Chem Question Enthalpy

Question

What is the ΔH^o_rxn for the production of propane, C₃H₈, given the following reaction enthalpies:

ΔH^o(combustion of propane) = –2219.9 kJ

ΔH^o _f CO₂(g)= –393.5 kJ

ΔH^o _f H₂O(l)= –285.8 kJ

Select one:

a.

Approx. +0.1040 × 10³ kJ

b.

Approx. –0.1040 × 10³ kJ

c.

Approx. +1.325 × 10³ kJ

d.

Approx. +1.541 × 10³ kJ

Answer 1

I believe the question should be asking for the ∆Hºformation of propane, not the ∆Hºreaction.

Looking at the combustion of propane:

C₃H₈ + 5O₂ ==> 3CO₂ + 4H₂O ... balanced equation for combustion of propane

∆Hº_rxn = ∑n∆H_fºproducts - ∑n∆H_fºproducts

∆Hº_rxn = [(3x-393.5 + 4x-285.8) - [∆H_fºC₃H₈) + 0]

-2219.9 = (-1180.5) + (-1143.2) - ∆H_fºC₃H₈

-2219.9 = -2323.7 - ∆H_fºC₃H₈

∆H_fºC₃H₈ = -103.8 kJ = -104 kJ = -0.1040 x10³ kJ (a very strange way to present the answer)

(Answer b)