Negative free energy is spontaneous due to the following reasons:
A spontaneous reaction is one that favors the formation of products in the circumstances in which it takes place. A rise in the system's entropy occurs during a combustion reaction. It is determined that spontaneous reactions are combustion processes by the interaction of decreasing energy and increasing entropy.
If the Gibbs Free Energy is negative, the reaction is random; if it is positive, the reaction is non-spontaneous.
When G: is negative, the process is random and will proceed forward according to the written instructions.
The process is not spontaneous as indicated, but it may happen at random to go the other way favours. Random reactions are followed by an increase in total entropy or chaos. You may determine qualitatively when a reaction will have an increase in entropy by observing phase transition, temperature change, a change in the number of moles involved, and an increase in length.