Question

The enthalpy of formation of CO₂(g), H₂O(l) and propene (g) are -393.5, -285.8 and 20.42 kJ mol^-1 , respectively. The enthalpy of isomerization of cyclopropane to propene is -33.0 kJ mol^-1 . The enthalpy change for combustion of cyclopropane is

a) -2058.32 kJ

b) -2091.32 kJ

c) +2058.32 kJ

d) +2091.32 kJ

Answer 1

Correct option is b) -2091.32 kJ

First, calculate the enthalpy of combustion of propene using Hess's law. i.e.

6. 

Enthalpy of combustion of cyclopropane can be found on adding equations (4) and (6)

∆_cH = (-2058.32 – 33.0) kJ

∆_cH = -2091.32 kJ