Thermodynamic stability occurs when a system is in its lowest energy state, or in chemical equilibrium with its environment. State A is said to be more thermodynamically stable than state B if the Gibbs free energy of the change from A to B is positive.

Natural processes, such as a banana ripening or a lightening strike are spontaneous in the direction they occur and nonspontaneous in the opposite direction. By definition they are irreversible; they do not occur by reversible pathways.

A reaction is thermodynamically favorable when its enthalpy, ΔH, decreases and its entropy, ΔS, increases. Substituting the inequalities ΔH < 0 and ΔS > 0 into Equation 6.2. 2 shows that a reaction is thermodynamically favorable when ΔG is negative.

From a purely thermodynamic point of view, an emulsion is an unstable system because there is a natural tendency for a liquid/liquid system to separate and reduce its interfacial area and, hence, its interfacial energy. However, most emulsions demonstrate kinetic stability (i.e., they are stable over a period of time).

A system is called thermodynamically unstable when there exists a state where the system will have lower energy than it currently has. A dispersion can be thermodynamically unstable, but can still be kinetically stable.

Quickly said, the kinetic product is the one that forms the fastest and the thermodynamic product is the most stable.

A reaction yielding more thermodynamic product is under thermodynamic control, and likewise, a reaction that yields more kinetic product is under kinetic control.

Graphite is thermodynamically the most stable form of carbon. Graphite is more stable than diamond and fullerene because its enthalpy of formationΔHof is less than that of both diamond (1.98 kJmol-1) and fullerene (38.1 kJmol-1) at room temperature and atmospheric pressure.

A kinetically controlled reaction is one that does not occur because of a large activation energy barrier even though the products have a lower standard Gibbs free energy (i.e., the reaction is favorable because the product or products are more stable).

Kinetic and Thermodynamic Product Ratios At higher temperatures and longer reaction times, thermodynamic products are favored. On the contrary, at lower temperatures, one would tend to see a greater yield of kinetic products. These products are generally formed at or around 0°C.

If it doesn’t have enough kinetic energy to move out of its current position, we say that it is kinetically stable or kinetically trapped. If it has reached the global minimum, we say it is thermodynamically stable.

In such a case A is the kinetic product and is favoured under kinetic control and B is the thermodynamic product and is favoured under thermodynamic control.

Kinetic control: A reaction in which the product ratio is determined by the rate at which the products are formed. This E2 reaction is irreversible. The alkene products are not in equilibrium, so their relative stability does not control the amount of each product produced.

Thermodynamics describes the overall properties, behavior, and equilibrium composition of a system; kinetics describes the rate at which a particular process will occur and the pathway by which it will occur.

Thermodynamics has established in chemistry principally as a science determining possibility and direction of chemical transformations and giving conditions for their final, equilibrium state. Thermodynamics is usually thought to tell nothing about rates of these processes, their velocity of approaching equilibrium.