Materials, Corrosion, Haber Process and Fertilisers
Corrosion
Corrosion is the destruction of metals by chemical reactions with substances in the environment. The most familiar example is rusting, which occurs when iron reacts with both oxygen and water.
Rusting requires both oxygen and water. If either is absent, iron will not rust. The word equation is: iron + water + oxygen → hydrated iron(III) oxide (rust). Rusting is an oxidation reaction because the iron is oxidised.
Corrosion can be prevented in several ways:
- Painting, greasing, or oiling — creates a physical barrier that prevents oxygen and water reaching the metal surface.
- Galvanising — coating iron with a layer of zinc. Even if the zinc coating is scratched, the zinc corrodes preferentially because it is more reactive than iron (sacrificial protection).
- Electroplating — using electrolysis to coat a metal with a thin layer of a more attractive or less reactive metal (e.g. chromium plating on car parts).
Most metals used in everyday life are alloys — mixtures of metals (or a metal with a non-metal). Alloys are harder and stronger than pure metals because atoms of different sizes disrupt the regular layers, preventing them from sliding over each other easily.
Examples of alloys:
- Steel — iron + carbon (and sometimes other metals). Low carbon steel is easy to shape; high carbon steel is hard; stainless steel (iron + chromium + nickel) resists corrosion.
- Bronze — copper + tin. Hard and resistant to corrosion.
- Brass — copper + zinc. Easy to shape and has a gold-like appearance.
The Haber Process
The Haber process is one of the most important industrial chemical processes. It produces ammonia (\(\text{NH}_3\)) from nitrogen (from the air) and hydrogen (from natural gas or steam reforming).
The Haber process equation: \(\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3\). Conditions: temperature of 450 °C, pressure of 200 atmospheres, and an iron catalyst. The reaction is reversible, so these conditions are a compromise to give a reasonable yield at a reasonable rate.
The forward reaction (making ammonia) is exothermic. By Le Chatelier's principle:
- Temperature: A lower temperature would give a higher yield (favours the exothermic forward reaction), but the rate would be too slow. 450 °C is a compromise between yield and rate.
- Pressure: Higher pressure gives a higher yield (fewer gas molecules on the product side: 4 moles of reactants → 2 moles of products). 200 atm is used because very high pressures are expensive and dangerous.
- Iron catalyst: Speeds up both forward and reverse reactions equally, helping the reaction reach equilibrium faster. It does not change the yield, only the rate.
Any unreacted nitrogen and hydrogen are recycled back through the reactor to maximise overall conversion.
Fertilisers
NPK fertilisers provide the three essential elements that plants need for growth: nitrogen (N), phosphorus (P), and potassium (K). Ammonia from the Haber process is a key starting material for nitrogen-containing fertilisers.
The production of NPK fertilisers involves several steps:
- Ammonia + nitric acid → ammonium nitrate (\(\text{NH}_3 + \text{HNO}_3 \rightarrow \text{NH}_4\text{NO}_3\)) — provides nitrogen.
- Ammonia + sulfuric acid → ammonium sulfate — another nitrogen source.
- Phosphate rock + acid → soluble phosphates — provides phosphorus.
- Potassium chloride or potassium sulfate — provides potassium (mined from mineral deposits).
Plants need nitrogen to make proteins and chlorophyll, phosphorus for DNA and energy transfer, and potassium for enzyme function and disease resistance. Without fertilisers, crop yields would be significantly lower.
NPK fertilisers can be formulated in the laboratory by reacting an acid with an alkali to produce a soluble salt. The proportions of N, P, and K can be adjusted to suit different crops and soil conditions.