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AP Chemistry 2.4 Laws of Thermodynamics 11 Views


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AP Chemistry 2.4 Laws of Thermodynamics. What are the signs of ΔG, ΔH, and ΔS?

Language:
English Language

Transcript

00:04

And here's your Shmoop du jour, brought to you by the smoke machine, your weird uncle [Man smoking outside]

00:08

with a pack-a-day habit.

00:10

Okay, here's our question.

00:12

Solid CO2 is known as dry ice and is used in smoke machines and to keep food cold.

00:17

For the process at 25 °C: CO2 (s) ? CO2 (g)

00:24

What are the signs of ?G, ?H, and ?S

00:28

And here are our potential answers.

00:30

All right, what’s this question asking us? [Girl thinking about the question]

00:34

Well, it requires us to think about the thermodynamics of the solid to gas phase change for CO2.

00:40

Let’s think about what the sign of ?G, ?H, and ?S mean

00:46

qualitatively.

00:47

First up - ?G, or the change in Gibbs free energy. [Gibb's free energy table]

00:50

No relation to Barry, Robin, or Maurice.

00:52

We know that ?G tells us about the spontaneity of a chemical reaction.

00:57

Like whether or not the chemical reaction will take a spur of the moment trip to Disneyland [Man on a rollercoaster in DisneyLand]

01:01

with you.

01:02

We hope so…we've been waiting to wear our mouse ears for years now…

01:04

Anyway, if the ?G is negative, the reaction is spontaneous.

01:08

Now what about ?H, or the change in enthalpy?

01:11

The sign of ?H tells us if a reaction is endothermic or exothermic. [DeltaG sign with exothermic or endothermic]

01:16

A positive change in enthalpy implies that the reaction is endothermic and absorbs heat,

01:23

while a negative change in enthalpy implies that the reaction is exothermic and releases

01:28

heat.

01:29

As far as blankets go, you want the endothermic ones. [A dog wrapped in a blanket]

01:32

Anyone selling you an exothermic blanket is a bad guy..

01:36

And finally ?S, or the change in entropy.

01:40

Entropy tells us about the degree of dispersal, or disorder.

01:43

A positive change in entropy means that entropy has increased, and thus disorder has increased.

01:50

Your messy room? [Boy in a messy bedroom]

01:51

That’s a very positive change in entropy.

01:54

Now that we have this handy dandy chart, the problem becomes a lot easier.

01:58

That’s right, take it all in. [Man studying the chart]

02:00

We're pretty sure this chart is in contention to be the eighth wonder of the world…

02:03

… it's okay to cry.

02:04

Okay, so let’s consider our specific “reaction,” which is just a phase change: [Scientist drops substance into a beaker]

02:08

CO2 (s) ? CO2 (g)

02:13

We want to decide if each variable in our chart is positive or negative for this reaction. [Girl sat waiting]

02:17

Let’s walk through it.

02:18

And yes, we'll do it hand in hand.

02:20

Safety first.

02:21

Is our reaction spontaneous?

02:23

Yes—it happens at 25°C without any effort from us. [Pregnancy stick with 25 degrees shown]

02:27

Easy, breezy, beautiful.

02:29

Chemical reactions.

02:30

Is our reaction endothermic or exothermic?

02:33

AKA does it absorb or release heat? [fire burning]

02:35

Well, if it’s used to keep food cold, it must absorb heat from the food.

02:41

Thus, the reaction is endothermic.

02:44

And finally, is our reaction product, a gas, more or less disordered than our reactant,

02:49

a solid?

02:51

Gas molecules are further apart and more dispersed. [Gas bellowing]

02:53

Thus, they are more disordered than molecules in a solid.

02:57

We don’t blame them.

02:58

Everybody gets a little crazy when they’re far away from their pals. [Gas on a doctor's bed and doctor appears]

03:01

Now, looking at our handy chart, we see that ?G is negative, ?H is

03:05

positive, and ?S is positive.

03:08

So let’s mosey on down back to our potential answers…

03:11

It looks like choice (C) is the one we’re looking for. [Answer C circled and C dances in a club]

03:13

It’s okay, choice C, we can’t all be positive all of the time.

03:16

2 out of 3 times ain't half bad.

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