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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 Calculate 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.

ΔHf PCl3 (g) = −306.4 kJ mol−1

ΔHf PCl5 (g) = −398.9 kJ mol−1

[1]
c(i).

State the equilibrium constant expression, Kc, 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).

«Kc =» PCl5PCl3Cl2

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 5: Energetics/thermochemistry » 5.2 Hess’s Law
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