THERMODYNAMICS AND EQUILIBRIUM
•A spontaneous reaction (or favourable change) is a change that
has a natural tendency to happen under certain conditions.
•Eg. The oxidation of iron (rust)
•First law of Thermodynamics: The amount of energy in the
universe is constant. Energy can be neither created nor
destroyed, but can be transferred from one object or place to
another, or transformed from one form to another.
•In a chemical reaction, changes in the potential energy of
reactants and products result in the transfer of energy, as heat,
from the system to the surroundings (exothermic change) or the
surroundings to the chemical system (endothermic change). The
total energy of the system remains constant.
Enthalpy Changes and Spontaneity
reactant
ΔH (-)
product
exothermic
(fwd direction usually favoured)
Enthalpy H
Enthalpy H
• When products have less enthalpy than reactants,
the reaction releases energy, therefore Reaction is exothermic.
product
ΔH (+)
reactant
endothermic
(rev direction usually favoured)
• In general, exothermic reactions tend to proceed
spontaneously and endothermic reactions tend to be
non-spontaneous, occurring only when energy is
supplied.
• However, some endothermic reactions ARE
spontaneous even though the products are less
energetically stable than the reactants. Why??
• The answer is because of Entropy, S. Entropy is the
measure of the randomness or disorder in a system.
Second law of Thermodynamics:
Total entropy of the universe is increasing.
• Favourable changes involve an increase in the total
entropy
In chemical reactions, entropy increases when:
1. The physical state of a system changes
(slaqg)
2. Fewer moles of reactant molecules form a greater
number of moles of product molecules.
(Eg 2 NH3(g) N2(g) + 3H2(g)) (less molmore mol)
 volume of container increases (gases only), temp
increases, complex substances are broken down into
simpler substances.
Ex)
Determine whether the entropy, S, is increasing (+) or
decreasing (-)
a)
H2O(s) → H2O(l)
change of state (s)  (l),
increase in entropy S = +
2 NH3(g) → N2(g) + 3 H2(g)
2 mol  4 mol , increase in moles of gas,
increase in entropy S = +
C2H4(g) + H2(g) → C2H6(g)
2 mol  1 mol , decrease in moles of gas,
decrease in entropy S = H2O2(g) →H2O2(l)
change of state (g)  (l),
decrease in entropy S = -
b)
c)
d)
GIBBS FREE ENERGY, G
How do we know if a Reaction is spontaneous or nonspontaneous?
The spontaneity of any reaction can be predicted by
calculating Gibbs Free Energy - the energy available to do
work
G = H - T S
where G = standard Gibbs free energy
H = standard enthalpy change
T = temperature (Kelvins)
S = standard entropy change
G < 0
change is spontaneous
G > 0
change is non-spontaneous
-H and + S
(exothermic,  entropy or disorder)
Reaction spontaneous in the forward direction
+ H and - S
(endothermic,  entropy or more ordered)
Reaction spontaneous in the reverse direction
- H and - S
Fwd rxn favourable at low temperatures
+ H and + S
Fwd rxn favourable at high temperatures
Note:
 if G <0, the reaction is spontaneous in the forward
direction
 if G >0, the reaction is non-spontaneous in the forward
direction but IS spontaneous in the reverse direction
 if G = 0, there is no “preferred” direction. The reaction is
occurring in the forward and reverse direction at the same
rate, therefore the system is at EQUILIBRIUM!
Ex) For the following reaction, CaCO3(s)  CaO(s) + CO2(g)
a) Determine whether the reaction is spontaneous if it
occurs at 25C, and the value of H is determined to be
10.5 kJ and the S is 30 J/K.
b) Determine the temperature at which a reaction, whose
H = 126 kJ, and S = 84 J/K, will be spontaneous.
a)
b)
T = 25°C = 298 K
ΔH = 10.5 kJ = 10500 J
ΔS = 30 J/K
ΔG = ΔH - TΔS
= 10500 J – (298)(30 J/K)
= 1560 J
not spontaneous
ΔG = ΔH - TΔS
0 = 126000 J – T(84 J/K)
T(84 J/K)= 126000 J
T = 1500 K
spontaneous when T > 1500 K
HOMEWORK
WS “Equilibrium Worksheet”
WS “Thermodynamics and Equilibrium”
P 327 #2-5
p333 #3-5