Reactivity (chemistry)

Reactivity refers to:

• the chemical reactions of a single substance,
• the chemical reactions of two or more substances that interact with each other,
• the systematic study of sets of reactions of these two kinds,
• methodology that applies to the study of reactivity of chemicals of all kinds,
• experimental methods that are used to observe these processes, and
• theories to predict and to account for these processes.

The chemical reactivity of a single substance (reactant) covers its behavior in which it:

• decomposes,
• forms new substances by addition of atoms from another reactant or reactants, and
• interacts with two or more other reactants to form two or more products.

The chemical reactivity of a substance can refer to the variety of circumstances (conditions that include temperature, pressure, presence of catalysts) in which it reacts, in combination with the:

• variety of substances with which it reacts,
• equilibrium point of the reaction (i.e., the extent to which all of it reacts), and
• rate of the reaction.

The term reactivity is related to the concepts of chemical stability and chemical compatibility.

An alternative point of view

'Reactivity' is a somewhat vague concept in chemistry. It appears to embody both thermodynamic factors and kinetic factors (i.e., whether or not a substance reacts, and how fast it reacts). Both factors are actually distinct, and both commonly depend on temperature. For example, it is commonly asserted that the reactivity of alkali metals (Na, K, etc.) increases down the group in the periodic table, or that hydrogen's reactivity is evidenced by its reaction with oxygen. In fact, the rate of reaction of alkali metals (as evidenced by their reaction with water for example) is a function not only of position within the group but also of particle size. Hydrogen does not react with oxygen—even though the equilibrium constant is very large—unless a flame initiates the radical reaction, which leads to an explosion.

Restriction of the term to refer to reaction rates leads to a more consistent view. Reactivity then refers to the rate at which a chemical substance tends to undergo a chemical reaction in time. In pure compounds, reactivity is regulated by the physical properties of the sample. For instance, grinding a sample to a higher specific surface area increases its reactivity. In impure compounds, the reactivity is also affected by the inclusion of contaminants. In crystalline compounds, the crystalline form can also affect reactivity. However, in all cases, reactivity is primarily due to the sub-atomic properties of the compound.

Although it is commonplace to make statements that "substance X is reactive," each substance reacts with its own set of reagents. For example, the statement that "sodium metal is reactive" suggests that sodium reacts with many common reagents (including pure oxygen, chlorine, hydrochloric acid, and water), either at room temperature or when using a Bunsen burner.

The concept of stability should not be confused with reactivity. For example, an isolated molecule of an electronically excited state of the oxygen molecule spontaneously emits light after a statistically defined period. The half-life of such a species is another manifestation of its stability, but its reactivity can only be ascertained via its reactions with other species.