Question
Question: Heat of combustion of ethanol at constant pressure and at temperature T K is found to be -qJ mol¹. H...
Heat of combustion of ethanol at constant pressure and at temperature T K is found to be -qJ mol¹. Hence, heat of combustion (in J mol¹) of ethanol at the same temperature at constant volume will be
A
RT-q
B
-(q+RT)
C
q-RT
D
q + RT
Answer
RT-q
Explanation
Solution
Ethanol combustion (when water is formed as liquid) has only the 3 moles of O₂ on the reactant side and 2 moles of CO₂ (gas) on the product side. Thus,
Δn_g = 2 − 3 = –1
At constant pressure, ΔH = –q. Hence, at constant volume:
ΔU = ΔH – (Δn_g)RT = –q – (–RT) = –q + RT
This is equivalent to “RT – q”.