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The standard enthalpy change of combustion of propanoic acid (C3H6O2) is -1527.2 kJ/mol. From this and the data given below, calculate the standard enthalpy change of formation of propanoic acid. Enter your answer in kJ mol-1 to appropriate accuracy.
ΔfH (H2O) = -285.8kJ/mol
ΔfH (CO2) = -393.5kJ/mol

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Original post by MiracleLeaf

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I'm guessing people don't want to do your homework for you, or at least not before they see what you've tried so far, so that they can help you.

Reply 3

MiracleLeaf

Original post by Pigster

I'm guessing people don't want to do your homework for you, or at least not before they see what you've tried so far, so that they can help you.

It's not even homework, it's a revision question. I'm not actually sure where to start with it, I've tried googling it and looking in books and can't find anything. I know the answer is -510.7, but I can't figure out how to get there. Are you able to help with the question?

Reply 4

Pigster

Original post by MiracleLeaf

ΔfH (H2O) = -285.8kJ/mol
ΔfH (CO2) = -393.5kJ/mol

I assume you can create Hess' Law cycles. A fact that may help you is
ΔfH (H2O)= ΔcH (H2), likewise ΔfH (CO2)= ΔcH (C).

Reply 5

MiracleLeaf

(Original post by Pigster)I assume you can create Hess' Law cycles. A fact that may help you is
ΔfH (H2O)= ΔcH (H2), likewise ΔfH (CO2)= ΔcH (C).

So it would be -393.5 — -285.8? But that’s -108.5 but the answer was given as -510.7?

Reply 6

MiracleLeaf

Original post by Pigster

I assume you can create Hess' Law cycles. A fact that may help you is
ΔfH (H2O)= ΔcH (H2), likewise ΔfH (CO2)= ΔcH (C).

So it would be -393.5 — -285.8? But that’s -108.5 but the answer was given as -510.7?

Reply 7

Pigster

Original post by MiracleLeaf

So it would be -393.5 — -285.8? But that’s -108.5 but the answer was given as -510.7?

You didn't include ΔcH (PrOOH), which makes me think you didn't do a cycle.

Reply 8

MiracleLeaf

Original post by Pigster

You didn't include ΔcH (PrOOH), which makes me think you didn't do a cycle.

I tried. This question is confusing so would you just tell me how to work it out please?

Reply 9

Pigster

When constructing Hess' Law cycles, I find it best to always start with the one you are trying to work out written horizontally (like a normal equation). In this case ΔfH (PrOOH). Underneath it, (if you can be bother to work them out) write the common combustion products. Then you need arrows going from the left and the right of the inital equation pointing up or down to the stuff at the bottom. You should have three arrows. Next to each of these add the values and don't forget to multiply them, if needed. Then do the algebra.