1. Chapter 20 Thermodynamics
Spontaneity, entropy and free energy

2. Thermodynamics is based on a few laws that summarize centuries of experimental observations
The First Law of Thermodynamics – internal energy may be transferred as heat or work, but cannot be created or destroyed – was originally discussed in Chapter 7

3. The change in internal energy is defined in terms of heat (q) and work (w)
Internal energy is a state function which means that its value does not depend on how the change from one state to another was carried out

4. Thermodynamics vs. Kinetics
Domain of Kinetics
Rate of a reaction depends on the pathway from reactants to products.
Thermodynamics tells us whether a reaction is spontaneous based only on the properties of reactants and products.
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5. Spontaneous A reaction that will occur without outside intervention.
We can’t determine how fast.
We need both thermodynamics and kinetics to describe a reaction completely.
Thermodynamics compares initial and final states.
Kinetics describes pathway between.
Spontaneous

6. Most, but not all, exothermic reactions are spontaneous
Some endothermic reactions are spontaneous
Heat changes alone are not sufficient to predict if a process will proceed spontaneously

7. Entropy, S Defined in terms of probability.
Substances take the arrangement that is most likely.
The most likely is the most random.
Calculate the number of arrangements for a system.
Entropy, S

8. 2 possible arrangements
50 % chance of finding the left empty

9. 4 possible arrangements
25% chance of finding the left empty
50 % chance of them being evenly dispersed

10. 4 possible arrangements
8% chance of finding the left empty
50 % chance of them being evenly dispersed

11. Spontaneous processes tend to proceed from states of low probability to states of higher probability
The entropy (symbol S) is used in thermodynamics to describe the number of equivalent ways that the energy can be distributed
The greater the statistical probability of a particular state, the greater the entropy

12. A gas expands into a vacuum to give a uniform distribution because the expanded state has the highest positional probability of states available to the system.
Therefore: Ssolid < Sliquid << Sgas
Positional Entropy

13. Temperature must be considered when determining if a process is spontaneous
Liquid water spontaneously freezes when the temperature is below 0oC to form solid water (ice)
Solid water (ice) spontaneously melts when the temperature is above 0oC
The three factors that can influence spontaneity are the enthalpy change, the entropy change, and the temperature

14. The evaporation of alcohol The freezing of water
CONCEPT CHECK!
Predict the sign of ΔS for each of the following, and explain:
The evaporation of alcohol
The freezing of water
Compressing an ideal gas at constant temperature
Heating an ideal gas at constant pressure
Dissolving NaCl in water + –
a) + (a liquid is turning into a gas)
b) - (more order in a solid than a liquid)
c) - (the volume of the container is decreasing)
d) + (the volume of the container is increasing)
e) + (there is less order as the salt dissociates and spreads throughout the water)
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15. Factors affecting Entropy
Volume: For gases, the entropy increases as the volume increases.
Temperature: The higher the temperature, the higher the entropy.
Physical state: the entropy of gases is highest followed by liquids and then solids.
Molecular complexity: An increase in the number of particles increases the entropy.
Factors affecting Entropy

16. Second Law of Thermodynamics
In any spontaneous process there is always an increase in the entropy of the universe.
The entropy of the universe is increasing.
The total energy of the universe is constant, but the entropy is increasing.
Suniverse = ΔSsystem + ΔSsurroundings
It can be shown that the entropy change of the surroundings is equal to the heat transferred to the surroundings from the system divided by the Kelvin temperature
Second Law of Thermodynamics
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17. Remember that T is never negative

18. - - - Yes + + + No, Reverse + - ? At High temp. + - ? At Low temp.
DSsurr
DSuniv
Spontaneous?
DSsys - - -
Yes + + +
No, Reverse + - ?
At High temp. + - ?
At Low temp.

