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

The equation for the catalytic cracking of tetradecane (C14H30) can be represented as follows:

$\text{C}_{14}\text{H}_{30} \rightarrow \text{C}_8\text{H}_{18} \text{ (octane)} + \text{C}_6\text{H}_{12} \text{ (cycloalkane)}$

An autocatalyst is a substance that acts as a catalyst for a reaction while being produced during the reaction itself. This means that as the reaction progresses, the concentration of the autocatalyst increases, which can speed up the reaction.

The reaction is slow at first because the concentration of the reactants is initially low, and there may not be enough autocatalyst present. As the reaction proceeds and more products (such as Mn^2+) are formed, they can further facilitate the reaction. This can be illustrated by the reaction equation:

$2\text{MnO}_4^- + 5\text{C}_2\text{O}_4^{2-} + 16\text{H}^+ \rightarrow 2\text{Mn}^{2+} + 10\text{CO}_2 + 8\text{H}_2\text{O}$

As more Mn^2+ is produced, the reaction rate accelerates, leading to a rapid color change.

The electrochemical potentials show that the Co^2+ ions can readily be reduced to Co(s) based on its standard electrode potential (+1.82 V). This reduction process can help facilitate the oxidation of iodide ions and the subsequent production of I2. The presence of Co^2+ ions lowers the activation energy required for the reaction to occur, thereby allowing the overall process to proceed more quickly.