Question

Hydrogen and iodine react to form hydrogen iodide.

H2 (g) + I2 (g) 2HI (g)

The following experimental data was obtained.

Consider the reaction of hydrogen with solid iodine.

H2 (g) + I2 (s) 2HI (g)     ΔH = +53.0 kJ mol−1

Deduce the order of reaction with respect to hydrogen.

[1]
a(i).

Deduce the rate expression for the reaction.

[1]
a(ii).

Calculate the value of the rate constant stating its units.

[2]
a(iii).

State two conditions necessary for a successful collision between reactants.

[1]
b.

State the equilibrium constant expression, Kc, for this reaction.

[1]
c.

Calculate the entropy change of reaction, ΔS, in J K−1 mol−1.

[1]
d(i).

Predict, giving a reason, how the value of the ΔSreaction would be affected if I2 (g) were used as a reactant.

[1]
d(ii).

Calculate the Gibbs free energy change, ΔG, in kJ mol−1, for the reaction at 298 K. Use section 1 of the data booklet.

[1]
d(iii).

Calculate the equilibrium constant, Kc, for this reaction at 298 K. Use your answer to (d)(iii) and sections 1 and 2 of the data booklet.

(If you did not obtain an answer to (d)(iii) use a value of 2.0 kJ mol−1, although this is not the correct answer).

[2]
d(iv).

Markscheme

first order ✔

a(i).

Rate=k [H2] [I2]

a(ii).

k=«1.2×10-6moldm-3s-12.0×10-3moldm-3×3.0×10-3moldm-3=»0.20 ✔

mol–1 dm3 s–1

a(iii).

EEa AND appropriate «collision» geometry/correct orientation ✔

b.

Kc=HI2H2I2 ✔

c.

«ΔSreaction = 2 × 206.6 – (130.6 + 116.1) =» 166.5 «J K–1 mol–1» ✔

d(i).

ΔSreaction lower/less positive AND same number of moles of gas

OR

ΔSreaction lower/less positive AND a solid has less entropy than a gas ✔

d(ii).

«ΔG = 53.0 kJ mol–1 – (298K × 0.1665 kJ K–1 mol–1) =» 3.4 «kJ mol–1» ✔

d(iii).

«ln Kc= – (3.4 × 103 J mol–1 /8.31 J K–1 mol–1 × 298 K)» = –1.37 ✔

«Kc =» 0.25 ✔

Award [2] for “0.45” for the use of 2.0 kJ mol–1 for ΔG.

d(iv).

Examiners report

4(a)(i)-(iii): Deduction of rate orders and rate expression were very well done overall, with occasional errors in the units of the rate constant, but clearly among the best answered questions.

a(i).
[N/A]
a(ii).
[N/A]
a(iii).

Generally well answered by all but very weak candidates. Some teachers thought this should be a 2-mark question but actually the marks were generally missed when students mentioned both required conditions but failed to refer the necessary energy to Ea.

b.

One of the best answered questions.

c.

ΔS was well calculated in general except for some inverted calculations or failure to consider the ratios of the reactants.

d(i).

Some candidates confused the entropy change in this situation with absolute entropy of a solid and gas, or having realised that entropy would decrease lacked clarity in their explanations and lost the mark.

4(d)(ii)-(d)(iv): marks were lost due to inconsistency of units throughout, i.e., not because answers were given in different units to those required, but because candidates failed to convert all data to the same unit for calculations.

d(ii).
[N/A]
d(iii).
[N/A]
d(iv).

Syllabus sections

Additional higher level (AHL) » Topic 15: Energetics/thermochemistry » 15.2 Entropy and spontaneity
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