AP Chemistry 2.1 Laws of Thermodynamics

APAP Chemistry
AP ChemistryLaws of Thermodynamics
LanguageEnglish Language
Test PrepAP Chemistry

Transcript

00:21

By the way, that N.O. in answer D?

00:25

Yeah, it’s not the opposite of “YES.” [A girl looking confused]

00:27

It’s nitric oxide, people.

00:28

That’s not to be confused with nitrous oxide, better known as laughing gas. [Students laughing in class and teacher asks what is funny]

00:34

But who needs laughing gas when you’ve got us, and our dazzling sense of humor?

00:39

Anyway, let’s get this thing going.

00:41

To answer this question, we probably need to know what the heck entropy is.

00:43

Well, according to the unbreakable second law of thermodynamics, the overall entropy [Image of a galaxy in a thermodynamics book]

00:48

in the universe never decreases.

00:51

Increasing entropy means a dispersal of energy or matter.

00:54

This leads to increased disorder or messiness. [A boys bedroom door opens and lots of mess inside]

00:58

So yes, your bedroom is messy and the Universe is, too.

01:03

Try that one on Mom and Dad. Good luck!

01:05

All right, we have to think about which of our answer choices describes an increase in

01:08

disorder or dispersal.

01:11

We know that the molecules in a gas are more spread out than in a liquid, and the molecules [Molecules in liquid bouncing up and down]

01:14

in a liquid are more spread out than in a solid.

01:17

The molecules in a gas are the most dispersed and most disordered…gases have the highest

01:22

entropy.

01:23

Okay, so let’s look at choice A. Freezing water represents a liquid to solid

01:27

phase change, which corresponds to decreased disorder and thus decreased entropy. [Molecules inside an ice cube]

01:32

So we can lose A. Those solid molecules sure are orderly, but

01:36

jeez, they don’t have much fun.

01:38

Could it be C - crystallization of salt from a supersaturated solution?

01:42

Well, like freezing water, this answer describes a transition from the liquid phase to the [Molecules bouncing in a wine glass]

01:46

solid phase, which corresponds to a decrease in disorder and thus a decrease in entropy.

01:52

So say bye-bye to C. Choice D describes a reaction in the gaseous [Gaseous molecules in disorderly fashion]

01:56

phase.

01:57

We have two molecules combining to form one—matter that was once dispersed is now more compact.

02:03

This is a decrease in disorder, and thus a decrease in entropy, and thus a decrease in

02:07

our interest in choice D. That leaves us with choice B, boiling water. [A pan of boiling water]

02:12

Boiling water represents a liquid to vapor phase change, which corresponds to increased

02:16

disorder and thus increased entropy.

02:18

That means that B is our answer.

02:20

Hey, now that you know your second law of thermodynamics…maybe you could – we don’t

02:24

know - clean your room? [boy using a vacuum in his room and mother watches]

02:25

Just a thought.