These results lead to a profound statement regarding the relation between entropy and spontaneity known as the second law of thermodynamics: all spontaneous changes cause an increase in the entropy of the universe.

Spontaneous reactions release free energy as they proceed. Recall that the determining factors for spontaneity of a reaction are the enthalpy and entropy changes that occur for the system. The free energy change of a reaction is a mathematical combination of the enthalpy change and the entropy change.

Spontaneous processes are irreversible because they can be reversed only by taking a different path to get back to their original state. A reversible process can take the same path to return to its original state. An irreversible process must take a different path to get back to its original state.

Entropy ( S ) is a measure of the disorder in a system. In a closed system, entropy always increases over time. In an open system, energy can be added to a system to cause a decrease in entropy, but this is not necessarily a spontaneous reaction. A reaction will be spontaneous if the change in G , ΔG , is negative.

The second law of thermodynamics states that for any spontaneous process, the overall ΔS must be greater than or equal to zero; yet, spontaneous chemical reactions can result in a negative change in entropy.

Are these processes non-spontaneous? Not really. These reactions are simply too much slow at room temperature to be visible, but if they are triggered with a little spark or flame, they start quickly, producing enough thermal energy to be self-sustained and actually getting along spontaneously.

Spontaneity is a thermodynamic phenomenon, and the rate/speed of a reaction is a kinetic phenomenon.

A negative change in entropy indicates that the disorder of an isolated system has decreased. For example, the reaction by which liquid water freezes into ice represents an isolated decrease in entropy because liquid particles are more disordered than solid particles.

A spontaneous reaction is more likely to be exothermic but can be endothermic. Non-spontaneous reactions are more likely to be endothermic but can be exothermic. The deciding factor for these systems is the temperature.

Are all exothermic reactions spontaneous? No, but all reactions that are spontaneous at standard state (interpreted to mean that at standard state, equilibrium lies in the forward direction, i.e. K > 1 and ΔrG∘<0) are either exothermic (ΔrH∘<0) or show a positive standard entropy of reaction (ΔrS∘>0), or both.

Definition of a Spontaneous Process A spontaneous process is one that occurs on its own, without any energy input from the outside. For example, a ball will roll down an incline; water will flow downhill; ice will melt into water; radioisotopes will decay; and iron will rust.

Hence, ΔStotal​ is positive for spontaneous reaction.

Therefore, flow of heat from a cold body to a hot body is a non-spontaneous process as it requires external work as per Clausius.