1. The Effect of Temperature on Spontaneity.
Consider the following:
H2O(l)  H2O(g)
Water is the system, everything else is the surroundings.
Consider the entropy of liquid and gaseous water.
Explain what happens if this is left at a constant temperature, heated or cooled.

2. Direction of heat flow and the sign of S
At a constant temperature (from everything other than the reaction), an exothermic process releases heat to the surrounding area.
This increases the random motion and thus the entropy of the surroundings.
For exothermic reactions, Ssurr is always positive.
For endothermic reactions, Ssurr is always negative.
Therefore, endothermic reactions normally need to be heated to account for this and make them spontaneous.

3. The magnitude of DSsurr depends on the temperature
That is to say that an area that is already “hot”, will already have a high level of entropy.
Adding heat energy will increase the randomness, however, you will not see the same level of increase as you would if you added the heat energy to something that is “cold” and therefore has a lower level of entropy.

4. Energy will cause entropy changes
Therefore the energy changes will be the driving force of the reaction
DSsurr = _ DHsys T

5. Determining DSsurr
In the metallurgy of antimony, the pure metal is recovered via different reactions, depending on the composition of the ore. For example, iron is used to reduce antimony in the sulfide ores:
 Sb2S3(s) + 3Fe(s) 2Sb(s) + 3FeS(s) DH = -125 kJ
Carbon is used as the reducing agent for oxide ores:
Sb4O6(s) + 6C(s)  4Sb(s) + 6CO(g) DH = 778 kJ
Calculate DSsurr for each of these at 25o C and 1 atm.

6. Role ofSsys and Ssurr in Determining the Sign of Suniv

7. Gibbs Free Energy
Some reactions are spontaneous because they release energy, which increases the entropy of the surrounding area.
Some reactions are spontaneous because they involve an increase in the entropy of the system.
Gibbs Free energy (G) is a function that combines the system’s enthalpy and entropy.
G = H - TS
Where H is enthalpy, T is temperature, and S is entropy.

8. Free Energy
The free energy change (DG) is a measure of the spontaneity of a process and of the useful energy available from it.
DG = DH - TDS
This is the driving force of a reaction, and what you will use to determine if a reaction is spontaneous or not.
A NEGATIVE value for DG means the process is spontaneous.
POSITIVE means the reaction is not spontaneous.

9. Exergonic/Endergonic
Exothermic and endothermic are describing the enthalpy of the system, H.
Exergonic reactions are one that release free energy to the system for work or have a negative DG. These are spontaneous, or thermodynamically favored.
Endergonic reactions are one that absorb free energy from the system for work or have a positive DG. These are not spontaneous, or are not thermodynamically favored.

10. Relating Gibbs Free Energy to spontaneity
DG = DH - TDS divide each side by (-T)
-DG/T = - DH/T + DSsys DSsurr = - DH/T
- DG = DSsurr + DSsys T
DSuniv = - DG
Conclusion: A process (at constant T and P) is spontaneous in the direction in which the free energy decreases
A negative DG means there is a positive DSuniv

11. Spontaneous reactions
Exothermic reactions tend to be spontaneous.
DH is negative.
Reactions that involve and increase in their entropy tend to be spontaneous.
DS is positive.
Temperature will always be positive (Kelvin Scale)
DG = DH – T DS
negative positive
A negative number minus a positive number so DG will always be negative and these reactions will always be spontaneous.

12. Nonspontaneous reactions
Endothermic reactions tend to not be spontaneous.
DH is positive.
Reactions that involve and decrease in their entropy tend to not be spontaneous.
DS is negative.
DG = DH – T DS
positive negative
Minus a negative number means adding, a positive number plus a positive number means DG will always be positive and this reaction will never be spontaneous.

13. Reactions dependent on the temperature
What if there is an endothermic reaction that involves an increase in entropy or and exothermic reaction that involves a decrease in entropy?
DG = DH – T DS
positive positive
Or negative negative
The temperature and value of DH, DS determine whether it is spontaneous or not.

14. Temperature
For the endothermic reaction with an increase in entropy, the reaction will be spontaneous at high temperatures. When T DS is larger DH.
How high of a temperature depends on the value of DH and DS
For the exothermic reaction with a decrease in entropy, the reaction will be spontaneous at low temperatures. When DH has a higher magnitude than T DS.
The magnitude of DH and DS is important to determining if a reaction is spontaneous.

