1. Entropy, Spontaneity, and Free Energy
Thermodynamics
Entropy, Spontaneity, and Free Energy

2. Laws of Thermodynamics
Thermodynamics is the study of the energy changes in reactions.
Thermodynamics allows us to predict:
How “far” a reaction goes
Whether or not a reaction is spontaneous

3. First Law of Thermodynamics
Energy can neither be created nor destroyed, only changed from one form to another
Also known as “Conservation of Energy”

4. Second Law of Thermodynamics
All spontaneous processes involve an increase in entropy (disorder) in the Universe.
ΔS univ = ΔS system + ΔS surroundings
Increase in entropy is +ΔS

5. Note
Spontaneity of a reaction does not imply anything about the rate of the reaction.
Some spontaneous reactions occur very slowly (diamond  graphite)
Thermodynamics only depends on the energies of the initial and final states, not the pathways (this is the province of kinetics).

6. Entropy Entropy is a measure of randomness or disorder
A highly disordered system has high entropy
Entropy is associated with probability. A highly disorder state is more probable than an ordered state

7. Entropy Example Your Bedroom Your room tends to get messy!
Why? There are many more ways for things to be out of place than in place
To keep things in order, we need to put work into the system (CLEAN YOUR ROOM!!!)

8. Second Law Restated It is natural for things to become disordered
There is a natural tendency from order to disorder
Nature spontaneously proceeds towards states that have the highest probability of existing: more disordered

9. Predicting Changes in Entropy
Predict which phase, solid, liquid, or gas, would have the highest entropy. Why?
Predict the effect of increased volume on entropy, why?
As the temperature of a substance increases, predict how entropy will change. Why?

10. Predicting Changes in Entropy
Predict whether the following reactions will have an increase in entropy (+ΔS) or a decrease in entropy (-ΔS)
N2(g) + 3 H2(g)  2 NH3(g)
2 Hg(l) + O2(g)  2 HgO(s)
H2(g)  2 H(g)

11. Reaction Entropy More molecules mean more entropy
Phase changes mean more/less entropy
Decomposition means more entropy

12. What to Look For
Compare the # moles of gas on each side of the equation
Look for phase changes
gas>>>liquid>solid
Look for decomposition

13. Predicting Spontaneity
Remember: All spontaneous processes involve an increase in entropy (disorder) in the Universe. (ΔS universe must be “+”)
What happens to the entropy in a system as a gas condenses to a liquid?
Is it possible for a substance to spontaneously condense at temps below the boiling point. How?

14. Predicting Spontaneity
Relationship between ΔS (surroundings) and the ΔH (system)
ΔS surr = -ΔH sys/T
In an exothermic system (-ΔH), heat is released to the surrounding, so ΔS surr is “+”.

15. Predicting Spontaneity
WHY? As the molecules in the surroundings absorb the heat released by the reaction, their random motion and speed increase = more entropy.

16. Predicting Spontaneity
Gibbs Free Energy (ΔG)
Instead of using ΔS univ to predict spontaneity, ΔG is used because it relates the entropy, temperature, and enthalpy of a system.
A process is spontaneous if ΔG is “-”

17. Gibbs Free Energy ΔS univ = -ΔG sys/T ΔG sys = ΔH sys – TΔS sys
Advantage: Using ΔG to predict spontaneity centers on the system only.
(The equation only holds true for a system at constant Temp and Pressure)

18. Questions Are all exothermic processes spontaneous?
Are all processes which have an increases in the entropy of the system spontaneous?
Is the answer to #2 consistent with the 2nd Law of Thermodynamics?