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Chemical synthesis · Lesson 3

From laboratory to plant

Rate against yield, catalysts, recycling, atom economy and green chemistry.

In a laboratory, a chemist chooses conditions that give the purest product. In a plant that makes a million tonnes a year, the question changes: which conditions make the most product per dollar, safely, with the least waste? Every industrial process is an answer to that question, and green chemistry asks us to include the planet in the cost.

Rate against yield

For an exothermic equilibrium such as N2 + 3H2 ⇌ 2NH3, a low temperature gives a high equilibrium yield but a sluggish rate. A high temperature is fast but shifts the equilibrium back. Industry chooses a compromise: a moderate temperature with a catalyst, which gives an acceptable yield quickly. High pressure favours the side with fewer gas particles and speeds up the reaction, but very high pressures need expensive, thick-walled equipment and a lot of energy for compression. Unreacted gases are recycled, so even a 15% yield per pass becomes about 98% overall.

  • Haber process: about 450 °C, 20 MPa, iron catalyst, ammonia condensed out, gases recycled.
  • Contact process: about 450 °C, just above atmospheric pressure (the yield is already above 95%), V₂O₅ catalyst, gas cooled between beds.
  • Methanol: about 250 °C, 5–10 MPa, copper-based catalyst.

Why catalysts matter

A catalyst provides a pathway with a lower activation energy for both the forward and reverse reactions. Equilibrium is reached faster, at a lower temperature, saving energy; the equilibrium yield is unchanged and the catalyst is not used up.

Green chemistry

Green chemistry designs products and processes that reduce or eliminate the use and generation of hazardous substances. Key ideas include: high atom economy, renewable feedstocks, catalysts rather than reagents that become waste, safer solvents and conditions, and less energy.

Atom economy = (molar mass of the desired product ÷ total molar mass of all reactants) × 100. Hydration of ethene has an atom economy of 100%; fermentation, which also makes CO₂, has 51.1%. Compare some real routes below.

The hydrogen comparison shows why atom economy is not the whole story: solar-powered electrolysis has a lower atom economy than steam reforming but uses a renewable feedstock and releases no CO₂.

Fuels from renewable feedstocks

Bioethanol is made by fermenting sugars with yeast at 30–40 °C without air; fermentation stops at about 15% ethanol, so the mixture is fractionally distilled. Biodiesel is made by transesterifying plant oils with methanol, using NaOH or the enzyme lipase as the catalyst, giving methyl esters and glycerol.

Now put it to workMethods for every question type, five worked examples, twenty exam questions and ten extended questions.
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