1. Entropy and Free Energy
How to predict if a reaction can occur, given enough time?
THERMODYNAMICS
How to predict if a reaction can occur at a reasonable rate?
KINETICS
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2. Thermodynamics Most product-favored reactions are exothermic.
Is the state of a chemical system such that a rearrangement of its atoms and molecules would decrease the energy of the system?
If yes, system is favored to react — a product-favored system.
Most product-favored reactions are exothermic.
Often referred to as spontaneous reactions.
Spontaneous does not imply anything about time for reaction to occur.

3. Thermodynamics and Kinetics
Diamond is thermodynamically favored to convert to graphite, but not kinetically favored.

4. Thermodynamics and Kinetics
Diamond is thermodynamically favored to convert to graphite, but not kinetically favored.
Paper burns — a product-favored reaction. Also kinetically favored once reaction is begun.

5. Product-Favored Reactions
In general, product-favored reactions are exothermic.
Fe2O3(s) + 2 Al(s) ---> 2 Fe(s) + Al2O3(s)
DH = kJ

6. Product-Favored Reactions
But many spontaneous reactions or processes are endothermic or even have DH = 0.
NH4NO3(s) + heat ---> NH4NO3(aq)

7. Entropy, S Spontaneity is related to an increase in randomness.
One property common to product-favored processes is that the final state is more DISORDERED or RANDOM than the original.
Spontaneity is related to an increase in randomness.
The thermodynamic property related to randomness is ENTROPY, S.
Reaction of K with water

8. The entropy of liquid water is greater than the entropy of solid water (ice) at 0° C.

9. Directionality of Reactions
How probable is it that reactant molecules will react?
PROBABILITY suggests that a product-favored reaction will result in the dispersal of energy or of matter or both.

10. Directionality of Reactions
Probability suggests that a product-favored reaction will result in the dispersal of energy or of matter or both.
Matter Dispersal

11. Directionality of Reactions
Probability suggests that a product-favored reaction will result in the dispersal of energy or of matter or both.
Matter Dispersal

12. Directionality of Reactions
Probability suggests that a product-favored reaction will result in the dispersal of energy or of matter or both.
Energy Dispersal

13. Directionality of Reactions
Probability suggests that a product-favored reaction will result in the dispersal of energy or of matter or both.
Energy Dispersal

14. Directionality of Reactions — Energy Dispersal
Exothermic reactions involve a release of stored chemical potential energy to the surroundings.
The stored potential energy starts out in a few molecules but is finally dispersed over a great many molecules.
The final state—with energy dispersed—is more probable and makes a reaction product-favored.

15. Entropy, S S (gases) > S (liquids) > S (solids) So (J/K•mol)
H2O(liq) 69.91
H2O(gas) 188.8

16. Entropy, S Entropy of a substance increases with temperature.
Molecular motions of heptane, C7H16
Molecular motions of heptane at different temps.

17. Entropy, S
Increase in molecular complexity generally leads to increase in S.
So (J/K•mol) CH
C2H
C3H

18. Entropy, S Entropies of ionic solids depend on coulombic attractions.
So (J/K•mol)
MgO 26.9
NaF 51.5

19. Entropy, S
Entropy usually increases when a pure liquid or solid dissolves in a solvent.

20. Entropy Changes for Phase Changes
For a phase change, DS = q/T
where q = heat transferred in phase change
For H2O (liq) ---> H2O(g)
DH = q = +40,700 J/mol

21. Entropy Changes for Phase Changes
For a phase change, DS = q/T
where q = heat transferred in phase change
For H2O (liq) ---> H2O(g)
DH = q = +40,700 J/mol

22. Calculating DS for a Reaction
DSo =  So (products) -  So (reactants)
Consider 2 H2(g) + O2(g) ---> 2 H2O(liq)
DSo = 2 So (H2O) - [2 So (H2) + So (O2)]
DSo = 2 mol (69.9 J/K•mol) [2 mol (130.7 J/K•mol) mol (205.3 J/K•mol)]
DSo = J/K
Note that there is a decrease in S because 3 mol of gas give 2 mol of liquid.

23. 2nd Law of Thermodynamics
A reaction is spontaneous (product-favored) if ²S for the universe is positive.
DSuniverse = DSsystem + DSsurroundings
DSuniverse > 0 for product-favored process
First calc. entropy created by matter dispersal (DSsystem)
Next, calc. entropy created by energy dispersal (DSsurround)

24. 2nd Law of Thermodynamics
Dissolving NH4NO3 in water—an entropy driven process.

25. 2nd Law of Thermodynamics
2 H2(g) + O2(g) ---> 2 H2O(liq)
DSosystem = J/K

26. 2nd Law of Thermodynamics
2 H2(g) + O2(g) ---> 2 H2O(liq)
DSosystem = J/K

27. 2nd Law of Thermodynamics
2 H2(g) + O2(g) ---> 2 H2O(liq)
DSosystem = J/K
Can calc. that DHorxn = DHosystem = kJ

28. 2nd Law of Thermodynamics
2 H2(g) + O2(g) ---> 2 H2O(liq)
DSosystem = J/K
Can calc. that DHorxn = DHosystem = kJ

29. 2nd Law of Thermodynamics
2 H2(g) + O2(g) ---> 2 H2O(liq)
DSosystem = J/K
Can calc. that DHorxn = DHosystem = kJ
DSosurroundings = J/K

30. 2nd Law of Thermodynamics
2 H2(g) + O2(g) ---> 2 H2O(liq)
DSosystem = J/K
DSosurroundings = J/K
DSouniverse = J/K
The entropy of the universe is increasing, so the reaction is product-favored.

31. 2nd Law of Thermodynamics
2 H2(g) + O2(g) ---> 2 H2O(liq)
DSosystem = J/K
DSosurroundings = J/K
DSouniverse = J/K
The entropy of the universe is increasing, so the reaction is product-favored.