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AP Chemistry 3.1 Laws of Thermodynamics 18 Views


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AP Chemistry 3.1 Laws of Thermodynamics. What is the change in enthalpy of this reaction?

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Transcript

00:04

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

00:08

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

00:12

Which is also super cool and exciting, right?

00:14

…Right?

00:15

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

00:16

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

00:18

Okay, here's today’s question.

00:19

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

00:26

And here are your potential answers.

00:32

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

00:36

avocados. [Hess kicks an avocado]

00:37

That’s Hass

00:39

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

00:44

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

00:50

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

00:54

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

00:56

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

00:59

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

01:04

No.

01:05

We're calling "reaction 1" Peter.

01:07

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

01:14

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

01:19

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

01:25

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

01:31

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

01:37

of C.

01:39

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

01:44

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

01:49

path chosen.

01:51

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

01:56

for Sarah and Phil.

01:58

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

02:00

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

02:04

breakdown. [Man crying in a rocking chair]

02:05

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

02:11

Here’s what we came up with.

02:13

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

02:18

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

02:25

reaction.

02:26

We add the ?H values in the same way.

02:29

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

02:34

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

02:39

by that number too.

02:41

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

02:46

Looks like our answer is B, 886 kJ.

02:50

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

02:53

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

02:58

…We're not great at naming things.

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