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[ShadowLink64]

Chemistry Question:

If 512 g of propane are burned in an automobile engine, the amount of energy released (to the nearest tenth) is __________ MJ.

Incorrect. Your answer=25.8, Correct answer=23.7

Here's my working below:

Enthalpy Change Given in the Data Sheet: (-2219.9 kJ/mol)

Enthalpy Change (kJ) = (moles) * (molar enthalpy)
25773.04 kJ = (11.61 mol of C3H8) * (-2219.9 kJ/mol)

25773.04 kJ = 25.8 MJ

However, I tried forming the combustion formula for propane and used individual formation enthalpies instead:

C3H8(g) + 7(O2) --> 3(CO2)(g) + 4(H2O)(g)

I found the net enthalpy change per mole of propane (using Hess's law) to be -2043.9 kJ/mol and that multiplied by the moles (11.61) gives the correct amount of energy (23724.25 kJ which is 23.7 MJ)

Is the molar enthalpy of combustion calculated differently than the net enthalpy of formation? I'm not sure why both molar enthalpies are different (-2219.9 kJ/mol and -2043.9 kJ/mol are very close).

A person who knows what I'm talking about is probably going to find something blatantly dumb about my question because it's something obvious that I'm missing.

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EDIT: Figured it out.... the given enthalpy is if liquid water and carbon dioxide gas are the products. In the question I was doing, water vapour and carbon dioxide were the products (water vapour and liquid water vary a bit) which is why I had to manually calculate it.


Thanks anyways.