Monitoring and controlling chemical reactions Study Guide

A catalyst is a substance that changes the rate of a chemical reaction but is not used up in the reaction. Catalysts work by providing an alternative reaction pathway with a lower activation energy. The activation energy is the minimum amount of energy that reacting particles must possess in order to undergo a successful collision. By lowering this energy barrier, more particles will have sufficient energy to react at a given temperature, leading to a higher frequency of successful collisions. This results in an increased rate of reaction. Importantly, catalysts remain chemically unchanged at the end of the reaction, meaning they can be reused many times. They do not affect the products formed or the overall energy change of the reaction (enthalpy change), only how quickly the reaction reaches completion. Different catalysts are specific to different reactions; for example, iron is used in the Haber process to synthesise ammonia, while enzymes are biological catalysts specific to reactions within living organisms. Increasing the surface area of a solid catalyst can further increase the reaction rate, as more active sites are exposed for reactant particles to interact with.

Atom economy is a measure of how efficiently reactant atoms are converted into desired products. In many chemical reactions, not all the atoms in the reactants end up in the desired product; some are incorporated into unwanted by-products. A high atom economy indicates that most atoms from the reactants are incorporated into the useful product, leading to less waste. This is important for sustainability and economic reasons. To calculate atom economy, you need the balanced chemical equation, which shows the molar masses of all reactants and products. The formula for atom economy is: (Mass of desired product / Total mass of all reactants) × 100%. Alternatively, it can be calculated as (Mass of desired product / Total mass of all products) × 100%. Both formulations yield the same result because, by the law of conservation of mass, the total mass of reactants equals the total mass of products. A higher atom economy is desirable because it means less raw materials are wasted, energy consumption for separating desired products from by-products is reduced, and less waste needs to be disposed of, making the process more 'green' and cost-effective. For example, addition reactions typically have 100% atom economy as all reactant atoms form a single desired product, while substitution or elimination reactions often yield by-products, resulting in lower atom economy.