Spontaneous Reactions and Gibb’s Free Energy
Free Energy – the energy given off by a reaction that is available
to do work
spontaneous reaction – a reaction that proceeds to form the
products without outside intervention
 All spontaneous reactions release free energy
and are said to be exergonic. ( - G )
 Whether or not a reaction is spontaneous depends on its entropy
and enthalpy.
Entropy – measure of the disorder of a system + S is favorable
(S)
 All systems strive toward increased entropy 
prefer greater disorder
Enthalpy – heat content of a system
(H)
 All systems strive to release energy 
prefer to be exothermic
–
H is favorable
In order to determine the spontaneity of a reaction we use the equation for
Gibb’s free energy which is related to both the change in entropy (S) and the
change in enthalpy (H).
G = H –T S
G is the Gibb’s Free Energy
T is the temp. in Kelvin
H is the Enthalpy
S is the Entropy
If
G is negative, the reaction is spontaneous.
(Rxn is exergonic.)
If
G is positive, the reaction is not spontaneous.(Rxn is endergonic.)
If
G is zero, the reaction is at equilibrium.
A reaction that is not spontaneous, that is, a reaction with a + G
at a given temperature may become spontaneous if the temperature
is changed.
H
(favorable)
(unfavorable)
S
G
Temperature
spontaneous at
ALL temperatures
(favorable)
+ or –
spontaneous at
LOW temperatures
+ or –
spontaneous at
HIGH temperatures
(unfavorable)
spontaneous at
NO temperatures
1. State whether the values for H and S would be positive or negative
and favorable or unfavorable. Use these estimates to predict whether G
would be sponataneous at all temps, high temps, low temps, or no temps.
a. 2 H2 (g) + O2 (g)  2 H2O (g) + 242 kJ
Exothermic reaction so H is negative; favorable
Reactants more disordered so S is negative; unfavorable
Spontaneous at low temperatures
b. 6 CO2 (g) + 6 H2O (l) + heat  C6H12O6 (s) + 6 O2 (g)
Endothermic reaction so H is positive; unfavorable
Reactants more disordered so S is negative; unfavorable
Spontaneous at no temperatures
G is the Gibb’s Free Energy
G =
H is the Enthalpy
H–T S
T is the temp in Kelvin
S is the Entropy
Ho = Ho(products) – Ho (reactants)
So = So (products) – So (reactants)
2. Using the values for standard entropy and standard enthalpy, determine
whether the formation of Carbon Dioxide gas from the reaction of solid.
Carbon (graphite) with Oxygen gas is spontaneous (standard temp = 25oC)
C (s, graphite) + O2 (g)
Hfo (kJ/mol)
Substance
C (s, graphite)
0.0
O2 (g)
0.0
CO2 (g)
- 393.5

CO2 (g)
So (J/K mol)
5.69 =
kJ/mol
205 =
kJ/mol
214 =
kJ/mol
.0057
.205
.214
So = So (products) – So (reactants)
S CO2
SC
SO2
= .214 kJ/mol – (.0057 kJ/mol + .205 kJ/mol) = .003 kJ/mol
Ho = Ho(products) – Ho (reactants)
H CO2
HC
HO2
= –393.5kJ/mol – (0.0 kJ/mol + 0.0kJ/mol) =–393.5 kJ/mol
T = 25oC + 273 = 298 K
G =
H –T S = – 393.5 kJ/mol – ( 298 K x .003 kJ/mol )
= – 394.4 kJ/mol
Gibb’s free energy is negative, therefore the reaction is spontaneous.