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SAChemistrySyllabus dot point

How can chemical processes be designed to reduce waste and environmental impact?

Apply green chemistry principles, including atom economy, to evaluate the efficiency and sustainability of chemical processes.

The principles of green chemistry, how to calculate and interpret atom economy, and how to evaluate the efficiency and sustainability of an industrial process.

Generated by Claude Opus 4.77 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. What green chemistry is
  3. Calculating atom economy
  4. Atom economy vs yield
  5. Evaluating a process

What this dot point is asking

You must know the main green chemistry principles, calculate atom economy, and use it (alongside yield and other factors) to evaluate a process.

What green chemistry is

Green chemistry aims to reduce the environmental impact of making chemicals. The guiding ideas relevant at SACE level include:

  • Prevent waste rather than treat or clean it up afterwards.
  • Maximise atom economy so most reactant atoms end up in the product.
  • Use renewable feedstocks and safer solvents and reagents.
  • Improve energy efficiency (run reactions near room temperature and pressure where possible).
  • Use catalysts rather than large amounts of reagents.
  • Design products that degrade safely after use.

Calculating atom economy

Atom economy=molar mass of desired producttotal molar mass of all reactants×100%\text{Atom economy} = \frac{\text{molar mass of desired product}}{\text{total molar mass of all reactants}} \times 100\%

Use the balanced equation and molar masses. A reaction with a single product (an addition reaction) has 100%100\% atom economy; reactions that also make by-products have less.

Atom economy vs yield

Evaluating a process

A genuine sustainability judgement weighs many factors: atom economy, yield, energy demand, the toxicity and renewability of feedstocks, the hazards of solvents and by-products, and what happens to waste. A process can score well on one measure and poorly on another.

Exam-style practice questions

Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2023 SACE Stage 22 marksNickel(II) sulfide is heated in oxygen-enriched air to produce nickel(II) oxide and the by-product sulfur dioxide. Explain what it means to call sulfur dioxide a by-product rather than a waste product.
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The distinction is whether the substance has a use.

  1. A by-product is a substance produced alongside the main product that can be collected and used (or sold) for another purpose, rather than being discarded.

  2. The sulfur dioxide produced can be captured and used, for example to manufacture sulfuric acid, instead of being released as waste.

  3. Because it has a commercial or further use rather than simply being disposed of, it is correctly called a by-product, not a waste product. One mark for the definition, one for applying it to SO2.

2024 SACE Stage 22 marksHydrogen gas is produced during the electrolysis of a solution containing gallium cations. Explain why this hydrogen gas is classified as a by-product rather than a waste product.
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Recall the by-product versus waste distinction.

  1. The hydrogen is produced in addition to the main product (gallium), but it is not discarded.

  2. As the passage states, the hydrogen can be collected, purified, and sold for use in other industries and processes.

  3. Because it has a commercial use rather than being thrown away, it is a by-product, not a waste product. One mark for the definition, one for applying it to the saleable hydrogen.

2024 SACE Stage 22 marksResearchers found a bacterium that uses the water gas shift reaction to produce H2 at room temperature and standard atmospheric pressure, with Kc = 5 x 10^4. Explain one benefit of using this biological system to produce H2 compared with the commonly used method.
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Compare the conditions and outcomes against the conventional high-temperature process.

  1. The biological system operates at room temperature and standard pressure, so it requires far less energy input (no high temperatures or pressures to maintain), reducing energy costs and the associated greenhouse emissions.

  2. The very large Kc (5 x 10^4) means the reaction strongly favours products, giving a high yield of hydrogen. Either point, clearly explained as a benefit, earns the 2 marks.

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