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 (state) to the reactants. In this case, the solid zeolite catalyst is in a different phase than the gaseous alkanes.

An autocatalyst is a product of the reaction that also acts as a catalyst. It speeds up the reaction after its formation, meaning that as the reaction proceeds, the rate of reaction increases due to the presence of this product.

The reaction starts slowly because the initial reactants (ethanoate ions and manganese ions) are less reactive. As the reaction progresses, the concentration of the product (Mn²⁺ ions) increases, which acts as an autocatalyst:

• Initially, the reaction can be represented as: $2MnO_4^- + 5C_2H_3O_2^- + 16H^+ \rightarrow 2Mn^{2+} + 10CO_2 + 8H_2O$
• As Mn²⁺ accumulates, it catalyzes further reduction of MnO₄⁻, speeding up the reaction.

Thus, the introduction of additional potassium manganate(VII) later enhances the reaction rate significantly.

Co²⁺ ions can act as catalysts because they participate in redox reactions. The relevant half-reactions are:

• For Co²⁺: $Co^{2+} + 2e^- \leftrightarrow Co$
• The standard electrode potential shows that Co²⁺ can easily get reduced, providing a pathway for the reaction that requires lower activation energy, thus facilitating the reaction between peroxodisulfate and iodide ions.