A Level Chemistry OCR Practice Exam

What is heterogeneous catalysis?

1. A reaction in which all reactants and catalyst share the same physical state.

2. A reaction where the catalyst is in a different physical state than the reactants.

3. A process that does not involve any change in energy states.

4. A type of catalysis that only occurs in gas phase reactions.

The correct answer is: A reaction where the catalyst is in a different physical state than the reactants.

Heterogeneous catalysis refers to a process in which the catalyst and the reactants are in different phases, promoting the reaction at their interface. The catalyst typically exists in a solid state while the reactants are in either gas or liquid phases. This physical separation allows for distinct reaction surfaces where the reactants can interact with the catalyst, facilitating the reaction while the catalyst itself remains unchanged in composition. Since the catalyst is not in the same phase as the reactants, this contributes to the efficiency of the reaction and often improves the selectivity of the products. Heterogeneous catalysis is widely employed in various industrial processes, such as the Haber process for ammonia synthesis, where solid iron serves as a catalyst for the reaction between gaseous nitrogen and hydrogen. The other options do not accurately represent the concept of heterogeneous catalysis. For instance, reactions where all components share the same physical state describe homogeneous catalysis instead. The statement about change in energy states is unrelated, as catalysis specifically aims to lower the activation energy required for a reaction, rather than implying that no energy change occurs. Lastly, saying that it only occurs in gas phase reactions is too restrictive, as heterogeneous catalysis can take place in various systems involving different states including solids, liquids, and