The rate of a reaction is the change in concentration of the reactants or the change in concentration of the products per unit time. The definition of chemical equilibrium when the forward reaction rate and the reverse reaction rate are equal. The result of this equilibrium is that the concentrations of the reactants and the products do not change. The rate constant, k, measures how fast a chemical reaction reaches equilibrium assuming the reactants were supplied with enough activation energy to enable the reaction to proceed in the forward direction from reactants to products. The need for energy is due to the fact that the reactants are unreactive under certain conditions. In this case, the reaction must have some sort of energy input before it can proceed; otherwise, the reactants cannot cross the activation energy threshold and convert to products. The reaction is activated by energy supplied to the reactants by different energy sources. The rate of reaction, the rate constant, and the kinetic energy required for activation of reaction indicate how fast the reaction reaches equilibrium.

For all chemical reactions there is a kinetic and thermodynamic component. The quantity related to kinetics is the rate constant k. The rate constant is associated with the activation energy required for the reaction to proceed according to the reactivity of the reactants. The thermodynamic quantity is the energy difference resulting from the free energy (ΔG) given off during a chemical reaction: how stable the products are relative to the reactants. Although kinetics describes the rate of reaction and how fast equilibrium is reached, it provides no information about conditions once the reaction equilibrates. In the same measure, thermodynamics only gives information regarding the equilibrium conditions of products after the reaction takes place, but does not explain the rate of reaction. Chemical reactivity and stability are two related but distinct concepts 1) kinetics describes whether reactants need to be energized to produce products. It does not consider the overall energy on the activation energy what is required. Conversely, thermodynamics is a state function concerned with the energy of both reactant(s) and product(s). Whether a reaction is spontaneous depends on both the initial and final state. Those reactions with a higher free energy or higher potential chemical energy require energy to drive the reaction as they are not capable of liberating energy and thus being spontaneous.  The most stable states of a kinetic reaction are those of the reactants, in which an input of energy is required to move the reaction from a state of stability, to that of reacting and converting itself to products. Kinetics is related to reactivity. In contrast, the most stable state of a thermodynamically favorable reaction is the products, because the reaction occurs spontaneously, without the need for energy to be added. Thermodynamics is related to stability.