Heterogeneous catalysis: on bathroom mirrors and boiling stones

Abstract

A catalyst is defined as a substance that accelerates a process without undergoing a net change due to that process. Most chemistry students learn about catalysts in the context of chemical reactions, such as the enzymes in biochemistry or the heterogeneous metal catalysts in inorganic chemistry (1, 2). However, it is both important and instructive to show students that the concept of catalysis is broader than molecular chemistry as it also includes physical processes such as crystallization, freezing, boiling, and liquid condensation. Students are familiar with measures to accelerate these processes but probably without realizing that they apply a catalyst, according to the definition given above. In basic laboratorywork, for example, students learn about the precautions to prevent “bumping” when heating solutions or distilling liquids. This bumping is the sudden boiling that results in hot liquid and vapor shooting uncontrollably out of the heating vessel. Addition of boiling stones to the liquid before heating will prevent this uncontrolled boiling; alternative antibumping measures are stirring of the liquid with amechanical stirrer ormagnetic flea or the use of an inert gas (nitrogen) capillary bleed in the case of vacuum distillation (3). The effect of these precautions is that they facilitate the nucleation, that is, the formation of vapor bubbles in the heated liquid, such that the liquid boils at its normal boiling point (for either ambient or reduced pressure). Without antibumping measures the liquid temperature may rise above the liquid's boiling point. Any bubble nucleation in this so-called superheated liquid will lead to the uncontrollable boiling mentioned above. Ern e (4) describes an instructive classroom experiment, using amicrowave oven, that demonstrates this superheating of clean liquids and the effect of antibumping agents.