A Level Chemistry OCR Practice Exam

In which type of catalysis do gas-phase reactants typically interact with a solid catalyst?

1. Homogeneous catalysis.

2. Heterogeneous catalysis.

3. Equilibrium catalysis.

4. Enzymatic catalysis.

The correct answer is: Heterogeneous catalysis.

Heterogeneous catalysis is the process where reactants in different phases interact, specifically when the reactants are typically gases (or sometimes liquids) and the catalyst is a solid. In this scenario, the gaseous reactants adsorb onto the surface of the solid catalyst, where the reaction occurs. Once the reaction has taken place, the products desorb from the catalyst surface and return to the gas phase. This type of catalysis is very common in industrial processes, such as in the Haber process for ammonia synthesis or catalytic converters in automobiles, where pollutants are converted into less harmful emissions. The solid catalyst remains unchanged in mass and chemical composition after the reaction, allowing it to be used repeatedly, which is a significant advantage of heterogeneous catalysis. In contrast, homogeneous catalysis involves reactants and catalysts in the same phase, typically both are in liquid form. Equilibrium catalysis relates to situations where the position of equilibrium can be altered but does not specifically describe a phase interaction, while enzymatic catalysis involves biological catalysts (enzymes) and typically occurs in an aqueous environment.