Green chemistry, atom economy and percentage yield, sustainable feedstocks and energy, the carbon footprint of processes, and the social and economic impact of chemical manufacture.

What this dot point is asking

WJEC wants you to evaluate the impact of chemical processes using green-chemistry ideas: atom economy and percentage yield, sustainable feedstocks and energy, carbon footprint, and the social and economic consequences of manufacture.

The answer

Green chemistry, yield and atom economy

Sustainable feedstocks and energy

A sustainable feedstock is renewable or recyclable rather than depleting (for example bioethanol from fermentation rather than from crude-oil-derived ethene). Sustainable energy sources lower the carbon footprint of a process, the total greenhouse gas emitted across its life cycle.

Social and economic impact

Chemical manufacture brings benefits (medicines, fertilisers, materials, jobs) and costs (pollution, resource depletion, hazards). Decisions weigh economic viability, regulation and sustainability together.

Why atom economy matters beyond yield

Percentage yield and atom economy answer different questions, and a process can score well on one but badly on the other. Yield asks how much of the theoretical product you actually obtained, which depends on the reaction reaching completion and on losses in handling. Atom economy is fixed by the equation: it asks what fraction of the reactant atoms end up in the desired product, regardless of yield. A substitution reaction with a 90 percent yield can still have a low atom economy if it produces a bulky by-product. Green chemistry favours high atom economy because the wasted atoms cost money to buy and to dispose of.

Sustainability trade-offs

Real decisions weigh several factors at once: the source of the feedstock (renewable or finite), the energy required (and whether it is low-carbon), the toxicity and disposal of by-products, and the economic cost. A renewable feedstock is not automatically the greenest choice if processing it consumes large amounts of energy. Evaluating these competing factors, rather than reciting a single rule, is what extended-answer questions on the wider impact of chemistry reward.

Examples in context

Bioethanol versus hydration of ethene. Bioethanol uses a renewable feedstock (sugar) but is energy-intensive to distil; ethene hydration is 100 percent atom-economic but uses a fossil feedstock, illustrating the trade-offs in real decisions. Catalysts and green chemistry. Replacing stoichiometric reagents with catalysts reduces waste and improves atom economy, a principle driving cleaner industrial processes.

Try this

Q1. State what atom economy measures. [1 mark]

• Cue. The fraction of reactant mass that ends up in the desired product.

Q2. State why addition reactions tend to have a 100 percent atom economy. [1 mark]

• Cue. They form a single product, so no atoms are wasted as by-products.

Q3. Calculate the percentage yield if $4.0$ g of product is obtained when the theoretical maximum is $5.0$ g. [1 mark]

• Cue. $(4.0/5.0) \times 100 = 80$ percent.

Q4. Explain why a high yield does not guarantee a high atom economy. [1 mark]

• Cue. Yield measures how much product forms, but a reaction can still waste atoms as bulky by-products, lowering atom economy.

Q5. State one reason a renewable feedstock might still not be the greenest option. [1 mark]

• Cue. Processing it may consume large amounts of energy, raising the overall carbon footprint.