15. Question
At what temperature is the following process spontaneous at 1 atm (not a number, words. What do we call this temperature)?
Br2(l)  Br2(g)
Any temperature above the boiling point, this process becomes spontaneous. Therefore, DG must be negative above the boiling point. If DG is positive, then the reverse will occur (condensation).
If you are at the boiling point, DG is zero.

16. Problem Br2(l)  Br2(g) DHo = 31.0 kJ/mol, and DSo = 93.0 J/K*mol
Determine what is the normal boiling point of liquid Br2? Set DG =0
Unit agreement!!!!!
Enthalpy is in kilojoules/mole, entropy is in joules/mole kelvin

17. Review
DSuniv is positive for all spontaneous processes. 2nd Law of Thermodynamics.
This is impractical to chemistry, don’t use it
DG is the free energy change of the system.
This is much more practical to chemists.
DG is NEGATIVE when DSuniv is positive.
DG represents the usable work done by a system.
DGo = DHo - TDSo

18. Entropy Changes in Chemical Reactions.
Standard Molar Entropies, So.
In thermodynamics, the change in a certain function is usually what is important.
Absolute values for H and G cannot be determined.
From the third law of thermodynamics we find that at 0 K, the entropy of a pure crystal is equal to 0.

19. Because this provides a starting point to compare all other entropies, an absolute entropy scale has meaning.
Standard molar entropy, So, is the entropy of one mole of a substance in its standard state.
The degree symbol means at standard state which is 25o C and 1.0 atm.
So it is the change in entropy from 0 K to 298 K

20. Entropy Changes in Chemical Reactions.
Predicting Relative So Values of the System.
Temperature changes.
For a given substance, So increases as the temperature increase.
Physical states and phase changes.
For a given substance, So increases as the substance changes from a solid to a liquid to a gas (the change from a liquid to a gas is greater than from a solid to a liquid).

21. Dissolution of a solid or a liquid.
The entropy of a dissolved solid or liquid solute is greater than the entropy of the pure solute.
The type of solute and solvent and the nature of the solution process affects the overall entropy change.

22. Entropy Changes in Chemical Reactions.
Predicting Relative So Values of the System.
Dissolution of a gas.
A gas always becomes more ordered when dissolved in a liquid and gas.

23. Complexity
In general, difference in entropy values for substances in the same phase are based on atomic size and molecular complexity.
For elements within a periodic group, those with higher molar masses have higher entropy.
For compounds, the chemical complexity increases as the number of atoms (ions) in a compound increases, and so does the entropy.
For larger molecule, entropy increases with size.
IN GENERAL, THE LARGER THE MOLECULE, THE MORE ELECTRONS, THE MORE “POLARIZABLE” THE MOLECULE IS.

24. Entropy Changes in Chemical Reactions.
Predicting the Sign of DSo.
Predict the sign of DSo for each of the following reactions.
The thermal decomposition of solid calcium carbonate:
CaCO3(s)  CaO(s) + CO2(g)
The oxidation of SO2 in air:
2 SO2(g) + O2(g)  2 SO3(g)

25. Predicting Relative Entropy Values
Choose the member with the higher entropy in each of the following pairs, and justify your choice.
1 mol SO2(g) or 1 mol SO3(g)
1 mol CO2(s) or 1 mol CO2(g)
2 mol O2(g) or 2 mol O3(g)
1 mol KBr(s) or 1 mol KBr(aq)
Sea water in winter at 2 oC or in summer at 23 oC
1 mol HF(g) or 1 mol HI(g)

26. Entropy Changes in Chemical Reactions.
Because entropy is a state function, the property is what it is regardless of pathway, the entropy change for a given reaction can be calculated by taking the difference between the standard entropy values of products and those of the reactants.
DSoreaction = SnpDSoproducts - SnrDSoreactants

27. Entropy Changes in Chemical Reactions.
Calculating DSo.
Calculate DSo at 25o C for the reaction
2NiS(s) + 3O2(g)  2SO2(g) + 2NiO(s)

28. Entropy Changes II
Calculate DSo for the reaction of aluminum oxide by hydrogen gas: Al2O3(s) + 3H2(g)  2Al(s) + 3H2O(g)