Learning Goals
9.1
Learning Goals
9.2
Textbook Reading
“Chemistry: 2e” Second Edition, 2014. A digital version of the textbook will be available here.
Reading: Chapter 16 - Sections 1-3
AP Daily Videos for 9.1
AP Daily Videos for 9.2
Extra Reference Sheet for Entropy Values
Kids love entropy!
Entropy is a measure of disorder (or randomness) in a system.
These beads were in the box. Now they’re all over the floor… This is “greater entropy.”
Also, we could pour these beads onto the floor 1000 times and they’ll never be in this exact same arrangement. This is also “greater entropy.”
S represents the “number of possible ways that molecules can be arranged with respect to each other.”
Evaporation has a positive ΔS. Dissolving ionic compounds generally gives a positive ΔS.
Entropy values are constant under constant pressure and temperature.
In this unit we’ll be describing reactions as “thermodynamically favorable.”
What we’ll learn is that “thermo favorability” is a combination effect between enthalpy (heat), entropy (disorder), and Gibb’s free energy (spontaneity).
For now, understand that greater entropy is favorably.
It takes work to decrease entropy.
You have to put in work to keep your home clean. It gets dirty naturally; dust, dirt by the door, daily activity, etc.
You could not make a stir fry without entropy - nothing would mix! It would take a lot of work to pick out the pieces.
Law of Conservation of Energy: Energy cannot be created or destroyed, just transferred.
OK, 1st Law is understood.
2nd Law: Why would entropy be increasing???
The universe is constantly expanding. So we have a “greater volume” for things to expand into.
Think of the beads poured onto the living room floor versus cafeteria floor. The beads would have must greater entropy in a cafeteria.
ΔS > 0
Positive Entropy
More Dispersed / Random
ΔS > 0
Positive Entropy
More Dispersed / Random
A solid does have some entropy because the particles are vibrating.
ΔS > 0
Positive Entropy
More Dispersed / Random
ΔS > 0
Positive Entropy
More Dispersed / Random
Watch the phases!
If 4 gaseous molecules produce 5 liquid molecules, the entropy would decrease because of the associated phase change.
ΔS > 0
Positive Entropy
More Dispersed / Random
ΔS < 0
Negative Entropy
Less Dispersed / Less Random
This is the only exception.
ΔS > 0
Positive Entropy
More Dispersed / Random
Check out this entropy versus temperature graph!
Reminder: Temperature is the average kinetic energy of the particles.
This is a Maxwell-Boltzmann distribution graph.
Even gases can have greater entropy (become more random) at higher temperatures.
This is like the universe expanding. If there is a greater volume, than there is a greater space for the particles to disperse/be more random.
This works at a constant temperature or if temp increases.
This is the same idea as the enthalpy of formation equation:
ΔHorxn
You can look up entropy values on this reference sheet!
You can look up entropy values on this reference sheet!
Do you think your entropy will be positive or negative for this reaction?
The negative answer was expected. 3 moles of a gas change into 1 mole of a gas.
You can look up entropy values on this reference sheet!
You can look up entropy values on this reference sheet!
Do you think your entropy value will be positive or negative for this reaction?
You can look up entropy values on this reference sheet!
It makes sense that it was positive because you went from a liquid to a gas.
Learning Goals
9.1
Learning Goals
9.2
Homework
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