1. Thermodynamics
Chapter 18

2. The First Law of Thermodynamics
q = heat w = work
E = q + w = heat + work = Energy
Everything that is not part of the system (sys) is the surroundings (surr),
and vice versa:
Euniv = Esys + Esurr
The total energy of the universe is constant and, therefore,
energy cannot be created or destroyed.

3. Thermodynamics
One of the main objectives of thermodynamics is to predict whether or not a reaction will occur when reactants are brought together under specific conditions.
A reaction that does occur under the specific set of conditions is called a spontaneous reaction.
The reverse of a spontaneous reaction cannot occur under the same set of conditions.
What kind of rule can we come up with to characterize
spontaneous processes?
Perhaps spontaneous processes decrease energy.

4. Spontaneous Physical and Chemical Processes
A waterfall runs downhill
A lump of sugar dissolves in a cup of coffee
At 1 atm, water freezes below 0 0C and ice melts above 0 0C
Heat flows from a hotter object to a colder object
A gas expands in an evacuated bulb
Iron exposed to oxygen and water forms rust
spontaneous
nonspontaneous
18.2

5. Sign of H Cannot Predict Spontaneous Change
All combustion reactions are spontaneous and exothermic:
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O(g)
Horxn = -802 kJ
Iron rusts spontaneously and exothermically:
2 Fe(s) O2 (g) Fe2O3 (s) 3 2
Horxn = -826 kJ
Ionic compounds form spontaneously from their elements with a large
release of heat: 1 2
Horxn = -411 kJ
Na(s) + Cl2 (g) NaCl(s)
At ordinary pressure, water freezes below 0°C but melts above 0°C.
Both processes are spontaneous, but the first is exothermic and the
second endothermic.
H2O(l) H2O(s) Horxn = kJ
(exothermic; spontaneous at T < 0oC)
H2O(s) H2O(l) Horxn = kJ
(endothermic; spontaneous at T > 0oC)

6. A Spontaneous, Endothermic Chemical Reaction

7. EH Assignment
Due
Unit 21
Minimum score
Sec 1 Enthalpy Change
2 points extra credit
Sec 2 Entropy Change 60
Sec 3 Free Energy Change
Sec 4 Spontaneity of Rxns 90
Sec 5 Free Energy and Conc.

8. Does a decrease in enthalpy mean a reaction proceeds spontaneously?
Spontaneous reactions
CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH0 = kJ
H+ (aq) + OH- (aq) H2O (l) DH0 = kJ
H2O (s) H2O (l) DH0= 6.01 kJ
NH4NO3 (s) NH4+(aq) + NO3- (aq) DH0 = 25 kJ
H2O
18.2

9. No energy change! Probability factor.
spontaneous
nonspontaneous
No energy change! Probability factor.
18.2

10. Entropy and the Second Law of Thermodynamics
Entropy (S)–Entropy is a measure of the randomness or
disorder of a system
A system with relatively few equivalent ways to arrange its
components, such as a crystalline solid or a deck of cards in a
specific sequence, has relatively small disorder and low entropy.
A system with many equivalent ways to arrange its components,
such as a gas or a shuffled deck of cards, has relatively large
disorder and high entropy.
Sdisorder > Sorder
Ssys = Sfinal - Sinitial

11. Entropy (S) is a measure of the randomness or disorder of a system.
DS = Sf - Si
If the change from initial to final results in an increase in randomness
Sf > Si
DS > 0
For any substance, the solid state is more ordered than the liquid state and the liquid state is more ordered than gas state
Ssolid < Sliquid << Sgas
H2O (s) H2O (l)
DS > 0
18.2

12. Processes that lead to an increase in entropy (DS > 0)
18.2

13. Predicting Relative Entropy Values
Problem: Choose the member with the higher entropy in each of the
following pairs, and justify your choice.
(a) 1 mol NaCl(s) or 1 mol NaCl(aq)
(b) 1 mol O2 and 2 mol H2 or 1 mol H2O
(c) 1 mol H2O(s) or 1 mol H2O(g)
(d) Lipton’s noodle soup at 24oC or at 95oC
Is the entropy change greater or less than zero for:
(a) freezing ethanol
(b) evaporating a beaker of bromine
(c) dissolving urea in water
(d) cooling N2 gas

14. (a) Condensing water vapor
How does the entropy of a system change for each of the following processes?
(a) Condensing water vapor
Randomness decreases
Entropy decreases (DS < 0)
(b) Forming sucrose crystals from a supersaturated solution
Randomness decreases
Entropy decreases (DS < 0)
(c) Heating hydrogen gas from 600C to 800C
Randomness increases
Entropy increases (DS > 0)
(d) Subliming dry ice
Randomness increases
Entropy increases (DS > 0)
18.2

15. The Second Law of Thermodynamics - the
Entropy and the Second Law of Thermodynamics
The Second Law of Thermodynamics - the
entropy of the universe (system + surroundings)
always increases in a spontaneous process and
remains unchanged in an equilibrium process.
Any process in which the entropy of the universe
would decrease is forbidden.
DSuniv = DSsys + DSsurr > spontaneous
DSuniv = DSsys + DSsurr < forbidden

16. Entropy Changes in the System (DSsys)
The standard entropy of reaction (DS0 ) is the entropy change for a reaction carried out at 1 atm and 250C.
rxn
aA + bB cC + dD
DS0
rxn
dS0(D)
cS0(C) = [ + ] -
bS0(B)
aS0(A)
DS0
rxn
nS0(products) = S
mS0(reactants) -
What is the standard entropy change for the following reaction at 250C? 2CO (g) + O2 (g) CO2 (g)
S0(CO) = J/K•mol
S0(CO2) = J/K•mol
S0(O2) = J/K•mol
DS0
rxn
= 2 x S0(CO2) – [2 x S0(CO) + S0 (O2)]
DS0
rxn
= – [ ] = J/K•mol
18.3

