1. Unit 10:
Thermodynamics

2. Review: Substance ∆Hf S
N2O4
9.08 KJ/mol
J/K mol
NO2
33.1 KJ/mol
J/K mol
Review:
Answer the following concerning the reaction shown below:
N2O4 (g)  2 NO2 (g)
a. What is the standard enthalpy change? Is heat released or absorbed?
b. If the reaction takes place and 560 kJ of heat is released or absorbed, what mass of N2O4 must have reacted?
c. Is entropy increasing or decreasing? Do you expect the sign of ∆Srxn to be positive or negative?
d. Calculate the ∆Srxn.

3. Second Law of Thermodynamics:
A spontaneous process is one that results in an increase of the entropy of the universe.
What this means to you …

4. Spontaneity:
A chemical reaction is said to be spontaneous if …
It occurs on it’s own,
without any outside intervention, (w/ the exception of applying an activation energy)
Show match burning, leading to candle lighting, leading to paper burning, but only after started, as examples of spontaneous things.
in the direction of equilibrium.
A 3rd and final thermodynamic property indicates whether or not a process is spontaneous …

5. Free Energy: (Gibbs Free Energy)
A third and final thermo. property that directly indicates whether or not a rxn will be spontaneous!!!

6. Free Energy: (Gibbs Free Energy)
A third and final thermo. property that directly indicates whether or not a rxn will be spontaneous!!!
Defined as the energy available to do useful work
(∆G = wmax)
Free energy is the “left-over” energy or the “available” energy

7. Free Energy: (Gibbs Free Energy)
What is “Free” Energy anyway???
2 H2 (g) + O2 (g)  2 H2O (g) ∆H = kJ/mol
more entropy
less
Is all this heat energy available to do useful work???
NO !!!
A portion of the energy released as heat must be used to “concentrate” the energy in the less entropic products
The energy left over can then be used to do work, and is called “free” energy

8. Calculating ∆G values:
Many of the same calculations can be used in calculating free energy …
1. Hess’ Law
2. ∆G = ∑∆Gf (products) - ∑∆Gf (reactants)
Since enthalpy and entropy both play a role in determining spontaneity …
3. ∆G = ∆H -
(T) (∆S)

9. Calculating ∆G values:
Many of the same calculations can be used in calculating free energy …
1. Hess’ Law
Since enthalpy and entropy both play a role in determining spontaneity …
∆G = ∆H -
(T) (∆S)
What ∆G tells you …

10. What does ∆G tell you? If ∆G = negative spontaneous as written
If ∆G = positive non-spontaneous as written
If ∆G = not spontaneous in any direction, at EQM!!!

11. Free Energy: (Gibbs Free Energy)
There is one type of rxn that is always spontaneous, and another type that is never spontaneous. Can you find them?
Enthalpy
Sign of ∆H
Entropy
Sign of ∆S
Expression
Sign of ∆G
Spont?
Exothermic
Increase
Endothermic
Decrease

12. SKIP 
Entropy Video