Green chemistry and sustainability, atom economy and percentage yield as measures of efficiency, the use of catalysts and renewable feedstocks, and reducing the environmental impact of chemical processes.

What this topic is asking

Eduqas topic C2.4 is a distinctive feature of the specification: it asks you to judge chemical processes by their wider impact, using the principles of green chemistry and sustainability. You apply atom economy and percentage yield as efficiency measures, weigh the role of catalysts and renewable feedstocks, and explain how chemists reduce the environmental cost of industrial reactions.

Green chemistry and sustainability

Atom economy and yield as efficiency measures

A process can be efficient in two distinct senses, and Eduqas expects both to be considered.

A reaction that converts most of its reactants into the desired product (high atom economy) and loses little of that product in practice (high yield) is the most efficient and least wasteful.

Catalysts and feedstocks

Catalysts lower the activation energy, so a process can run at lower temperature and pressure, cutting energy use and cost; many also improve selectivity, reducing by-products. Choosing a renewable feedstock (such as plant biomass) instead of a finite one (such as crude oil) lowers the long-term resource and carbon impact, though it may bring trade-offs in land use or yield.

Reducing environmental impact

Chemists reduce impact by recycling solvents and catalysts, capturing or reusing by-products, designing reactions that avoid toxic reagents, and using waste heat. These choices, alongside atom economy and yield, decide whether a process is sustainable.

Examples in context

Example 1. The ethanol routes. Hydration of ethene has a $100\%$ atom economy and gives a pure product continuously, but uses a crude-oil feedstock; fermentation has about a $51\%$ atom economy but uses renewable sugar at low temperature. Each is "greener" by a different measure, which is exactly the kind of evaluation Eduqas rewards.

Example 2. Ibuprofen synthesis. The modern catalytic route to ibuprofen replaced an older six-step process, raising the atom economy and cutting waste dramatically; it is a standard case study of green-chemistry redesign.

Try this

Q1. State why addition reactions are generally more atom-economical than substitution reactions. [1 mark]

• Cue. In an addition, all the reactant atoms end up in the single product, so the atom economy is $100\%$; a substitution loses atoms as a by-product.

Q2. Calculate the atom economy for producing ethanol by fermentation, $\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$. [2 marks]

• Cue. $\dfrac{2 \times 46.0}{(2 \times 46.0) + (2 \times 44.0)} \times 100 = \dfrac{92.0}{180.0} \times 100 = 51.1\%$.