Monitoring and controlling chemical reactions Study Guide

The rate of a chemical reaction, particularly involving a solid reactant, can be significantly influenced by the size of the solid pieces. This is explained by considering the surface area to volume ratio. When a solid is broken down into smaller pieces, its total surface area increases for the same overall volume or mass. For a reaction to occur, reactant particles must collide with sufficient energy (activation energy). In a heterogeneous reaction involving a solid and a liquid or gas, these collisions can only happen at the exposed surface of the solid. With a larger total surface area, more reactant particles from the surrounding fluid (liquid or gas) can come into contact with the solid reactant simultaneously. This increases the frequency of successful collisions per unit time. Consequently, the rate of reaction increases because more product can be formed in the same amount of time. Conversely, if a solid reactant is in large, unbroken pieces, its surface area to volume ratio is low, meaning fewer sites are available for collision at any given moment. This leads to a lower frequency of successful collisions and thus a slower reaction rate. This principle is widely used in industry; for example, grinding catalysts into fine powders or sugar into granulated forms before use to speed up processes.