1. Chemistry: The Central Science
Fourteenth Edition
Chapter 19
Chemical Thermodynamics
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2. First Law of Thermodynamics
You will recall from Chapter 5 that energy cannot be created or destroyed.
Therefore, the total energy of the universe is a constant.
Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa.

3. Enthalpy/Entropy
Enthalpy is the heat absorbed by a system during a constant-pressure process.
Entropy is a measure of the randomness in a system.
Both play a role in determining whether a process is spontaneous.

4. Spontaneous Processes
Spontaneous processes proceed without any outside assistance.
Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.
Don’t confuse spontaneous with fast. There are fast and slow reactions that are spontaneous.
Don’t confuse nonspontaneous with impossible. Energy can be used to make some reactions spontaneous that otherwise would not be.

5. Experimental Factors Affect Spontaneous Processes
Temperature and pressure can affect spontaneity.
An example of how temperature affects spontaneity is ice melting or freezing.

6. Reversible and Irreversible Processes (1 of 2)
Reversible process: The system changes so that the system and surroundings can be returned to the original state by exactly reversing the process. This maximizes work done by a system on the surroundings.

7. Reversible and Irreversible Processes (2 of 2)
Irreversible processes cannot be undone by exactly reversing the change to the system or cannot have the process exactly followed in reverse. Also, any spontaneous process is irreversible!

8. Entropy
Entropy can be thought of as a measure of the randomness of a system.
It is a state function:
It can be found by heat transfer from surroundings at a given temperature:

9. Second Law of Thermodynamics
The entropy of the universe increases in any spontaneous processes.
This results in the following relationships:

10. Entropy on the Molecular Scale (1 of 2)
Entropy on the molecular level (Boltzmann)
Gas molecule expansion: Two molecules start on one side above. What is the likelihood they end up there?
If one mole is used?

11. Entropy on the Molecular Scale (2 of 2)
Gases spontaneously expand to fill the volume given.
Most probable arrangement: approximately equal molecules in each side.

12. Statistical Thermodynamics
Thermodynamics looks at bulk properties of substances (the big picture).
We have seen what happens on the molecular scale.
How do they relate?
We use statistics (probability) to relate them. The field is called statistical thermodynamics.
Microstate: A single possible arrangement of position and kinetic energy of molecules

13. Boltzmann’s Use of Microstates
Because there are so many possible microstates, we can’t look at every picture.
W represents the number of microstates.
Entropy is a measure of how many microstates are associated with a particular macroscopic state.
The connection between the number of microstates and the entropy of the system is:

14. Entropy Change
Since entropy is a state function, the final value minus the initial value will give the overall change.
In this case, an increase in the number of microstates results in a positive entropy change (more disorder).

15. Effect of Volume and Temperature Change on the System
If we increase volume, there are more positions possible for the molecules. This results in more microstates, so increased entropy.
If we increase temperature, the average kinetic energy increases. This results in a greater distribution of molecular speeds. Therefore, there are more possible kinetic energy values, resulting in more microstates, increasing entropy.

16. Molecular Motions Molecules exhibit several types of motion.
Translational: Movement of the entire molecule from one place to another
Vibrational: Periodic motion of atoms within a molecule
Rotational: Rotation of the molecule about an axis
Note: More atoms means more microstates (more possible molecular motions).

17. Entropy on the Molecular Scale
The number of microstates and, therefore, the entropy tend to increase with increases in
temperature.
volume.
the number of independently moving molecules.

18. Entropy and Physical States (1 of 2)
Entropy increases with the freedom of motion of molecules.
S(g) > S(l) > S(s)
Entropy of a system increases for processes where:
gases form from either solids or liquids.
liquids or solutions form from solids.
the number of gas molecules increases during a chemical reaction.

19. Entropy and Physical States (2 of 2)

20. Third Law of Thermodynamics
The entropy of a pure crystalline substance at absolute zero is 0.
Consider all atoms or molecules in the perfect lattice at 0 K; there will only be one microstate.

