Global challenges Study Guide

The Haber process is a crucial industrial chemical process that synthesises ammonia (NH₃) from nitrogen gas (N₂) and hydrogen gas (H₂). Its immense importance in agricultural production stems from ammonia being a key ingredient in the manufacture of nitrogen-based fertilisers. Nitrogen is one of the three macronutrients (nitrogen, phosphorus, potassium) essential for plant growth and development. Plants cannot directly absorb nitrogen gas from the atmosphere, despite it making up about 78% of the air. Instead, they require it in a 'fixed' form, such as nitrates or ammonium compounds, which are soluble in water and can be absorbed through their roots. Ammonia produced by the Haber process is converted into various fertilisers like ammonium nitrate, urea, and ammonium sulfate. These fertilisers replenish nitrogen in the soil, which is depleted by crop harvesting. By providing readily available nitrogen, the Haber process enables farmers to grow more food on the same amount of land, supporting a growing global population and preventing widespread famine. Without this process, agricultural yields would be significantly lower, leading to food scarcity, increased food prices, and a greater need for agricultural land, potentially impacting biodiversity and natural habitats.

A chemical cell, often referred to as a galvanic or voltaic cell, is a device that converts chemical energy into electrical energy through a spontaneous redox (reduction-oxidation) reaction. At the heart of a chemical cell are two different metals, or a metal and a non-metal, immersed in an electrolyte. The difference in reactivity between these two materials is crucial. The more reactive material will preferentially lose electrons (undergo oxidation), acting as the negative electrode (anode). These released electrons flow through an external circuit to the less reactive material, which gains electrons (undergoes reduction), acting as the positive electrode (cathode). This movement of electrons constitutes an electric current, producing a potential difference (voltage) across the cell terminals.

The electrolyte is a substance containing free-moving ions, typically an ionic solution, which completes the circuit by allowing ions to move between the two half-cells. This ionic movement maintains charge neutrality and ensures the continuous flow of electrons in the external circuit. As the redox reaction proceeds, the reactants are consumed. The reactive metal at the anode will gradually corrode or dissolve as it loses electrons, and the chemical species at the cathode will be reduced as it gains electrons. The cell continues to produce an electrical potential difference until one or both of the reactants are used up, or the electrolyte becomes depleted, at which point the chemical reaction can no longer proceed, and the cell 'dies' or runs out of charge.