|AP Chemistry||Laws of Thermodynamics|
|Test Prep||AP Chemistry|
By the way, that N.O. in answer D?
Yeah, it’s not the opposite of “YES.” [A girl looking confused]
It’s nitric oxide, people.
That’s not to be confused with nitrous oxide, better known as laughing gas. [Students laughing in class and teacher asks what is funny]
But who needs laughing gas when you’ve got us, and our dazzling sense of humor?
Anyway, let’s get this thing going.
To answer this question, we probably need to know what the heck entropy is.
Well, according to the unbreakable second law of thermodynamics, the overall entropy [Image of a galaxy in a thermodynamics book]
in the universe never decreases.
Increasing entropy means a dispersal of energy or matter.
This leads to increased disorder or messiness. [A boys bedroom door opens and lots of mess inside]
So yes, your bedroom is messy and the Universe is, too.
Try that one on Mom and Dad. Good luck!
All right, we have to think about which of our answer choices describes an increase in
disorder or dispersal.
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]
in a liquid are more spread out than in a solid.
The molecules in a gas are the most dispersed and most disordered…gases have the highest
Okay, so let’s look at choice A. Freezing water represents a liquid to solid
phase change, which corresponds to decreased disorder and thus decreased entropy. [Molecules inside an ice cube]
So we can lose A. Those solid molecules sure are orderly, but
jeez, they don’t have much fun.
Could it be C - crystallization of salt from a supersaturated solution?
Well, like freezing water, this answer describes a transition from the liquid phase to the [Molecules bouncing in a wine glass]
solid phase, which corresponds to a decrease in disorder and thus a decrease in entropy.
So say bye-bye to C. Choice D describes a reaction in the gaseous [Gaseous molecules in disorderly fashion]
We have two molecules combining to form one—matter that was once dispersed is now more compact.
This is a decrease in disorder, and thus a decrease in entropy, and thus a decrease in
our interest in choice D. That leaves us with choice B, boiling water. [A pan of boiling water]
Boiling water represents a liquid to vapor phase change, which corresponds to increased
disorder and thus increased entropy.
That means that B is our answer.
Hey, now that you know your second law of thermodynamics…maybe you could – we don’t
know - clean your room? [boy using a vacuum in his room and mother watches]
Just a thought.