ShmoopTube

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Here’s your Shmoop du jour, brought to you by oil refining.

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Once it's finished being refined, we hear it has a lovely debutante ball. [Person walks down staircase in a gown]

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Okay, here’s our question:

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Propene is a byproduct of the oil refining process.

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What is the heat of combustion of propene, C3H6, given the following information?

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We’re given several chemical reaction equations and reaction enthalpies.

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And here are the potential answers: Okay, let's not be scared off by all those

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letters and numbers.

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After all, a refined Shmooper never lets a little hard work scare them off. [Boy smashes laptop]

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So this question requires us to use Hess’s Law, which says that the total enthalpy change

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of a chemical reaction is equal to the sum of enthalpy changes of each of the steps in

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the reaction.

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We also need to recognize that the first reaction given in the problem statement is the combustion [Propene combustion equation]

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of propene.

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So we want to find the enthalpy of this reaction.

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If we can figure out how to add up some combination of the three reactions given in the problem

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statement to make the overall combustion reaction, we can add up the enthalpies in the same way

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to find the total reaction enthalpy.

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No problem, right?

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So let’s label these reactions I, II, and III and start adding them up to try to make

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the overall combustion reaction. [Three reactions added together]

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And remember, we have to follow Hess’s Law.

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If you violate Hess’s Law, you have to go to Hess’s prison, and believe us, you don’t [People inside a prison cell]

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want to go to Hess’s prison… that’s a mistake we’ll never make again.

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First, we’ll add three times reaction II.

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Then, add three times reaction III.

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Finally, we’ll subtract reaction I, which is the same as adding the reverse reaction.

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When we add all this up, we see some terms appear on both sides of the equation, so we

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can subtract 3 H2 and 3 C from both sides.

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Once we’ve done this, we’ve got the overall propene combustion reaction.

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Now comes the most important part of Hess’s Law. [Woman teacher discussing Hess's Law]

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We have to add up the enthalpies of reactions I, II, and III in exactly the same way as

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we added up the reactions.

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So if we take three times reaction II plus three times reaction III minus reaction I,

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plug in the values, and do the math, we’ve got our answer:

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-2093.3 kJ

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If we round to the nearest ten, we can see that this is answer choice (D). [Answer D circled green]

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Looky there, we did it, and without landing in Hess's Jail.

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Hey, it's no laughing matter.

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Last week our neighbor called Hess’s cops to come bust up a chemistry study group. [Study group eating chips together and cops appear]

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We hear they're serving twenty to Avogadro's number.

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