Chapter 19 (not 19.7!) – Chemical Thermodynamics
The BIG question: ____________________________________________________________________
Sample Problem 1:
Which of the following processes are spontaneous?
(a) a ball rolling up a hill
(b) the freezing of water at -3oC and 1 atm pressure
(c) the freezing of water at 3oC and 1 atm pressure
(d) the decomposition of carbon dioxide into carbon and oxygen at 25oC and 1 atm pressure
(e) the dissolving of NaCl in water
Processes that occur spontaneously in one direction are_________________________________________
The two driving forces for reactions are
(1) ___________________________________
(2) ________________________________________
Reactions move toward__________________________________________________________________
Sample Problem 2:
At 25oC, the ΔH value for the reaction
2 H2(g) + O2(g)  2 H2O(l)
is - 572 kJ
Can you conclude from the sign of ΔH that the reaction is spontaneous?
Entropy is a ___________________________________________________________________________
Entropy is a measure of the number of ways energy is distributed in a system. The more ways energy is
distributed in a system, the more random or the more disordered the system is said to be. The greater the
entropy of a system, the greater is its randomness or disorder.
Entropies are____________________________________________________________________________
The units of entropy are J/mole•K
We usually look at the change in entropy during a process:
ΔS = Sfinal - Sinitial ( _______________ )
ΔS > 0 (is positive) if ____________________________________________________________________
ΔS < 0 (is negative) if ___________________________________________________________________
2nd Law of Thermodynamics:
In any spontaneous process, the entropy of the universe always increases.
A process that involves an increase in order (ΔSsystem < 0) CAN be spontaneous provided it ____________
_____________________________________________________________________________________
An isolated system (no exchange of energy between the system and surroundings) always increases its entropy
when it undergoes a spontaneous change. For such a change, ΔSsurroundings = 0
Standard entropies (S°) have been determined for many substances, and these values are based on the 3rd law of
thermodynamics.
rd
3 Law of Thermodynamics
The entropy of a perfect, pure crystalline substance at 0K is ___________.
Qualitative "Rules" About Entropy:
1) Entropy increases as one goes from a solid to a liquid, or even more dramatically, a liquid to a gas
2) Entropy increases as the number of moles of gas increases
3) The Entropy of any material increases with increasing temperature
4) Entropy increases if a solid or liquid is dissolved in a solvent.
5) Entropy is higher for weakly bonded compounds than for compounds with very strong covalent bonds
6) Entropy increases as the number of particles in a system increases
7) Entropy increases as the number of electrons increases:
Entropy increases as the mass, # of atoms, # of heavier atoms, etc. of a molecule increases
Entropy changes dramatically at a ____________________________.
Sample Problem 3
For each of the following pairs, which has the greater entropy:
(a) CO2(s) or CO2(g)
(b) NH3(l) or NH3(g)
(c) a crystal of pure Mg at 0 K or a crystal at 200 K
Sample Problem 4
Predict whether ΔS is > 0 or < 0 for each of the following:
(a) sugar + water  sugar dissolved in water
(b) Na2SO4(s)  2 Na+(aq) + SO4-2(aq)
(c) 2 H2(g) + O2(g)  2 H2O(g)
Entropy is a state function and can be calculated for a reaction:
ΔS°rxn =
Sample Problem 5
Calculate the entropy change for the following at 25oC
2 SO2(g) + O2(g)  2 SO3(g)
given the following standard entropy values at 25oC: (from appendix C)
SO2(g)
248.5 J/K mole
O2(g)
205.0 J/K mole
SO3(g)
256.2 J/K mole
Exothermic processes (ΔH < 0) are _________________________________________________________
Processes that involve an increase in entropy (ΔS > 0) are________________________________________
Enthalpy and entropy are connected together by the thermodynamic state function called
free energy (G). The change in free energy for any process or reaction at constant temperature and
pressure is:
if ΔG < 0, _____________________________________________________________________________
if ΔG = 0, _____________________________________________________________________________
if ΔG > 0, _____________________________________________________________________________
ΔG is a state function and tabulated values exist for Gof for substances in appendix C
G°f for any element in its most stable state is zero.
G°rxn =
Sample Problem 6
Given the ΔG value for the following phase changes at 1 atm, predict whether each change is
spontaneous:
(a) at 283 K, ΔG = -250 J/mole for H2O(s)  H2O(l)
(b) at 273 K, ΔG = 0 J/mole for H2O(s)  H2O(l)
(c) at 263 K, ΔG = 210 J/mole for H2O(s)  H2O(l)
Sample Problem 7
Given:
G°f for C6H12O6(s) = -907.9 kJ/mole,
G°f for CO2(g) = -394.6 kJ/mole
G°f for H2O(l) = -237.2 kJ/mole
calculate G° for the oxidation of glucose
C6H12O6(s) + 6 O2(g)  6 CO2(g) + 6 H2O(l)
G°rxn = Σ n G°f (products) - Σ m G°f (reactants)
Effect of Temperature on Reaction Spontaneity
ΔH
ΔS
ΔG
Rxn Characteristics
+
+
+
+
Sample Problem 8
What is the temperature at which sodium chloride reversibly melts (solid and liquid states are in
equilibrium)? The enthalpy of melting is 30.3 kJ/mole and the entropy change upon melting is 28.2
J/mole K.





Sample Problem 9
At 298 K, G° = -190.5 kJ and H° = -184.6 kJ for the reaction
H2(g) + Cl2(g)  2 HCl(g)
Calculate the standard entropy S° for the reaction
Sample problem 10
Given the following data:
Substance
Hof (kJ/mol)
So (J/K mol)
Fe2O3(s)
-826
90.0
Fe(s)
0
27.0
O2(g)
0
205.0
o
Calculate G for the reaction: 4 Fe(s) + 3O2(g)  2 Fe2O3(s)