17. Entropy Changes in the System (DSsys)
When gases are produced (or consumed)
If a reaction produces more gas molecules than it consumes, DS0 > 0.
If the total number of gas molecules diminishes, DS0 < 0.
If there is no net change in the total number of gas molecules, then DS0 may be positive or negative BUT DS0 will be a small number.
What is the sign of the entropy change for the following reaction? 2Zn (s) + O2 (g) ZnO (s)
The total number of gas molecules goes down, DS is negative.
18.3

18. Entropy Changes in the Surroundings (DSsurr)
Exothermic Process
DSsurr > 0
Endothermic Process
DSsurr < 0
18.3

19. Third Law of Thermodynamics
The entropy of a perfect crystalline substance is zero at the absolute zero of temperature.
18.3

20. For a constant-temperature process:
Gibbs Free Energy
Spontaneous process:
DSuniv = DSsys + DSsurr > 0
Equilibrium process:
DSuniv = DSsys + DSsurr = 0
For a constant-temperature process:
Gibbs free energy (G)
DG = DHsys -TDSsys
DG < The reaction is spontaneous in the forward direction.
DG > The reaction is nonspontaneous as written. The
reaction is spontaneous in the reverse direction.
DG = The reaction is at equilibrium.
18.4

21. Entropy and Free Energy
Gibbs free energy–This is a function that combines the systems
enthalpy and entropy:
G = H - TS
The change in free energy of a system at constant temperature and
pressure is given by the Gibbs equation:
Gsys = Hsys T Ssys
The Second Law can be stated in terms of G
for the system.
Suniv > 0 for a spontaneous process G < 0 for a spontaneous process
Suniv < 0 for a nonspontaneous process G > 0 for a nonspontaneous process
Suniv = 0 for a process at equilibrium G = 0 for a process at equilibrium

22. Reaction Spontaneity and the Signs of Ho, So, and Go
Ho So T So Go Description
Spontaneous at all T
Nonspontaneous at all T
or Spontaneous at higher T;
Nonspontaneous at lower T
or Spontaneous at lower T;
Nonspontaneous at higher T
An endothermic reaction can be spontaneous only if
there is an entropy increase (an increase in disorder).

23. Effect of Temperature on Reaction Spontaneity
The temperature at which a reaction occurs influences the magnitude of
the T S term. By scrutinizing the signs of H and S, we can predict
the effect of temperature on the sign of G and thus on the spontaneity
of a process at any temperature.
Temperature-independent cases (opposite signs)
1. Reaction is spontaneous at all temperatures: Ho < 0, So > 0
2. Reaction is nonspontaneous at all temperatures: Ho > 0, So < 0
2 H2O2 (l) H2O(l) + O2 (g) Ho = -196 kJ and So = 125 J/K
3 O2 (g) O3 (g) Ho = 286 kJ and So = J/K
Temperature-dependent cases (same signs)
3. Reaction is spontaneous at higher temperature: Ho > 0 and So > 0
4. Reaction is spontaneous at lower temperature: Ho < 0 and So < 0
2 N2O(g) + O2 (g) NO(g) Ho = kJ and So = J/K
2 Na(s) + Cl2 (g) NaCl(s) Ho = kJ and So = J/K

24. DG0 of any element in its stable form is zero.
The standard free-energy of reaction (DG0 ) is the free-energy change for a reaction when it occurs under standard-state conditions.
rxn
aA + bB cC + dD
DG0
rxn
dDG0 (D) f
cDG0 (C) = [ + ] -
bDG0 (B)
aDG0 (A)
DG0
rxn
nDG0 (products) f = S
mDG0 (reactants) -
Standard free energy of formation (DG0) is the free-energy change that occurs when 1 mole of the compound is formed from its elements in their standard states. f
DG0 of any element in its stable form is zero. f
18.4

25. 2C6H6 (l) + 15O2 (g) 12CO2 (g) + 6H2O (l)
What is the standard free-energy change for the following reaction at 25 0C?
2C6H6 (l) + 15O2 (g) CO2 (g) + 6H2O (l)
DG0
rxn
nDG0 (products) f = S
mDG0 (reactants) -
DG0
rxn
6DG0 (H2O) f
12DG0 (CO2) = [ + ] -
2DG0 (C6H6)
DG0
rxn =
[ 12x– x–237.2 ] – [ 2x124.5 ] = kJ
Is the reaction spontaneous at 25 0C?
DG0 = kJ
< 0
spontaneous
18.4

26. There is an important relationship between
the Gibbs Free Energy change and the
equilibrium constant.
Go = -RT ln K
Homework - pages
#1, 2, 5, 6, 7, 10, 14, 15, 16, 19, 22, 44, 66

27. The Relationship Between Go and K at 25oC
Go (kJ) K Significance
x Essentially no forward reaction;
x reverse reaction goes to
x completion.
x 10 -2
x 10 -1
Forward and reverse reactions
proceed to same extent.
x 101
x 108
x Forward reaction goes to
x completion; essentially no
reverse reaction.

28. DG = DH - TDS
18.4

29. Temperature and Spontaneity of Chemical Reactions
CaCO3 (s) CaO (s) + CO2 (g)
DH0 = kJ
DS0 = J/K
DG0 = DH0 – TDS0
At 25 0C, DG0 = kJ
DG0 = 0 at 835 0C
18.4

30. DG0 = - RT lnK
18.4