9701 MCQ Solution 47

Question 47: [Physical > Chemical Energetics]

Gaseous phosphorus pentachloride can be decomposed into gaseous phosphorus trichloride and

chlorine by heating. The table below gives the bond energies.

Bond	Bond energy / kJ mol–1
P-Cl (in both chlorides)	330
Cl-Cl	240

What is the enthalpy change in the decomposition of PCl5 to PCl3 and Cl2?

A –420 kJ mol–1
B –90 kJ mol–1
C +90 kJ mol–1
D +420 kJ mol–1

Reference: Past Exam Paper - June 2003 Paper 1 Q7

Solution 47:

Answer: D.

Form a balanced equation.

PCl5(g)-->PCl3(g)+Cl2(g)

Enthalpy Change = Bond Broken - Bonds Formed

2 P-Cl bonds were broken and a Cl-Cl bond was formed

= (2*330) - 240 = +420 kJ/mol