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Date	November 2021	Marks available	1	Reference code	21N.2.sl.TZ0.3
Level	SL	Paper	2	Time zone	TZ0
Command term	Explain	Question number	3	Adapted from	N/A

Question

White phosphorus is an allotrope of phosphorus and exists as P₄.

An equilibrium exists between PCl₃ and PCl₅.

PCl₃(g) + Cl₂(g) $⇌$ PCl₅(g)

Sketch the Lewis (electron dot) structure of the P₄ molecule, containing only single bonds.

[1]

a(i).

Write an equation for the reaction of white phosphorus (P₄) with chlorine gas to form phosphorus trichloride (PCl₃).

[1]

a(ii).

Deduce the electron domain and molecular geometry using VSEPR theory, and estimate the Cl–P–Cl bond angle in PCl₃.

[3]

b(i).

Explain the polarity of PCl₃.

[1]

b(ii).

Calculate the standard enthalpy change (ΔH^⦵) for the forward reaction in kJ mol⁻¹.

ΔH^⦵_f PCl₃(g) = −306.4 kJ mol⁻¹

ΔH^⦵_f PCl₅(g) = −398.9 kJ mol⁻¹

[1]

c(i).

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

[1]

c(ii).

State, with a reason, the effect of an increase in temperature on the position of this equilibrium.

[1]

c(iii).

Markscheme

Accept any diagram with each P joined to the other three.

Accept any combination of dots, crosses and lines.

a(i).

P₄(s) + 6Cl₂ (g) → 4PCl₃ (l) ✔

a(ii).

Electron domain geometry: tetrahedral ✔

Molecular geometry: trigonal pyramidal ✔

Bond angle: 100«°» ✔

Accept any value or range within the range 91−108«°» for M3.

b(i).

polar AND unsymmetrical distribution of charge
OR
polar AND dipoles do not cancel
OR
«polar as» dipoles «add to» give a «partial» positive «charge» at P and a «partial» negative «charge» at the opposite/Cl side of the molecule ✔

Accept “polar AND unsymmetrical molecule”.

b(ii).

«−398.9 kJ mol⁻¹ − (−306.4 kJ mol⁻¹) =» −92.5 «kJ mol⁻¹» ✔

c(i).

«K_c =» $\frac{[{PCl}_{5}]}{[{PCl}_{3}] [{Cl}_{2}]}$ ✔

c(ii).

«shifts» left/towards reactants AND «forward reaction is» exothermic/ΔH is negative ✔

c(iii).

Examiners report

[N/A]

a(i).

[N/A]

a(ii).

[N/A]

b(i).

[N/A]

b(ii).

[N/A]

c(i).

[N/A]

c(ii).

[N/A]

c(iii).

Syllabus sections

Core » Topic 4: Chemical bonding and structure » 4.3 Covalent structures

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Core » Topic 4: Chemical bonding and structure