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Date	May 2017	Marks available	2	Reference code	17M.2.sl.TZ2.3
Level	SL	Paper	2	Time zone	TZ2
Command term	Deduce	Question number	3	Adapted from	N/A

Question

PCl₅(g) and Cl₂(g) were placed in a sealed flask and allowed to reach equilibrium at 200 °C. The enthalpy change, ΔH, for the decomposition of PCl₅(g) is positive.

M17/4/CHEMI/SP2/ENG/TZ2/03

Deduce the equilibrium constant expression, K_c, for the decomposition of PCl₅(g).

[1]

a.i.

Deduce, giving a reason, the factor responsible for establishing the new equilibrium after 14 minutes.

[2]

a.ii.

Deduce the Lewis (electron dot) structure and molecular geometry of PCl₃.

[2]

b.

Markscheme

«K_c» = $\frac{{\left[ {{\text{PC}}{{\text{l}}_3}} \right]\left[ {{\text{C}}{{\text{l}}_2}} \right]}}{{\left[ {{\text{PC}}{{\text{l}}_5}} \right]}}$

[1 mark]

a.i.

decrease in temperature

endothermic «reaction» AND «equilibrium» shifts to the left/reactants
OR
endothermic «reaction» AND K_c decreases
OR
endothermic «reaction» AND concentration of PCl₅ increased/concentration of PCl₃ and Cl₂ decreased
OR
«equilibrium» shifts in exothermic direction

Do not accept “temperature change”.

Accept “ΔH positive” in place of “endothermic”.

Accept “products” instead of “PCl₃ and Cl₂”.

[2 marks]

a.ii.

Lewis structure:

Molecular geometry:

trigonal/triangular pyramidal

Penalize missing lone pairs once only between this question and 4(b).

Accept any combination of lines, dots or crosses to represent electrons.

Do not apply ECF.

[2 marks]

b.

Examiners report

[N/A]

a.i.

[N/A]

a.ii.

[N/A]

b.

Syllabus sections

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

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Core » Topic 4: Chemical bonding and structure