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
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