This question is about catalysis - AQA - A-Level Chemistry - Question 5 - 2022 - Paper 3

The term ‘heterogeneous’ refers to a catalyst that is in a different phase (solid, liquid, gas) compared to the reactants. In this case, zeolites, which are solid, act as catalysts while the reactants (alkanes) are in the vapor phase.

An autocatalyst is a substance that catalyzes its own production. In a reaction, once the autocatalyst is formed, its presence speeds up the reaction rate. This occurs because the product of the reaction also acts as a catalyst, enhancing the rate of reaction.

Initially, the reaction is slow due to the low concentration of the reactants and lack of available autocatalyst. As the reaction proceeds, product formation leads to the production of autocatalyst, which accelerates the reaction rate. For example, if the reaction is:

$2MnO_{4}^{-} + 5C_2O_{4}^{2-} \rightarrow 2Mn^{2+} + 10CO_{2} + 8H_{2}O$

As $ext{Mn}^{2+}$ ions accumulate, they facilitate the reaction more efficiently, leading to an increase in the reaction rate.

The standard electrode potentials indicate that Co$^{2+}$ ions can be reduced to Co. The reaction:

$Co^{2+} + 2e^- \rightarrow Co$

has a high reduction potential, suggesting that Co$^{2+}$ is a good electron acceptor. By accepting electrons, Co$^{2+}$ ions facilitate the oxidation of iodide ions and enhance the overall reaction rate, demonstrating catalytic behavior.