Chemistry at industrial scale
Making a chemical in a test tube and making it by the megatonne are different problems. Industry must balance yield (how much product), rate (how fast), energy cost and raw-material cost — and the best conditions for one are rarely best for another. Every large process is a compromise, usually with unreacted material recycled and waste heat reused.
The Haber process — ammonia
The Haber process makes ammonia from nitrogen (from the air) and hydrogen: N₂ + 3H₂ ⇌ 2NH₃. As a reversible, exothermic reaction it is pure Le Chatelier economics — high pressure (~200 atm) favours ammonia, but a very low temperature (which would also favour it) makes the reaction too slow, so a moderate ~450 °C is used with an iron catalyst.
Ammonia matters because it feeds the world: most of it becomes fertiliser. Ammonia is oxidised to nitric acid, and ammonia plus acids give ammonium salts — the nitrogen crops need to grow.
The Solvay process — sodium carbonate
The Solvay process makes sodium carbonate (soda ash) — vital for glass, detergents and many chemicals — from cheap, abundant raw materials: salt (brine) and limestone. Its elegance is recycling: the ammonia it uses and the carbon dioxide from the limestone are recovered and fed back in, so little is wasted, which is what makes it economic.
Refining petroleum
The petrochemical industry starts by refining crude oil — fractional distillation splits it into fractions (petrol, diesel, kerosene, bitumen) by boiling point. Because demand for the short, useful fractions outstrips supply, long fractions are cracked into smaller molecules, producing more petrol and the alkenes used to make plastics.
So a barrel of crude becomes fuels, lubricants, road surfaces and the feedstock for polymers — the same chemistry as the organic topic, run at enormous scale.
The theme of industrial chemistry
Every process here reuses the earlier topics: the Haber process is equilibrium (Le Chatelier), refining is organic chemistry (distillation and cracking), and fertilisers are acid–base chemistry (ammonia neutralised to ammonium salts). Industry is school chemistry scaled up and costed.
A worked example
Why does the Haber process use ~450 °C rather than a much lower temperature, when the forward reaction is exothermic and low temperature would give more ammonia?
- A low temperature does favour the exothermic forward reaction, raising the equilibrium yield of ammonia.Le Chatelier: cooling shifts toward the heat-releasing side.
- But at low temperature the reaction is far too slow to be useful.Rate depends strongly on temperature — few collisions have the activation energy when cold.
- So ~450 °C is a compromise: a workable rate for a still-reasonable yield, with unreacted gases recycled.Industry optimises yield AND rate AND cost together, not yield alone.
It is a compromise — a lower temperature gives more ammonia but far too slowly, so a moderate temperature trades some yield for a usable rate (and the leftovers are recycled).
The Contact process — sulfuric acid
Sulfuric acid is the world's most-produced chemical, made by the Contact process. Sulfur is burned to sulfur dioxide, which is then oxidised in the key reversible step 2SO₂ + O₂ ⇌ 2SO₃ over a vanadium(V) oxide catalyst at about 450 °C. The sulfur trioxide is absorbed into concentrated sulfuric acid and diluted to give more acid.
Its uses are everywhere — fertilisers, detergents, paints, and as the electrolyte in car batteries.
Fertilisers and nitric acid
Ammonia from the Haber process is the starting point for nitrogen fertilisers. Some ammonia is oxidised (the Ostwald process) to make nitric acid, which is then neutralised by more ammonia to give ammonium nitrate — a fertiliser rich in nitrogen. Fertilisers labelled NPK supply nitrogen, phosphorus and potassium, the elements plants most need.
But over-use has a cost: fertiliser washed into rivers and lakes causes eutrophication — algae bloom, then die and decay, using up the oxygen and suffocating fish.
Green chemistry and sustainability
A modern industrial process is judged on more than yield. Green chemistry asks how much of the reactant mass ends up in the product (atom economy), how much energy is used, whether the feedstock is renewable, and what happens to the waste. Recycling unreacted materials, reusing heat, and choosing catalysts that lower energy demand all cut cost and environmental impact together.
Yield is not the whole story
A reaction can have a high yield yet a poor atom economy if it makes a lot of unwanted by-product. Sustainable industry optimises yield, atom economy, energy and waste all at once.