1. Chapter 19 Chemical Thermodynamics

2. Spontaneous vs Nonspontaneous Change
Spontaneous process – any process, once started, that proceeds without the external input of energy
Nonspontaneous process – any process which requires the continual, external input of energy to keep the process going
Any process that is spontaneous in one direction will be nonspontaneous in the reverse direction!

3. Nonspontaneous Change
Occurs only with outside assistance
Never occurs by itself:
Room gets straightened up
Pile of bricks turns into a brick wall
Decomposition of H2O by electrolysis
Continues only as long as outside assistance occurs:
Person does work to clean up room
Bricklayer layers mortar and bricks
Electric current passed through H2O

4. Spontaneous Change What factors influence spontaneity?
Occurs by itself
Without outside assistance until finished
e.g.
Water flowing over waterfall
Melting of ice cubes in glass on warm day

6. Spontaneous processes tend to favor:
1. Decrease in Energy
2. Increase in Disorder

8. Disorder is driven by statistical probability
Statistical probability – refers to the number of possible arrangements of a system
A disordered state is more probable because it can be achieved in more statistical ways
The larger the number of different possible combinations, the greater the probability of getting a disordered state

9. Entropy (Symbol S ) Thermodynamic quantity
Describes number of equivalent ways that energy can be distributed
Can be thought of as a measure of the randomness or disorder of a system
The greater the statistical probability of a particular state the greater the entropy!
Larger S, means more possible ways to distribute energy and that it is a more probable result

10. There are 2,598,960 possible five-card poker hands

11. more disorder S = larger #
Entropy, S
less disorder
S = smaller #
Ludwig Boltzmann 1844 – 1906
more disorder S = larger #

12. Gases have greater entropy than liquids which, in turn, have greater entropy than solids.

13. Order Disorder ΔS = + # Disorder Order ΔS = - #
Order Disorder is favored to occur spontaneously
Order Disorder
ΔS = + #
Disorder Order
ΔS = - #
2nd Law of Thermodynamics – the entropy of the universe is increasing over time

14. Effect of Volume on Entropy
For gases, entropy increases as volume increases
Gas separated from vacuum by partition
Partition removed, more ways to distribute energy
Gas expands to achieve more probable particle distribution
More random, higher probability, more positive S

15. Effect of Temperature on Entropy
3rd Law of Thermodynamics – entropy decreases as temperature decreases (and vice-versa)

16. Effect of Physical State on Entropy
Crystalline solid very low entropy
Liquid higher entropy, molecules can move freely
More ways to distribute KE among them
Gas highest entropy, particles randomly distributed throughout container
Many, many ways to distribute KE

17. Entropy Affected by Number of Particles
Adding particles to system
Increase number of ways energy can be distributed in system
So all other things being equal
Reaction that produces more particles will have positive S

18. = [(sum S products) – (sum S reactants)]
Standard Entropy of Reaction – the ΔS (entropy, or disorder) that accompanies ANY reaction under standard conditions (units are J/K)
= [(sum S products) – (sum S reactants)]
1. write and balance the reaction
2. use table of thermodynamic data of S to calculate
Figure: 19-13
Standard Entropy, S – disorder of a substance at standard conditions (units are J/mole·K)

19. 3 4 = – 93.0 J/K Determine the Standard Entropy of Reaction, 2
N2O (g) O2 (g) NO2 (g) 3 4
= – 93.0 J/K
= – #, suggests the reaction is nonspontaneous

20. Recap: 2 forces in nature which drive processes to occur spontaneously
ΔHrxn only slightly varies with temperature
ΔSrxn is highly dependent on temperature
Recap: 2 forces in nature which drive processes to occur spontaneously
1. Decrease in energy ΔHrxn = – # (exo)
2. Increase in disorder ΔSrxn = + # (disorder)

21. ΔGrxn = ΔHrxn – T ΔSrxn Willard Gibbs 1839 – 1903
ΔHrxn is the heat of reaction
T is the temperature in Kelvin
Willard Gibbs
1839 – 1903
ΔSrxn is the entropy of reaction
ΔGrxn is the Gibbs Free Energy

22. ΔGrxn or Gibbs Free Energy, is the ultimate, final deciding factor as to whether a reaction will occur spontaneously, anywhere in the universe
Bottom Line:
When ΔGrxn = – #, the reaction is spontaneous.
When ΔGrxn = + #, the reaction is nonspontaneous.
When ΔGrxn = 0, the reaction is at equilibrium (has no tendency to go one way or the other)

23. Is the following reaction spontaneous at 175 °C?4 2
N2O (g) O2 (g) NO2 (g) 3
= – 28.0 kJ
favors spontaneous
= – 93.0 J/K
favors nonspontaneous
Who wins ??
ΔGrxn = ΔHrxn – T ΔSrxn
ΔGrxn = kJ
ΔGrxn = + #, the reaction is nonspontaneous

24. Figure: 19-T04