Global challenges Study Guide

Fuel cells are electrochemical devices that convert the chemical energy from a fuel directly into electrical energy through a chemical reaction with an oxidising agent. Unlike batteries, fuel cells do not run down or need recharging; they produce electricity continuously as long as fuel and oxidant are supplied. The most common type is the hydrogen-oxygen fuel cell, which reacts hydrogen with oxygen to produce water, generating electricity in the process. The reaction is 2H₂(g) + O₂(g) → 2H₂O(l) + electrical energy. Methanol fuel cells and other types can also be used, which involve different fuel sources but the same principle of converting chemical energy to electrical energy. The key advantage of a hydrogen-oxygen fuel cell is that its only byproduct is water, making it a very clean energy source at the point of use. This makes them attractive for vehicles and portable power generation where low emissions are important. They are also very efficient at converting fuel to electricity compared to combustion engines. However, there are significant disadvantages. Producing hydrogen often requires energy from fossil fuels, which can generate greenhouse gases. Storing hydrogen safely and efficiently is challenging due to its low density and flammability. Fuel cells can also be expensive to manufacture, and the infrastructure for hydrogen refueling is limited. The lifespan of fuel cells can also be a concern and they can be sensitive to impurities in the fuel. These factors mean that while fuel cells offer significant environmental benefits in certain applications, their widespread adoption is hindered by economic and practical challenges related to fuel production, storage, and initial cost.

The industrial production of fertilisers is a complex process that integrates several chemical reactions to convert raw materials into products essential for agriculture. Ammonia is a crucial intermediate in the production of many fertilisers. It is manufactured from nitrogen (from the air) and hydrogen (from natural gas or steam reforming of hydrocarbons) using the Haber process. This process is reversible and exothermic, typically carried out at high pressures (around 200 atmospheres) and high temperatures (around 450°C) with an iron catalyst to maximise yield and reaction rate. The ammonia produced can then be reacted with acids to form various ammonium salts, which are common fertilisers. For example, ammonia reacts with nitric acid to form ammonium nitrate, and with sulfuric acid to form ammonium sulfate. Phosphate fertilisers are produced from phosphate rock, which contains insoluble phosphates. This rock is treated with acids to make the phosphates soluble and therefore available to plants. For instance, reacting phosphate rock with sulfuric acid produces 'single superphosphate' (a mixture of calcium sulfate and calcium dihydrogen phosphate), while reacting it with phosphoric acid produces 'triple superphosphate' (primarily calcium dihydrogen phosphate). Potassium fertilisers are generally mined as potassium chloride or potassium sulfate and simply processed for purity. These various processes require careful management of raw materials, reaction conditions, and safety protocols to ensure efficient and sustainable production of high-quality fertilisers.