ShmoopTube

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And here's your Shmoop du jour, brought to you by Delta H. [Plane flying]

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Sounds like something cool and air force-y, but it’s just science.

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Which is also super cool and exciting, right?

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…Right?

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Thank you. [Scientist working in a lab and explosion occurs]

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Geez, what does it take around here to get an explosion...

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Okay, here's today’s question.

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Using Hess's law, what is the change in enthalpy for the following reaction?

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And here are your potential answers.

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So the question tells us to use Hess’s law, which, unfortunately, has nothing to do with

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avocados. [Hess kicks an avocado]

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That’s Hass

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Hess’s law comes from the fact that the overall enthalpy change of a reaction doesn’t

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depend on the reaction pathway, or how we get from reactants to products.

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For example, we could react one mole of A and 2 moles of B to produce 2 moles of C.

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Let’s call this Peter. [Formula example of Hess's law]

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What, you think all reactions want to be called "reaction"?

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Do you just want to be called "human six billion nine hundred and sixty-two thousand"?

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No.

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We're calling "reaction 1" Peter.

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We could also react one mole of A with one mole of B to produce two moles of D, then

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react these moles of D with one mole of B to produce two moles of C.

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We’ll call these reactions Sarah and Phil, because they're more than just 2 and 3.

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As you can see, reactions Sarah and Phil can be added to produce reaction Peter.

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The overall reaction is the same in both cases—one mole of A and two moles of B produce two moles

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of C.

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So in the same way that we added the actual reactions, we can add the enthalpy changes.

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Hess’s law says that the overall enthalpy change shouldn’t depend on the reaction

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path chosen.

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That means that the enthalpy change for Peter should equal the sum of the enthalpy changes [Hess's law for Peter, Sarah, Phil]

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for Sarah and Phil.

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Okay, so back to the problem at hand! [Man in a rocking chair]

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And we’re not referring to the fact we're naming reactions and may be having a mental

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breakdown. [Man crying in a rocking chair]

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We need to find a way to add the three reactions for which we have ?H to produce the top reaction.

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Here’s what we came up with.

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If we multiply the first equation by one half, reverse and multiply the second equation by

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3/2, and multiply the third equation by one half, then add them all up, we get the first [All equations added together]

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reaction.

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We add the ?H values in the same way.

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Just don’t forget that if you reverse a reaction, you have to negate the ?H value,

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and if you multiply a reaction by a factor, such as 1/2, you should multiply the ?H value [Man discussing ?H value]

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by that number too.

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So chugga chugga chug... chug through the math machine and…

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Looks like our answer is B, 886 kJ.

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Now it's time to brainstorm the pilot episode of our new sitcom. [People sitting on a couch watching a sitcom]

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It's about three young reactions living together…we're calling it, "Three's Company, but with Reactions".

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…We're not great at naming things.

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