Global challenges Study Guide

In many industrial chemical processes, a crucial challenge is to balance the rate of reaction with the position of equilibrium to maximise the economic efficiency of producing a desired product. This often involves a 'trade-off'. Changing reaction conditions like temperature and pressure can have opposing effects on the reaction rate and the equilibrium yield. For example, increasing temperature generally increases the rate of reaction (molecules have more kinetic energy, leading to more frequent and energetic collisions), but if the forward reaction is exothermic, a higher temperature will shift the equilibrium to the left, decreasing the yield of the desired product according to Le Chatelier's Principle. Conversely, a lower temperature might favour a higher equilibrium yield for an exothermic reaction but result in a very slow reaction rate, making the process economically unviable. Similarly, increasing pressure can increase the reaction rate (more molecules per unit volume, increasing collision frequency) and also shift the equilibrium towards the side with fewer moles of gas. If the desired product is on the side with fewer moles of gas, high pressure is beneficial for both rate and yield. However, using very high pressures requires stronger and more expensive equipment, increasing costs and safety risks. Catalysts are often used to increase the reaction rate without affecting the position of equilibrium, offering a way to mitigate some of these trade-offs by allowing lower temperatures or pressures to be used while maintaining an acceptable reaction speed. The optimal conditions chosen for industrial processes are therefore a compromise, aiming for an economically viable rate of production and an acceptable percentage yield, rather than necessarily the absolute maximum possible yield or the fastest possible rate.

Crude oil is a complex mixture of hydrocarbons, which are compounds made only of hydrogen and carbon atoms. When crude oil is separated into different fractions through fractional distillation, each fraction is still largely a mixture, not a single pure compound. A significant portion of these fractions consists of compounds belonging to the alkane homologous series. Alkanes are saturated hydrocarbons, meaning they contain only single carbon-carbon bonds. They follow the general formula CnH2n+2, where 'n' represents the number of carbon atoms in the molecule. For example, if n=1, the alkane is methane (CH4); if n=2, it's ethane (C2H6); if n=3, it's propane (C3H8). As the number of carbon atoms (n) in an alkane molecule increases, its boiling point, viscosity (resistance to flow), and density generally increase, while its flammability decreases. Different fractions from crude oil, like petrol, diesel, and bitumen, consist of alkanes with a specific range of carbon chain lengths, which gives them their characteristic properties and uses. Petrol, for instance, contains shorter-chain alkanes compared to diesel or bitumen. The general formula CnH2n+2 helps predict the molecular formula of any alkane if the number of carbon atoms is known, aiding in understanding the composition and properties of crude oil fractions.