Chapter 19
Chemical Thermodynamics
HW: 13 23 39 43 49 57 67 73 77
19.1 - Spontaneous Processes
• Spontaneous processes are
those that can proceed
without any outside
intervention.
• The gas in vessel B will
spontaneously effuse into
vessel A, but once the gas is
in both vessels, it will not
spontaneously effuse
Spontaneous Processes
Processes that are
spontaneous in one
direction are
nonspontaneous in
the reverse direction.
Spontaneous Processes
• Processes that are spontaneous at one temperature
may be nonspontaneous at other temperatures.
• Above 0C it is spontaneous for ice to melt.
• Below 0C the reverse process is spontaneous.
Enthalpy-based Spontaneity
• OFTEN, spontaneous reactions are
exothermic (DH is -)
• NOT ALWAYS, though!
• Ex – Ice melting
– It is ENDOTHERMIC, but occurs spontaneously
when T>0oC at standard pressure
19.2 - Entropy-based Spontaneity
• Entropy (S) is a term coined by Rudolph
Clausius in the 19th century.
Entropy
• Entropy can be thought of as a measure of
the randomness of a system.
• It is related to the various modes of motion
in molecules.
Entropy
• Like total energy (E) and enthalpy (H)
entropy is a state function.
• Therefore,
DS = Sfinal  Sinitial
• The more disorder the higher the S value.
• If DS is +, the reaction increases in disorder
• If DS is -, the reaction decreases in disorder
Solutions
Generally, when a
solid is dissolved in
a solvent, entropy
increases.
Entropy Changes
• In general, entropy
increases when
– Gases are formed from
liquids and solids.
– Liquids or solutions are
formed from solids.
– The number of gas
molecules increases.
– The number of moles
increases.
Second Law of Thermodynamics
The second law of thermodynamics states that
the entropy of the universe increases for
spontaneous processes, and the entropy of
the universe does not change for reversible
processes.
Second Law of Thermodynamics
In other words:
For equilibrium processes:
DSuniv = DSsystem + DSsurroundings = 0
For spontaneous processes:
DSuniv = DSsystem + DSsurroundings > 0
Second Law of Thermodynamics
These last truths mean that as a result of all
spontaneous processes the entropy of the
universe increases.
19.4 - Standard Entropies
• These are molar entropy
values of substances in
their standard states.
• Standard entropies tend
to increase with increasing
molar mass.
• Appendix in the back of
the book.
Standard Entropies
Larger and more complex molecules have
greater entropies.
Entropy Changes
Entropy changes for a reaction can be
estimated in a manner analogous to that by
which DH is estimated:
DS° = nDS°(products) - mDS°(reactants)
where n and m are the coefficients in the
balanced chemical equation.
Example:
3 O2 (g) -> 2 O3 (g)
Does this make sense??
Third Law of Thermodynamics
The entropy of a pure crystalline
substance at absolute zero is 0.
19.5 - Gibbs Free Energy
• Gibbs free energy, DG.
• DG is negative, a process is spontaneous.
• DG is positive, it is not spontaneous (it is
spontaneous in the reverse direction)
• DG = 0, it is at equilibrium
Standard Free Energy Changes
Standard free energies of formation, DGf is
the free energy change when 1 mole of
compound is synthe
DG = nDG(products)  mDG(reactants)
f
f
where n and m are the stoichiometric
coefficients.
19.6 - Free Energy Changes
At temperatures other than 25°C,
DG° = DH  TDS
How does DG change with temperature?
Free Energy and Temperature
• There are two parts to the free energy
equation:
– DH— the enthalpy term
– TDS — the entropy term
• The temperature dependence of free
energy, then comes from the entropy term.
Free Energy and Temperature
Temperature and Chemical Reactions
• It is possible to calculate the temperature at
which a reaction becomes favored.
CaCO3(s)
CaO(s) + CO2(g)
DH = +177.8 kJ
Favored or not favored with respect to…?
DS = +160.5 J/K
At what temp does the reaction become
favored???
During Phase Changes
DG = 0 = DH-TDS
So…
DS = +DH / T where DH is the molar heat of
vaporization / fusion / etc.
Before / After phase changes, heat is found
using q = msDT
19.7 - Free Energy and Equilibrium
Under any conditions, standard or
nonstandard, the free energy change can
be found this way:
DG = DG + RT lnQ
(Under standard conditions, all concentrations are 1 M, so Q
= 1 and lnQ = 0; the last term drops out.)
Free Energy and Equilibrium
• At equilibrium, Q = K, and DG = 0.
• The equation becomes
0 = DG + RT lnK
• Rearranging, this becomes
DG = RT lnK
or,
K = eDG/RT
Free Energy and Equilibrium
K
ln K
DG
Favored species
>1
+
-
Products
=1
0
0
Both
<1
-
+
Reactants