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AP Chemistry 1.5 Laws of Thermodynamics 10 Views


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AP Chemistry 1.5 Laws of Thermodynamics. What is the heat of combustion of propene C 3 H 6 ?

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English Language

Transcript

00:03

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

00:07

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

00:12

Okay, here’s our question:

00:13

Propene is a byproduct of the oil refining process.

00:17

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

00:22

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

00:30

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

00:37

letters and numbers.

00:38

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

00:41

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

00:46

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

00:50

the reaction.

00:52

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

00:56

of propene.

00:57

So we want to find the enthalpy of this reaction.

01:01

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

01:06

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

01:11

to find the total reaction enthalpy.

01:13

No problem, right?

01:15

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

01:20

the overall combustion reaction. [Three reactions added together]

01:22

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

01:24

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]

01:28

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

01:31

First, we’ll add three times reaction II.

01:35

Then, add three times reaction III.

01:37

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

01:43

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

01:46

can subtract 3 H2 and 3 C from both sides.

01:50

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

01:54

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

01:57

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

02:01

we added up the reactions.

02:03

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

02:07

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

02:12

-2093.3 kJ

02:14

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

02:18

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

02:21

Hey, it's no laughing matter.

02:22

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

02:27

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

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