1. Thermodynamics

2. Spontaneous and Non-spontaneous Processes
Spontaneity in chemical processes = the direction in which the reaction proceeds.
Not speed of reaction (kinetics)
A reaction can be thermodynamically favored but still be slow.
Example: Diamond converting to graphite

3. Entropy
Most spontaneous processes are exothermic, but some are endothermic.
Melting of ice above 0oC
Dissolution of sodium chloride in water
Why?
Second Law of Thermodynamics

4. Entropy (S) Informally—the disorder or randomness of a system
Formally—a measure of the molecular motional energy (plus any phase change energy) that has been dispersed in a system at a specific temperature.
System with highest entropy has the greatest dispersal of energy
For any spontaneous process, ΔSuniverse > 0
Units = J/K

5. Phase changes and Entropy
Ssolid < Sliquid < Sgas
In gas, more ways to distribute energy than a solid.
Energy in solid is due to vibrations between molecules.
Energy in gas is due to translational, rotational, and vibrational energy.

6. Predict the sign for ΔS for each process and justify your answer
Melting of ice to water
Sublimation of carbon dioxide
H2O(g)  H2O(l)
2N2O(g)  2N2(g) + O2(g)

7. Heat Transfer Exothermic process increases entropy of surroundings
Endothermic process increases entropy of system
ΔSuniv = ΔSsys + ΔSsurr
Process is spontaneous as long as entropy of the universe is positive.

8. Temperature dependence
Freezing of water is a nonspontaneous process above 0oC. Why?
Entropy is temperature dependent.
Impact of heat flow is greater at lower temperatures
Give money to a rich man vs. poor man

9. Entropy and Temperature

10. Entropy Changes in surroundings
Can use qsys to calculate Ssurr.
ΔSsurr = −ΔH/ T
If ΔH is positive, then ΔS is negative and non-spontaneous..
If ΔH is negative, then ΔS is positive and spontaneous.

11. Practice
Calculate the entropy change in the surroundings associated with the combustion of propane gas occuring at 25oC (ΔHrxn = kJ).
Determine sign of the entropy change for the system.
Determine the sign of the entropy change for the universe. Will the reaction be spontaneous?

12. Practice
A reaction has ΔHrxn = -107 kJ and a ΔSrxn = 285 J/K. At what temperature is the change in entropy for the reaction equal to the change in entropy of the surroundings?

13. Practice
Which reaction is most likely to have a positive ΔS? Justify your reasoning.
a. SiO2(s) + 3C(s)  SiC(s) + 2CO(g)
b. 6CO2(g) + 6H2O(g)  C6H12O6(s) + 6O2(g)
c. CO(g) + Cl2(g)  COCl2(g)
d. 3NO2(g) + H2O(l)  2HNO3(l) + NO(g)

14. Gibb’s Free Energy (G)
A measure of whether or not a process will occur without the input of outside energy.
Criterion of spontaneity (thermodynamically favored or unfavored)
AKA chemical potential

15. Entropy and Free Energy

16. Summary ΔG proportional to –ΔSuniv ΔG < 0 = spontaneous
ΔG > 0 = non-spontaneous
ΔG = ΔH −TΔS
Units for ΔG are generally kJ

17. Summary ΔG = ΔH −TΔS At low temperatures, enthalpy is dominant.
At high temperatures, entropy is dominant.

18. Concept Check
Which statement is true regarding the sublimation of dry ice (solid CO2)? Justify your reasoning.
ΔH is positive, ΔS is positive, and ΔG is positive at low temperature and negative at high temperature.
ΔH is negative, ΔS is negative, and ΔG is negative at low temperature and positive at high temperature.
ΔH is negative, ΔS is positive, and ΔG is negative at all temperatures
ΔH is positive, ΔS is negative, and ΔG is positive at all temperatures.

19. Calculating Entropy Third Law of Thermodynamics
In a perfect crystal (diamond), the entropy at 0K is 0 J/K.
Can use third law to develop standard molar entropy values
Factors that affect standard entropy:
State of substance
Molar mass of substance
If substance is in a particular allotrope
Molecular complexity
Extent of dissolution

20. Practice--Whiteboard
Get into groups of three. On your whiteboard, answer the following question.
Arrange these gases in order of increasing standard molar entropy: SO3, Kr, Cl2. Justify your reasoning.

21. Calculating ΔSorxn ΔSorxn = ΣnpSo(products) – ΣnrSo(reactants)
Calculate Δsorxn for the reaction between gaseous ammonia and oxygen producing nitrogen monoxide (gas) and water vapor.
Reactant or Product
So (J/mol K)
NH3(g)
192.8
O2(g)
205.2
NO(g)
210.8
H2O(g)
188.8

22. Calculating Standard Change in Free Energy for a reaction
One of the possible initial steps in the formation of acid rain is the oxidation of SO2 to SO3 via the following reaction: SO2(g) + ½ O2(g)  SO3(g) Calculate the ΔGorxn at 25oC and determine whether the reaction is spontaneous.
Reactant or Product
ΔHof (kJ/mol)
So (J/mol K)
SO2(g)
-296.8
248.2
O2(g)
205.2
SO3(g)
-395.7
256.8

23. Estimating Free Energy at Temperatures other than 25oC
For the same reaction, estimate the value of ΔGorxn at 125oC. Is the reaction more or less spontaneous at this elevated temperature?

24. Free Energy under non-standard conditions
ΔGrxn = ΔGorxn + R T lnQ
Q = reaction quotient (equilibrium concept)
T = Temperature in K
R = J/mol K
For aA + bB  cC + dD
Q = [C]c[D]d [A]a[B]b