1. Chapter 17: Free Energy & Thermodynamics
CHE 124: General Chemistry II
Dr. Jerome Williams, Ph.D.
Saint Leo University

2. Overview First Law of Thermodynamics Factors Affecting Spontaneity
Enthalpy
Entropy

3. First Law of Thermodynamics
You can’t win!
First Law of Thermodynamics: Energy cannot be created or destroyed
the total energy of the universe cannot change
though you can transfer it from one place to another
DEuniverse = 0 = DEsystem + DEsurroundings
Tro: Chemistry: A Molecular Approach, 2/e 3 3

4. First Law of Thermodynamics
Conservation of Energy
For an exothermic reaction, “lost” heat from the system goes into the surroundings
Two ways energy is “lost” from a system
converted to heat, q
used to do work, w
Energy conservation requires that the energy change in the system equal the heat released + work done
DE = q + w
DE = DH + PDV
DE is a state function
internal energy change independent of how done
Tro: Chemistry: A Molecular Approach, 2/e 4 4

5. The Energy Tax You can’t break even!
To recharge a battery with 100 kJ of useful energy will require more than 100 kJ
Second Law of Thermodynamics
Every energy transition results in a “loss” of energy
an “Energy Tax” demanded by nature
and conversion of energy to heat which is “lost” by heating up the surroundings
Tro: Chemistry: A Molecular Approach, 2/e 5 5

6. fewer steps generally results in a lower total heat tax
Tro: Chemistry: A Molecular Approach, 2/e 6 6

7. Thermodynamics and Spontaneity
Thermodynamics predicts whether a process will occur under the given conditions
processes that will occur called spontaneous
nonspontaneous processes require energy input to go
Spontaneity is determined by comparing the chemical potential energy of the system before the reaction with the free energy of the system after the reaction
if the system after reaction has less potential energy than before the reaction, the reaction is thermodynamically favorable.
Spontaneity ≠ fast or slow
Tro: Chemistry: A Molecular Approach, 2/e 7 7

8. Comparing Potential Energy
The direction of spontaneity can be determined by comparing the potential energy of the system at the start and the end
Tro: Chemistry: A Molecular Approach, 2/e 8 8

9. Reversibility of Process
Any spontaneous process is irreversible because there is a net release of energy when it proceeds in that direction
it will proceed in only one direction
A reversible process will proceed back and forth between the two end conditions
any reversible process is at equilibrium
results in no change in free energy
If a process is spontaneous in one direction, it must be nonspontaneous in the opposite direction
Tro: Chemistry: A Molecular Approach, 2/e 9 9

10. Thermodynamics vs. Kinetics
Tro: Chemistry: A Molecular Approach, 2/e 10 10

11. Diamond → Graphite
Graphite is more stable than diamond, so the conversion of diamond into graphite is spontaneous – but don’t worry, it’s so slow that your ring won’t turn into pencil lead in your lifetime (or through many of your generations)
Tro: Chemistry: A Molecular Approach, 2/e 11 11

12. Spontaneous Processes
Spontaneous processes occur because they release energy from the system
Most spontaneous processes proceed from a system of higher potential energy to a system at lower potential energy
exothermic
But there are some spontaneous processes that proceed from a system of lower potential energy to a system at higher potential energy
endothermic
How can something absorb potential energy, yet have a net release of energy?
Tro: Chemistry: A Molecular Approach, 2/e 12

13. Melting Ice
Melting is an Endothermic process, yet ice will spontaneously melt above 0 °C.
When a solid melts, the particles have more freedom of movement.
More freedom of motion increases the randomness of the system. When systems become more random, energy is released. We call this energy, entropy
Tro: Chemistry: A Molecular Approach, 2/e 13

