Question

By using the following reactions, calculate the heat of combustion of pentane:

C5H12 (g) + 8O2 (g) → 5CO2 (g) + 6H2O (l).



C (s) + O2 (g) → CO2 (g), ΔH = –395 kJ
H2 (g) + 1/2O2 (g) → H2O (l), ΔH = –285 kJ
5C (s) + 6H2 (g) → C5H12 (g), ΔH = –147 kJ


–6.80 × 103 kJ


–2.85 × 103 kJ


–5.42 × 103 kJ


–3.54 × 103 kJ

Answer 1

To answer this problem, we use Hess' Law to calculate the overall enthalpy of the reactions. The goal is to add all the reactions such that the final reaction is C5H12 (g) + 8O2 (g) → 5CO2 (g) + 6H2O (l) through cancellation adn multiplication. The first equation is multiplied by 5, the second one is multiplied by 6 and the third one is reversed. The final answer is -3538 J or -3.54 x10^3 kJ.