A reaction is said to be spontaneous if it proceeds without any
external energy being added. Some basic examples include:
The above examples show three important points
- Ice melting at 250C: H2O(s) -> H2O(l)
- Hydrogen burning: 2H2(g) + O2(g) ->
To determine if a
reaction is spontaneous, you must compute the change in the Gibbs free energy
for the reaction.
- Spontaneity depends on temperature: ice melts at 250C, but not
below 00C, for example.
- Spontaneous does not mean "fast": hydrogen and oxygen have a rate constant
of ~0 at room temperature, but the reaction is still spontaneous at that
- The reverse of a spontaneous reaction is not spontaneous: water does not
turn into hydrogen and oxygen unless you add energy.