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

In the context of catalysis, 'heterogeneous' refers to a catalyst that exists in a different phase (solid, liquid, or gas) from the reactants. For example, in catalytic cracking, the zeolite catalyst is typically a solid while the reactants are gaseous alkanes.

An autocatalyst is a substance that catalyzes its own production in a reaction. In other words, it is a product of the reaction that also increases the rate of that same reaction. This can lead to a rapid increase in reaction rate as the concentration of the autocatalyst increases.

Initially, the reaction is slow because there are few reactant molecules available to collide and react. As the reaction proceeds, the formation of products (which may be autocatalysts) accelerates the reaction. Once enough of the autocatalyst is formed, it significantly increases the reaction rate, leading to a rapid decrease in reactant concentration. The balanced reaction can be represented as:

$2MnO_{4}^{-} + 5C_{2}O_{4}^{2-} + 16H^{+} \rightarrow 2Mn^{2+} + 10CO_{2} + 8H_{2}O$

Co$^{2+}$ acts as a catalyst by providing an alternate reaction pathway with a lower activation energy. The standard electrode potential indicates that Co$^{2+}$ ions can reduce S$_2$O$_8^{2-}$ ions to SO$_4^{2-}$, and this facilitates the reaction with iodide ions. The potential reactions can be expressed as:

1. For the reduction: $Co^{2+} + 2e^{-} \rightarrow Co(s)$
2. For the overall reaction: $S_{2}O_{8}^{2-} + 2I^{-} \rightarrow 2SO_{4}^{2-} + I_{2}$

Thus, Co$^{2+}$ helps to speed up the conversion by providing a favorable pathway for the reaction.