First, let’s define the standard enthalpy values (H):
02(g) = 0 kj/mol
CO2 (g) = -393.5 kj/mol
H20(g) = -241.8 kj/mol
According to the instructions, heat per mole produced (H) equals 5094 kj
Therefore, -5094 kj is your H total.
H (reaction) = H (products) – H (reactants)
Let’s set the value of H for C8H18(g) to x (unknown)
then, -5094 kj = [8(CO2) + 9(H2O)] – [x +12.5(O2)]
If we use the standard enthalpy values,
5094kj = [8(-393.5) + 9(-241.8)] – [x + 12.5(0)]
-5094 kJ = -5324.2 – x
Answer: x = -230 kj/mol