14. Factors Affecting Whether a Reaction Is Spontaneous
There are two factors that determine whether a reaction is spontaneous. They are the enthalpy change and the entropy change of the system
The enthalpy change, DH, is the difference in the sum of the internal energy and PV work energy of the reactants to the products
The entropy change, DS, is the difference in randomness of the reactants compared to the products
Tro: Chemistry: A Molecular Approach, 2/e 14 14

15. Enthalpy Change DH generally measured in kJ/mol
Stronger bonds = more stable molecules
Reaction is generally exothermic if bonds in the products are stronger than bonds in the reactants
exothermic = energy released, DH is negative
A reaction is generally endothermic if the bonds in the products are weaker than the bonds in the reactants
endothermic = energy absorbed, DH is positive
The enthalpy change is favorable for exothermic reactions and unfavorable for endothermic reactions
Tro: Chemistry: A Molecular Approach, 2/e 15 15

16. Entropy
Entropy is a thermodynamic function that increases as the number of energetically equivalent ways of arranging the components increases, S
S generally J/mol
S = k ln W
k = Boltzmann Constant = 1.38 x 10−23 J/K
W is the number of energetically equivalent ways a system can exist
unitless
Random systems require less energy than ordered systems
Tro: Chemistry: A Molecular Approach, 2/e 16 16

17. W
These are energetically equivalent states for the expansion of a gas.
It doesn’t matter, in terms of potential energy, whether the molecules are all in one flask, or evenly distributed
But one of these states is more probable than the other two
Tro: Chemistry: A Molecular Approach, 2/e 17 17

18. Macrostates → Microstates
These microstates all have the same macrostate
So there are six different particle arrangements that result in the same macrostate
This macrostate can be achieved through
several different arrangements of the particles
Tro: Chemistry: A Molecular Approach, 2/e 18 18

19. Macrostates and Probability
There is only one possible arrangement that gives State A and one that gives State B
There are six possible arrangements that give State C
The macrostate with the highest entropy also has the greatest dispersal of energy
Therefore State C has higher entropy than either State A or State B
There is six times the probability of having the State C macrostate than either State A or State B
Tro: Chemistry: A Molecular Approach, 2/e 19 19

20. Changes in Entropy, DS DS = Sfinal − Sinitial
Entropy change is favorable when the result is a more random system
DS is positive
Some changes that increase the entropy are
reactions whose products are in a more random state
solid more ordered than liquid more ordered than gas
reactions that have larger numbers of product molecules than reactant molecules
increase in temperature
solids dissociating into ions upon dissolving
Tro: Chemistry: A Molecular Approach, 2/e 20 20

21. Increases in Entropy
Tro: Chemistry: A Molecular Approach, 2/e 21 21

22. DS
For a process where the final condition is more random than the initial condition, DSsystem is positive and the entropy change is favorable for the process to be spontaneous
For a process where the final condition is more orderly than the initial condition, DSsystem is negative and the entropy change is unfavorable for the process to be spontaneous
DSsystem = DSreaction = Sn(S°products) − Sn(S°reactants)
Tro: Chemistry: A Molecular Approach, 2/e 22

23. Entropy Change in State Change
When materials change state, the number of macrostates it can have changes as well
the more degrees of freedom the molecules have, the more macrostates are possible
solids have fewer macrostates than liquids, which have fewer macrostates than gases
Tro: Chemistry: A Molecular Approach, 2/e 23 23

24. Entropy Change and State Change
Tro: Chemistry: A Molecular Approach, 2/e 24 24

25. Practice – Predict whether DSsystem is + or − for each of the following
A hot beaker burning your fingers
Water vapor condensing
Separation of oil and vinegar salad dressing
Dissolving sugar in tea
2 PbO2(s)  2 PbO(s) + O2(g)
2 NH3(g)  N2(g) + 3 H2(g)
Ag+(aq) + Cl−(aq)  AgCl(s)
DS is +
DS is −
DS is −
DS is +
DS is +
DS is +
DS is −
Tro: Chemistry: A Molecular Approach, 2/e 25