19. CONCEPT CHECK!
Describe the following as spontaneous/non-spontaneous/cannot tell, and explain.
A reaction that is:
Exothermic and becomes more positionally random
Spontaneous
Exothermic and becomes less positionally random
Cannot tell
Endothermic and becomes more positionally random
Endothermic and becomes less positionally random
Not spontaneous
Explain how temperature affects your answers.
a) Spontaneous (both driving forces are favorable). An example is the combustion of a hydrocarbon.
b) Cannot tell (exothermic is favorable, positional randomness is not). An example is the freezing of water, which becomes spontaneous as the temperature of water is decreased.
c) Cannot tell (positional randomness is favorable, endothermic is not). An example is the vaporization of water, which becomes spontaneous as the temperature of water is increased..
d) Not spontaneous (both driving forces are unfavorable).
Questions "a" and "d" are not affected by temperature. Choices "b" and "c" are explained above.

20. ΔSsurr The sign of ΔSsurr depends on the direction of the heat flow.
The magnitude of ΔSsurr depends on the temperature.
ΔSsurr
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21. Gibb's Free Energy G=H-TS Never used this way.
DG=DH-TDS at constant temperature
Divide by -T
-DG/T = -DH/T-DS
-DG/T = DSsurr + DS
-DG/T = DSuniv
If DG is negative at constant T and P, the Process is spontaneous.
Gibb's Free Energy

22. At constant temperature and pressure, a change can only be spontaneous if it is accompanied by a decrease in the free energy of the system
General comments can be made about the possibility of a spontaneous process and the signs of the enthalpy and entropy changes

23. + - + + - - - + DG=DH-TDS DH DS Spontaneous? At all Temperatures
At high temperatures,
“entropy driven” + +
At low temperatures,
“enthalpy driven” - -
Not at any temperature,
Reverse is spontaneous - +

24. Summary of the effects of the sign of the enthalpy and entropy change on the spontaneity of a process.

25. A number of standard entropies have been tabulated
The Third Law of Thermodynamics makes the experimental determination of absolute entropies possible:
At absolute zero the entropy of a perfectly ordered crystalline substance is zero
S = 0 at T = 0 K (perfect crystal)
The entropy of 1 mol of a substance at a temperature of 298 K (25 oC) and a pressure of 1 atm is called the standard entropy, So
A number of standard entropies have been tabulated

26. Some of these are reported in
They can be used to calculate standard entropy changes for reactions
If the reaction under consideration corresponds to the formation of 1 mol of compound from its elements, the the calculated standard entropy change is called the standard entropy of formation,

27. Standard entropies of formation are not tabulated, so must be calculated when needed
Like for the entropy change, the standard free energy change is determined at 298 K and 1 atm
Some standard free energies of formation are tabulated in

28. This occurs when the free energy change is zero
A system that is neither spontaneous nor nonspontaneous is at equilibrium
This occurs when the free energy change is zero
Consider the equilibrium between ice and water at 0oC
The system will remain at equilibrium as long as no heat is added or removed
Both phases can exist together indefinitely

29. At equilibrium:
Given two of the three quantities (enthalpy change, entropy change, and temperature) the third can be calculated

30. Free energy change diagrams can be used to represent phase changes
The free energy diagram for the conversion of H2O(l) to H2O(s). At the left of each diagram the system consists entirely of H2O(l). At the right is H2O(s). The horizontal axis represents the extent of conversion between H2O(l) and H2O(s).

31. Free energy changes for reactions are usually more complex
The minimum on the curve indicates the composition of the reaction mixture at equilibrium. Because the standard free energy change is positive, the position of the equilibrium lies close to the reactants.

32. Free energy curve for a reaction having a negative standard free energy change. The reaction proceeds to equilibrium by moving “down-hill” from left to right. The position of the equilibrium lies close to the products because the standard free energy change for the reaction is negative.

33. The free energy change at nonstandard conditions is related to the change at standard condition by an expression that includes the reaction quotient Q
This important expression allows for any concentration or pressure

34. This provides a way to connect standard free energy changes for a reaction and the equilibrium constant
Thus, equilibrium constants at various temperatures can be quickly and efficiently be estimated

35. While the tables of thermodynamic quantities are extensive, they are incomplete
Reasonable estimates of the heat of reaction, for example, can be made from atomization energies and average bond energies
Atomization energies can be found in
A number of average bond energies are reported in

36. Free energy And Work Free energy is that energy free to do work.
The maximum amount of work possible at a given temperature and pressure.
Never really achieved because some of the free energy is changed to heat during a change, so it can’t be used to do work.
Free energy And Work