Chemical Thermodynamics
The concept of chemical thermodynamics deals with
how the enthalpy change and entropy change of a
chemical reaction are related. Some reactions occur
automatically at a certain temperature and pressure.
We call these reaction spontaneous. Spontaneous
reactions are not spontaneous in reverse.
Chemical Thermodynamics
• Spontaneous Reactions – Proceed on their own
without any assistance.
o Ice melting (when T > 0°C).
o The rusting of iron.
o Which one of these is spontaneous?
•
•
•
Water gets warmer if a hot object is placed into it.
The decomposition of water into H2 and O2 at room
temperature.
The sublimation of dry ice at -100°C. (The freezing point
of dry ice is -78°C.
Chemical Thermodynamics
• Entropy – A measure of the randomness of a
system.
o ΔS = Sfinal - Sinitial (The entropy of a system depends
only on the initial and final states of the system.)
o For an isothermal process: ΔS =
q reverse
T
Chemical Thermodynamics
• What is the entropy change of 50.0 g of liquid
mercury freezing? The freezing point of Hg = 38.9°C and ΔHfusion = 2.29 kJ/mol.
Chemical Thermodynamics
• Second Law of Thermodynamics –
• Spontaneous reactions, ones that are irreversible,
always lead to an increase in the entropy of the system
(ΔS = +).
• In other words, processes that occur naturally on their
own always lead to more ‘disorder’.
Chemical Thermodynamics
• Entropy and Phase Changes –
• Predict whether each of the following phase changes
produced an increase or decrease in the entropy of the
system;
o CO2(s)  CO2 (g)
o CaO(s) + CO2(g)  CaCO3(s)
o HCl(g) + NH3(g)  NH4Cl(s)
o 2SO2(g) + O2(g)  2SO3(g)
Chemical Thermodynamics
• Third Law of Thermodynamics –
• The entropy of a pure crystalline solid at absolute zero
is zero.
• At absolute zero, all molecular motion, rotational and
translational, would cease.
Chemical Thermodynamics
• Entropy Changes in Chemical Reactions
• Standard molar entropies (ΔS°) – The value of the
molar entropy of a substance at 298 K and 1 atm. They
are experimentally determined.
•
•
•
•
The ΔS° for an element in their free states is not zero.
The ΔS° for gases are higher than the ΔS° of liquids or solids
of the same element.
ΔS° usually increases with increasing molar mass.
ΔS° usually increases with increasing number of atoms in a a
compound.
Chemical Thermodynamics
• Entropy Changes in Chemical Reactions
ΔS° = Σ nΔS° (products) - Σ mΔS° (reactants)
• The standard entropy change of a chemical reaction is
equal to the difference of the the standard entropy
change of the products minus the standard entropy
change of the reactants.
Chemical Thermodynamics
• Using the standard entropies in appendix C of your
textbook, calculate the standard enthalpy change,
(ΔS°), for the following reaction at 298K;
Al2O3(s) + 3H2(g)  2Al(s) + 3H2O(g)
Chemical Thermodynamics
• Gibbs Free Energy
• The problem • Some endothermic reactions are spontaneous while
others are not.
• Some exothermic reactions are spontaneous
reactions while others are not.
• The entropy and enthalpy of a
chemical
• Reaction determine its spontaneity.
• So how can we tell if a reaction
will be spontaneous or not?
Chemical Thermodynamics
• Gibbs Free Energy (ΔG) – The amount of useable
energy either released or absorbed in a chemical
reaction.
ΔG = ΔH – TΔS
• If ΔG > 0, reaction is not spontaneous
• If ΔG = 0, reaction is at equilibrium
• If ΔG < 0, reaction is spontaneous
Chemical Thermodynamics
• Calculate the standard free energy change for the
formation of NO(g) from N2(g) and O2(g) at 298K.
2NO(g)  N2(g) + O2(g)
Given that ΔH° = 180.7 kJ and ΔS° = 24.7 J/K. Is the
reaction spontaneous under these conditions?
Chemical Thermodynamics
• A particular reaction has ΔH° = 24.6 kJ and ΔS° =
132 J/K at 298 K. Calculate ΔG° to determine the
spontaneity of the chemical reaction.
Chemical Thermodynamics
• Another way to calculate the ΔG° of a chemical
reaction:
ΔG° = Σ nΔGf° (products) - Σ mΔGf° (reactants)
The change in the free energy of a chemical
reaction is equal to the sum of the change of the
sum of the free energy of the products minus the
sum of the free energy of the reactants.
Chemical Thermodynamics
• Calculate the standard free energy change for the
following chemical reaction at 298 K:
P4(g) + 6Cl2(g)  4PCl3(g)
Chemical Thermodynamics
• What is the standard free energy change for the
reverse reaction at 298 K:
P4(g) + 6Cl2(g)  4PCl3(g)
Chemical Thermodynamics
• Free Energy and Temperature
ΔH
ΔS
-TΔS
ΔG
-
+
-
-
+
-
+
+
-
-
+
- or +
+
+
-
- or +
Reaction Characteristics
Spontaneous at all
temperatures.
Nonspontaneous at all
temperatures.
Spontaneous at low T
Nonspontaneous at high T
Spontaneous at high T
Nonspontaneous at low T
Chemical Thermodynamics
• The Haber process for the production of ammonia
involves the equilibrium N2(g) + 3H2(g)  2NH3(g).
Assume that ΔH° and ΔS° for this reaction do not
change with temperature.
• a.) Predict the direction in which ΔG° changes with
increasing temperature.
Chemical Thermodynamics
• The Haber process for the production of ammonia
involves the equilibrium N2(g) + 3H2(g)  2NH3(g).
Assume that ΔH° and ΔS° for this reaction do not
change with temperature.
• b.) Calculate ΔG° for this reaction at 25°C and 500°C.
Chemical Thermodynamics
• Free Energy and The Equilibrium Constant
•
Most chemical reactions occur at temperatures other than 298K and pressure
other than 1 atm.
ΔG = ΔG° + R T ln Q
ΔG = Free Energy at nonstandard conditions
ΔG° = Standard Free Energy
R = 8.314 J/K.mol
T = Temperature (K)
Q = Reaction Quotient
Q is calculated by setting up an equilibrium expression and
plugging in either the concentration or pressures of the reactants
or products at the current conditions.
Chemical Thermodynamics
• Free Energy and The Equilibrium Constant
•
Most chemical reactions occur at temperatures other than 298K and pressure
other than 1 atm.
ΔG = ΔG° + R T ln Q
ΔG = Free Energy at nonstandard conditions
ΔG° = Standard Free Energy
R = 8.314 J/K.mol
T = Temperature (K)
Q = Reaction Quotient
Q is calculated by setting up an equilibrium expression and
plugging in either the concentration or pressures of the reactants
or products at the current conditions.
Chemical Thermodynamics
• Free Energy and The Equilibrium Constant
•
If a chemical reaction is at equilibrium, then we know that ΔG =
0.
0 = ΔG° + R T ln K
(at equilibrium)
or
ΔG° = - R T ln K
K = e-ΔG°/RT
(at equilibrium)
or
(on equation sheet)