The release of free energy drives the spontaneous reaction.
Spontaneity can be determined using the change in Gibbs free energy (the thermodynamic potencial):
delta G=delta H – T*delta S
where delta H is the enthalpy and delta S is the entropy.
The direction (the sign) of delta G depends of the changes of enthalpy and entropy. If delta G is negative then the process is spontaneous.
In our case, both delta H and delta S are negative values, the process as said is spontaneous which means that it may proceed in the forward direction.