Reactivity (chemistry) - An Alternative Point of View

An Alternative Point of View

Reactivity is a somewhat vague concept used in chemistry which 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 are commonly temperature dependent. For example, it is commonly asserted that the reactivity of group one 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 particle size; and hydrogen will 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', all substances react with some reagents and not others. For example, in making the statement that 'sodium metal is reactive', we are alluding to the fact that sodium reacts with many common reagents (including pure oxygen, chlorine, hydrochloric acid, water) and/or that it reacts rapidly with such materials at either room temperature or using a bunsen flame.

'Stability' should not be confused with reactivity. For example, an isolated molecule of an electronically state of the oxygen molecule will spontaneously emit 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.