Spontaneity, Enthalpy, and
Entropy
Spontenaity
Learning Goal: Explain the role of energy
and entropy in determining the
spontaneity of a reaction.
Spontaneous reaction: a reaction that, once
begun, occurs with no outside
intervention
Examples:
(1)  Lighting a Bunsen burner
(2)  Putting a piece of zinc metal into HCl
(3)  Iron rusting
non-spontaneous reaction: a reaction that
does not occur unless there is a
continuous input of energy
Examples:
(1)  In order to decompose water, energy
must continuously be supplied
(2)  In order to boil water, energy must
continuously be supplied
It is possible to predict if a reaction is
spontaneous or not.
∆G = ∆H – T∆S
(T in kelvins)
If ∆G is negative, a reaction is spontaneous.
If ∆G is positive, a reaction is not spontaneous.
Example #1
If ∆H = -91.8 kJ and ∆S = -0.197 kJ/K at
25°C, is the reaction spontaneous?
Example #2
If ∆H = 145 kJ and ∆S= 0.377 kJ/K at 25°C,
is the reaction spontaneous?