Whether it is a crystal like metalic iron or a complex organic molicule like firewood the spontaneous reaction results in net work or heat produced and the products are part of the soil as rust or ash. To get back to metalic iron or to build a tree requires the system to be pushed or pumped with a net energy input.
Entropy and Physical Chemistry
Classical entropy plays a role in chemical reactions, and that role is exemplified in equation 4 below.
S = (H - F)/T
Equation 4
Of course, this looks just like equation 3 with different letters, and so it is. Here, we are not much interested in the physicists approach of describing the state of a "static" system, as does equation 1. The real interest for the chemist, is to predict whether or not a given chemical reaction will go. In equation 4, H is the enthalpy, and F is the free energy (also known as the Gibb's free energy). Likewise, H and F are incremental variations of those quantities, and S is an incremental change in the entropy of the chemical system, in the event of a chemical reaction.
A little algebra, leading to equation 5, will maybe make things just a little easier to see.
F = H - TS
Equation 5
The significance of this equation is that it is the value of F which tells you whether any give chemical reaction will go forward spontaneously, or whether it needs to be pumped. The enthalpy, H, is the heat content of the system, and so the change in enthalpy, H, is the change in heat content of the system. If that value is smaller than TS, then F will be negative, and the reaction will proceed spontaneously; the TS term represents the ability to do the work required to make the reaction happen. However, if F is positive, such that H is greater than TS, then the reaction will not happen spontaneously; we still need at least F worth of energy to make it happen.
Note that a positive free energy does not mean that the reaction will not happen, only that it will not happen spontaneously in the given environment. It can still be pushed or pumped into happening by adding energy, or setting the reaction in a higher temperature environment, making T larger as well as TS, and perhaps driving it far enough to make F negative.