1. Entropy, Free Energy and Equilibrium

2. Learning Objectives (LO’s) In this lesson you will learn….
1) What spontaneity is in reactions 2) The likelihood of exothermic reactions to be spontaneous compared to endothermic reactions 3) What entropy is and the 3rd Law of Thermodynamics 4) How to calculate entropy values

3. Entropy (7 min)

4. Spontaneous Processes
Often spontaneous processes are
exothermic (which is ok a ball rolls
downhill)
Water is made of hydrogen and oxygen, but it does not explode. This is NOT spontaneous…
2H2O(l) kJ →  2H2 (g) O2 (g)
So how do we get a ball to roll up hill? How can we make ENDOTHERMIC
reactions spontaenous?
(1 minute Ba(OH)2)

5. Spontaneous vs Nonspontaneous

6. ENTROPY SPONTANEOUS CHANGES - take place without the need for work
Exothermic reactions are usually spontaneous
- this is because they go from higher to lower enthalpy
However ...
Why should reactions with a positive DH value take place spontaneously ?
(some salts dissolve readily in water and the solution gets colder, not hotter)
ENDOTHERMIC
Basically, how does a ball roll UPHILL????????????

7. The Concept of Entropy (S)
Entropy refers to the state of order.
A change in order is a change in the number of ways of arranging the particles, and it is a key factor in determining the direction of a spontaneous process.
more order
less order
solid liquid gas
more order
less order
crystal + liquid ions in solution
more order
less order
crystal + crystal gases + ions in solution

8. Probability- what is the chance that the order of the cards can be restored by reshuffling?

9. Laws of Thermodynamics
1st Energy can neither be created nor destroyed. It can only change forms.
2nd: Entropy will increase over time; or entropy tends towards the maximum.
3rd The entropy of a perfect crystal at absolute zero is zero

10. Trends in Entropy
Entropy for gas phase is greater than that of liquid or solid of same substance
I2 (g) has greater entropy than I2 (s)
Dissolve a sugar cube creates disorder
What are some other examples???

11. Entropy Changes in a System Qualitative
Ssolid < Sliquid

12. Entropy Changes in a System Qualitative
Sliquid < Svapor

13. Entropy Changes in a System Qualitative
Ssolid < Saqueous

14. Entropy Changes in a System Qualitative
Slower temp < Shigher temp

15. Entropy Changes in a System Qualitative
Sfewer moles < Smore moles

16. Standard Entropies
Larger and more complex molecules have greater entropies.

17. Qualitative Entropy Changes
Determine the sign of S for the following, qualitatively
1. Liquid nitrogen evaporates
2. Two clear liquids are mixed and a solid yellow
precipitate forms
3. Liquid water is heated from 22.5 C to 55.8 C

18. Standard Entropies
These are molar entropy values of substances in their standard states.
These are in Joules not kJ
We can use these values to get a number for entropy

19. Entropy Changes in the System
Entropy can be calculated from the table of standard values just as enthalpy change was calculated.
Srxn = nS products  nS reactants

20. Standard Entropy
Calculate the standard entropy change for the following using the table of standard values. (first, predict the sign for S qualitatively)
2 NH3(g)  N2(g) H2(g)
oS NH3(g) = J/K
oS H2(g) = J/K
oS N2(g) = J/K

21. Standard Entropy 2NH3(g)  N2(g) + 3H2(g)
Answer: qualitatively (Entropy increases)
(2 mol gas  4 mol gas)
Srxn = nS products  nS reactants
= [(1)(191.5 J/K · mol) + (3)(131.0 J/K · mol)]
- [(2)(193.0 J/K · mol)]
= J/K · mol J/K · mol
Srxn = J/K · mol (Entropy increases)

22. Your Turn!
Calculate the standard entropy change for the following using the table of standard values. (first, predict the sign for S qualitatively)
2H2(g) + O2(g)  2H2O (g)
S H2O(g) = +188 J/K.mol
S H2 = +131 J/K.mol
S O2 = J/K.mol

23. Standard Entropy 2H2(g) + O2(g)  2H2O (g) Answer: qualitatively
(Entropy decreases, more organized; 3 mol gas  2 mol gas)
Srxn = nS products  nS reactants
= [(2)188 J/K·mol) – [(2)131 J/K·mol)+ (205 J/K·mol)]
= 376 J/K · mol J/K · mol
Srx = -91 J/K·mol (Entropy decreases, more organized; 3 mol gas  2 mol gas)