The State of Subdivision of the Reactants

The State of Subdivision of the Reactants

Except for substances in the gaseous state or in solution, reactions occur at the boundary, or interface, between two phases. Hence, the rate of a reaction between two phases depends to a great extent on the surface contact between them. A finely divided solid has more surface area available for reaction than does one large piece of the same substance.

Thus a liquid will react more rapidly with a finely divided solid than with a large piece of the same solid. For example, large pieces of iron react slowly with acids; finely divided iron reacts much more rapidly (see the figure below). Large pieces of wood smolder, smaller pieces burn rapidly, and saw dust burns explosively.

(a) Iron powder reacts rapidly with dilute hydrochloric acid and produces bubbles of hydrogen gas because the powder has a large total surface area: 2Fe(s) + 6HCl(aq) \(⟶\) 2FeCl₃(aq) + 3H₂(g). (b) An iron nail reacts more slowly.

Optional Video

Watch this video to see the reaction of cesium with water in slow motion and a discussion of how the state of reactants and particle size affect reaction rates.