Green chemistry principles: WACE Year 12 Chemistry

Syllabus dot point

“Describe the principles of green chemistry and evaluate syntheses against them”

8 min readUpdated 2 Jun 2026

Quick answer

Green chemistry is the design of chemical processes that reduce or eliminate hazardous substances and waste. Its principles include preventing waste, maximising atom economy, using safer solvents and reagents, using renewable feedstocks, using catalysts, and designing for energy efficiency and degradability. You evaluate a synthesis by judging it against these principles, often using atom economy and yield as measures.

What this dot point is asking

Green chemistry is an approach to designing chemical products and processes that minimises their environmental impact. Rather than cleaning up pollution after the fact, it aims to prevent it at the design stage.

Green chemistry

The design of chemical processes and products that reduce or eliminate the generation and use of hazardous substances, while improving efficiency in materials and energy.

The guiding principles

The principles relevant at WACE level can be grouped as follows:

Prevent waste: it is better to prevent waste than to treat or clean it up afterwards.
Maximise atom economy: design reactions so most of the reactant atoms end up in the product, favouring addition reactions over those with large by-products.
Use safer solvents and reagents: avoid toxic, volatile or hazardous chemicals; water is often a preferred solvent.
Use renewable feedstocks: prefer raw materials from plants and biomass over finite fossil resources.
Use catalysts: catalysts speed reactions and lower energy use, and are not consumed, reducing waste compared with stoichiometric reagents.
Design for energy efficiency: run reactions at ambient temperature and pressure where possible to save energy.
Design for degradation: make products that break down harmlessly after use rather than persisting in the environment.

Atom economy is the chemistry link

Atom economy is the principle most directly assessed quantitatively. A synthesis route with high atom economy (such as an addition reaction) wastes fewer atoms than one that produces large by-products (such as a substitution or condensation), so it is greener even before considering solvents and energy.

Evaluating a synthesis

To evaluate a synthesis against green chemistry you consider several questions together: How much waste is produced (atom economy and by-products)? Are the solvents and reagents safe and recoverable? Is the feedstock renewable? Does it use a catalyst? How much energy does it need? A route can score well on one principle and poorly on another, so judgement involves weighing the trade-offs, not just one number.

Comparing two routes to a product

Question

A compound can be made by an addition reaction with no by-product, or by a substitution reaction that releases hydrogen bromide. Using green chemistry principles, evaluate which route is preferable.

Solution

The addition route incorporates all reactant atoms into the product, so it has a high atom economy and produces no by-product to treat or dispose of. The substitution route releases hydrogen bromide, a corrosive and hazardous by-product, lowering its atom economy and creating waste that must be handled safely.

By the principles of preventing waste and maximising atom economy, the addition route is preferable. It also likely needs less treatment of by-products, supporting safer and more efficient processing.

Trade-offs and the limits of a single measure

The best answers recognise that no single number settles whether a process is green. A route with $100\%$ atom economy is wasteful if it runs at very high temperature and pressure (large energy cost) or relies on a hazardous, non-renewable solvent. Conversely, a slightly lower atom economy can be acceptable if the by-product is harmless and recyclable, the solvent is water, and the reaction proceeds at room temperature with a catalyst. Catalysis is especially valuable because a catalyst is not consumed, lowers the activation energy (saving energy), and can improve selectivity so fewer unwanted by-products form. Evaluating a synthesis therefore means weighing atom economy, energy, solvent safety, feedstock renewability and product degradability together, and explaining the dominant factor for the specific reaction in front of you.

Why this matters

Green chemistry connects the technical content of Unit 4 (reaction types, atom economy, catalysis) to real decisions about sustainability and cost in the chemical industry. Examination questions often ask you to justify the choice of one synthesis over another using these principles, so you must link the chemistry to the principle explicitly.

Listing principles without applying them

The common error is reciting green chemistry principles in the abstract without connecting them to the specific reaction in the question. To earn the marks you must apply a principle, for example explaining that an addition route is greener because it has higher atom economy and no by-product. Another slip is treating green chemistry as only about recycling waste, when its core idea is preventing waste by better design.

Exam-style questions

Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

WACE 2022-style6 marks

Two industrial routes make the same product. Route A is a substitution with

68\%

atom economy that uses a chlorinated solvent and runs at

250\ ^\circ\text{C}

. Route B is a catalytic addition with

100\%

atom economy that uses water as solvent at

60\ ^\circ\text{C}

. (a) Identify three green-chemistry principles that favour route B. (b) Despite route B's advantages, route A is sometimes still chosen. Suggest one reason. (c) Calculate the mass of waste atoms per

100\ \text{g}

of reactants for route A.

Show worked answer →

A 6 mark question rewards three applied principles, a balanced caveat, and a waste calculation.

(a) Route B is favoured by: (i) higher atom economy ( $100\%$ versus $68\%$ ), so it wastes no atoms; (ii) safer solvent (water replacing a toxic chlorinated solvent); and (iii) lower energy use (running at $60\ ^\circ\text{C}$ rather than $250\ ^\circ\text{C}$ ), plus the use of a catalyst rather than stoichiometric reagents.

(b) Route A may still be chosen if, for example, the starting materials are much cheaper or more readily available, the product is obtained at higher yield or purity, or the catalyst for route B is expensive or quickly poisoned. Green principles must be weighed against economic and practical factors.

(c) With $68\%$ atom economy, $68\ \text{g}$ of every $100\ \text{g}$ of reactants ends up in the product, so the waste atoms amount to $100 - 68 = 32\ \text{g}$ per $100\ \text{g}$ of reactants.

Markers reward three correctly applied principles, a sensible economic/practical caveat, and $32\ \text{g}$ of waste.

WACE 2020-style4 marks

Explain the difference between percentage yield and atom economy, and explain why a reaction with a high percentage yield may still be considered poor from a green-chemistry viewpoint.

Show worked answer →

A 4 mark answer needs both definitions and the link to green chemistry.

Percentage yield measures how much product was actually obtained compared with the theoretical maximum from the limiting reagent; it reflects practical losses, side reactions and incomplete reactions. Atom economy measures what proportion of the total reactant mass is built into the desired product by the balanced equation itself, regardless of how efficiently the reaction is run.

A reaction can have a high percentage yield (almost all the possible product is collected) yet a low atom economy if the equation inherently produces large by-products. From a green-chemistry viewpoint that is poor, because a large fraction of the reactant atoms becomes waste that must be treated or disposed of, even when little product is lost in practice.

Markers reward both definitions, the distinction (actual versus inherent), and the by-product-waste reason.

Practise this

Sources & how we know this

Chemistry ATAR course: syllabus and support materials — SCSA
Chemistry: past ATAR course examinations — SCSA