21. Standard Entropies (1 of 3)
The reference for entropy is 0 K, so the values for elements are not 0 J/m o l K at 298 K.
Standard molar enthalpy for gases are generally greater than liquids and solids. (Be careful of size!)
Standard entropies increase with molar mass.
Standard entropies increase with number of atoms in a formula. (Remember: More microstates!)

22. Standard Entropies (2 of 3)
Table 19.1 Standard Molar Entropies of Selected Substances at 298 K
Substance
S naught in joules per mole Kelvin
H 2gas.
130.6
N 2, gas
191.5
O 2, gas
205.0
H 2 O, gas
188.8
N H 3, gas
192.5
C H 3 O H, gas
237.6
C 6 H 6, gas
269.2
H 2 O, liquid
69.9
C H 3 O H, liquid
126.8

23. Standard Entropies (3 of 3)
[Table 19.1 continued]
Substance
S naught in joules per mole Kelvin
C 6 H 6, liquid
172.8
L I, solid
29.1
N ay, solid
51.4
K, solid
64.7
F e, solid.
27.23
F e C l 3, solid
142.3
N ay C l, solid
72.3

24. Entropy Changes
Entropy changes for a reaction can be calculated in a manner analogous to that by which H is calculated:
where n and m are the coefficients in the balanced chemical equation.

25. Entropy Changes in Surroundings
Heat that flows into or out of the system changes the entropy of the surroundings.
For an isothermal process
At constant pressure,
is simply H° for the
system.

26. Entropy Change in the Universe
The universe is composed of the system and the surroundings.
Therefore,
For spontaneous processes

27. Total Entropy and Spontaneity
Substitute for the entropy of the surroundings:
Multiply by −T:
Rearrange:
Call

28. Gibbs Free Energy
If G is negative, the forward reaction is spontaneous.
If G is 0, the system is at equilibrium.
If G is positive, the reaction is nonspontaneous, but spontaneous in the reverse direction.

29. Standard Free Energy Changes (1 of 2)
Analogous to standard enthalpies of formation are
standard free energies of formation,
where n and m are the stoichiometric coefficients.

30. Standard Free Energy Changes (2 of 2)
Table 19.2 Conventions Used in Establishing Standard Free Energies
State of Matter
Standard State
Solid
Pure solid
Liquid
Pure liquid
Gas
1 atm pressure
Solution
1 M concentration
Element
for element in standard state
Delta G naught of f = 0

31. Free Energy Changes (1 of 2)
Table 19.3 How Signs of ΔH and ΔS Affect Reaction Spontaneity
Delta H
delta S
negative T delta S
delta G = delta H minus T delta S
Reaction Characteristics
Example − +
Spontaneous at all temperatures
2 O 3, gas, yields 3 O 2, gas.
Nonspontancous at all temperatures
3 O 2, gas, yields 2 O 3, gas
+ or −
Spontaneous at low T; nonspontaneous at high T
H 2 O, liquid, yields H 2 O, solid
Spontaneous at high T; nonspontaneous at low T
H 2 O, solid, yields H 2 O, liquid.

32. Free Energy Changes (2 of 2)
How does ΔG change with temperature?
Since reactions are spontaneous if ΔG < 0, the sign of enthalpy and entropy and the magnitude of the temperature matters to spontaneity.

33. Free Energy and Equilibrium (1 of 2)
Under any conditions, standard or nonstandard, the free energy change can be found this way:
(Under standard conditions, concentrations are 1 M, so Q = 1 and ln Q = 0; the last term drops out.)

34. Free Energy and Equilibrium (2 of 2)
At equilibrium, Q = K, and G = 0.
The equation becomes
Rearranging, this becomes or

35. Coupling Reactions (1 of 2)
Many natural processes would NOT occur alone!
Nature couples them to offer the favorable energy conditions to cause something to occur, which is nonspontaneous alone.

36. Coupling Reactions (2 of 2)

37. Copyright