DP Chemistry (first assessment 2025)

Question 22M.2.HL.TZ2.4

Date	May 2022	Marks available	[Maximum mark: 11]	Reference code	22M.2.HL.TZ2.4
Level	HL	Paper	2	Time zone	TZ2
Command term	Calculate, Deduce, Justify, Predict, State	Question number	4	Adapted from	N/A

[Maximum mark: 11]

22M.2.HL.TZ2.4

Hydrogen and iodine react to form hydrogen iodide.

H₂ (g) + $I$ ₂ (g) $⇌$ 2H $I$ (g)

The following experimental data was obtained.

Consider the reaction of hydrogen with solid iodine.

H₂ (g) + $I$ ₂ (s) $⇌$ 2H $I$ (g) ΔH^⦵ = +53.0 kJ mol⁻¹

(a(i))

Deduce the order of reaction with respect to hydrogen.

[1]

Markscheme

first order ✔

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(ii))

Deduce the rate expression for the reaction.

[1]

Markscheme

Rate=k [H₂] [ $I$ ₂]

(a(iii))

Calculate the value of the rate constant stating its units.

[2]

Markscheme

$k = « \frac{1.2 \times 10^{- 6} mol {dm}^{- 3} s^{- 1}}{2.0 \times 10^{- 3} mol {dm}^{- 3} \times 3.0 \times 10^{- 3} mol {dm}^{- 3}} = » 0.20$ ✔

mol^–1 dm³ s^–1 ✔

(b)

State two conditions necessary for a successful collision between reactants.

[1]

Markscheme

E ≥ E_a AND appropriate «collision» geometry/correct orientation ✔

Examiners report

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 E_a.

(c)

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

[1]

Markscheme

$K_{c} = \frac{{[HI]}^{2}}{[H_{2}] [I_{2}]}$ ✔

Examiners report

One of the best answered questions.

(d(i))

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

[1]

Markscheme

«Δ ${S^{⦵}}_{reaction}$ = 2 × 206.6 – (130.6 + 116.1) =» 166.5 «J K^–1 mol^–1» ✔

Examiners report

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

(d(ii))

Predict, giving a reason, how the value of the ΔS^⦵_reaction would be affected if $I_{2}$ (g) were used as a reactant.

[1]

Markscheme

Δ ${S^{⦵}}_{reaction}$ lower/less positive AND same number of moles of gas

Δ ${S^{⦵}}_{reaction}$ lower/less positive AND a solid has less entropy than a gas ✔

Examiners report

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(iii))

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]

Markscheme

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

(d(iv))

Calculate the equilibrium constant, K_c, 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]

Markscheme

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

«K_c =» 0.25 ✔