ShmoopTube

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

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They're the difference between living in this room… [Boy playing in his bedroom]

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And this room… Alrighty, here’s our question:

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What is the standard enthalpy for the following reaction, given the standard enthalpies of

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formation?

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Here's the given reaction…lots of no's in there… [Chemical reaction equation]

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And we're given the heats of formation for NO, N2O and NO2.

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And here are our potential answers: This reaction is saying, “No, no, no!”

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But we’re saying, “yes, yes, yes” to finding the standard enthalpy of the reaction.

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Heh. Classic chemistry joke… Because nitric oxide is NO…? [Man thrown banana skins at him on stage]

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Well we thought it was classic.

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Anyway, to solve this problem, we need to find the standard enthalpy of reaction using

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the following relationship: The enthalpy of reaction is equal to the sum

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of the enthalpies of formation the products minus the sum of the enthalpies of formation [enthalpy of reaction definition]

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of the reactants. Alright, well unlike staying awake after a

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big lunch, this is actually pretty easy. In this reaction, our products are N2O and

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NO2. We need to look at the heats of formation for these molecules given in the problem statement [Heat formation of N2O and NO2]

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and add them up. Next, we see that the reactants are three

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N O molecules, so we need to subtract three times the heat of formation of N O.

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When we do the math, we find that the enthalpy of reaction is negative 154 kJ/mol, which

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is answer (A). And yup, that's all she wrote. We're done.

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Now that we’ve figured out this problem, it's time to reward ourselves with a big lunch…and [Boy eating food and falls asleep]

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maybe a nap…

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