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AP Chemistry 2.2 Laws of Thermodynamics 18 Views


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AP Chemistry 2.2 Laws of Thermodynamics. What is the ΔG and the spontaneity of the reacton?

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English Language

Transcript

00:04

And here's your Shmoop du jour, brought to you by spontaneity.

00:07

Otherwise known as why we're not going to finish this Shmoop du jour. [Shmoop employee at a desk and disappears]

00:10

We're going to Mexico, instead.

00:11

Vamos! [Car driving to Mexico]

00:12

Okay…maybe our spontaneous trip can wait.

00:16

Here's our question.

00:17

Consider the following reaction…right here…

00:19

Done checking it out?

00:21

Okay, moving right along…

00:23

The enthalpy of formation for the reaction is -92.0 kJ, and the change in

00:29

entropy is -55.4 J/K at 30 °C. What is the DeltaG of the reaction, and

00:37

what is the spontaneity of the reaction, respectively?

00:40

And here are your potential answers. [Potential answers to the reaction]

00:43

So we’re given a few different pieces of information in this problem. [Puzzle pieces separated]

00:48

We’ve got values for the enthalpy or heat of formation,..

00:51

…the entropy change,..

00:52

…and the temperature.

00:53

And we’re asked for the ?G, or the change in Gibbs free energy.

00:57

And no, by free energy, we don’t mean a blow-out sale on Red Bull. [Girl shopping for packs of Red Bull]

01:02

The Gibbs free energy tells us about the spontaneity of a reaction.

01:06

We need to know the value, and specifically the sign, of DeltaG.

01:11

We define DeltaG using the Gibbs free energy equation, DeltaG=DeltaH-T DeltaS

01:16

Anybody else craving alphabet soup? [Dinner lady serves boy alphabet soup]

01:20

We have values for all of the variables on the right hand side of this expression.

01:24

But before we plug them in, we have to consider their units.

01:27

There’s a few sneaky little guys that we need to convert before we plug them in, plug [Person plugs B into a wall socket]

01:31

them in.

01:33

The answer needs to be in kilojoules, so first convert DeltaS to kilojoules by multiplying

01:37

by the number of kilojoules in a joule.

01:40

And let’s not forget about temperature — DeltaS includes a kelvin unit and our given

01:45

temperature is in Celsius.

01:47

The kelvin scale is an absolute scale––it has a value of zero at absolute zero––so [Woman stood on a weighing scale]

01:53

we typically choose to use Kelvin in our equations.

01:55

Are we sure that’s better?

01:58

Absolutely.

01:59

Converting our temperature units, we find that our temperature is 303.15 Kelvin.

02:03

Now we just have to do the math…

02:04

Chugga-chugga-chugga, bing-bang-boom, and…

02:06

DeltaG= -75.2 kJ [Answer for the reaction]

02:12

Looking again at our choices, we see that choice D is what we want.

02:15

But we’re not done yet!

02:17

Is this reaction actually spontaneous? [Couple talking beside a stream]

02:20

Spontaneity depends on the sign of DeltaG

02:22

A negative DeltaG, a change in Gibbs free energy means energy is released and a reaction is

02:28

spontaneous, and a positive DeltaG means energy is added and

02:32

the reaction is not spontaneous.

02:35

So the negative DeltaG we calculated does correspond to a spontaneous reaction.

02:39

High five. [Boy holding out a hand for a high five]

02:40

Seriously.

02:41

Slap your screen. [Person slaps boys hand for high five]

02:42

Don't leave us hanging…

02:43

Now come on!

02:44

Mexico awaits!

02:45

…No?

02:46

You guys have responsibilities?

02:47

Drat.

02:48

Spontaneity looked awfully good in the movies… [Car spontaneously drives to Mexico]

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