Environmental Chemistry • Topic 3 of 3

Green Chemistry & Control

Once we understand pollution, the next step is to prevent and control it. This topic covers waste management strategies and the philosophy of green chemistry.

Strategies to control environmental pollution

The most effective approach follows the order reduce, reuse, recycle — cutting waste at the source is better than treating it afterwards.

  • Segregation of waste: separate biodegradable (vegetable peels, paper) from non-biodegradable (plastic, glass, metal) waste.
  • Treatment of biodegradable waste: composting and the production of biogas (methane) by anaerobic digestion.
  • Recycling: reprocessing paper, glass, metals and many plastics into new products.
  • Sewage and effluent treatment: treating waste water before it is released into rivers, and scrubbing industrial gases (e.g. removing SO2) before they reach the air.

Green chemistry

Green chemistry is the design of chemical processes that prevent pollution by reducing or eliminating the use and generation of hazardous substances. Rather than cleaning up waste afterwards, it avoids producing waste in the first place. Its core ideas include using safer reagents and solvents, designing reactions that are energy-efficient, and using renewable feedstocks.

Atom economy

A central idea is atom economy — the fraction of reactant atoms that end up in the desired product. A reaction with high atom economy wastes few atoms as by-products, so it is greener. It is expressed as:

atom economy (%) = (mass of atoms in desired product ÷ total mass of atoms in all reactants) × 100.

A reaction can have a high yield yet a poor atom economy if much of the reactant mass leaves as unwanted by-products; green chemistry favours reactions that score well on both.

Examples of green chemistry in action

  • Bleaching with hydrogen peroxide: paper and clothes are now bleached with H2O2 (which breaks down to water and oxygen) instead of chlorine, avoiding toxic organochlorine by-products.
  • Replacing harmful solvents: using water or supercritical/liquid carbon dioxide instead of volatile organic solvents.
  • Dry cleaning with liquid CO2: tetrachloroethene (PERC), a suspected carcinogen that contaminates groundwater, is being replaced by liquefied CO2 with a suitable detergent, a far safer cleaning medium.

Sustainable practices

Sustainable development means meeting present needs without compromising future generations: using renewable energy, conserving water, planting trees, and choosing processes that are clean by design. Green chemistry turns these goals into practical chemical choices.

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Solved Examples
1
Worked Example
Define green chemistry and explain how its approach differs from conventional pollution control.
Solution
  1. Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances.
  2. Conventional pollution control treats or cleans up waste after it has been produced (end-of-pipe).
  3. Green chemistry instead prevents waste at the source by choosing safer reagents, solvents and reaction designs.

Answer: Green chemistry prevents pollution at its source by design, whereas conventional control treats waste only after it is generated.

2
Worked Example
Why is bleaching with hydrogen peroxide (H2O2) considered greener than bleaching with chlorine?
Solution
  1. Chlorine bleaching can form toxic and persistent organochlorine by-products that pollute water.
  2. Hydrogen peroxide is an effective oxidising bleach that decomposes to water and oxygen: 2H2O2 → 2H2O + O2.
  3. Its by-products are harmless, so the process generates no hazardous waste.

Answer: H2O2 breaks down into harmless water and oxygen, avoiding the toxic chlorinated by-products of chlorine bleaching.

3
Worked Example
Explain the concept of atom economy and why a reaction with high yield may still be 'not green'.
Solution
  1. Atom economy is the fraction of reactant atoms that end up in the desired product, expressed as a percentage.
  2. Yield only tells us how much of the desired product forms relative to the maximum possible from the limiting reactant.
  3. A reaction can have high yield yet low atom economy if much of the reactant mass leaves as unwanted by-products, which is wasteful and not green.

Answer: Atom economy measures how efficiently reactant atoms become product; a high-yield reaction can still waste atoms as by-products, giving low atom economy.

4
Worked Example
How does dry cleaning with liquid (liquefied) carbon dioxide illustrate green chemistry?
Solution
  1. Traditional dry cleaning uses tetrachloroethene (PERC), a suspected carcinogen that contaminates groundwater.
  2. Green dry cleaning replaces it with liquefied CO2 together with a suitable detergent.
  3. Liquid CO2 is non-toxic, can be recovered and reused, and avoids hazardous solvent waste.

Answer: Replacing the harmful solvent PERC with safe, reusable liquid CO2 prevents toxic solvent pollution, a green-chemistry solution.

5
Worked Example
Describe two strategies for managing solid waste in a way that reduces environmental pollution.
Solution
  1. Segregate waste into biodegradable and non-biodegradable, then compost the biodegradable fraction (or convert it to biogas).
  2. Recycle the non-biodegradable fraction — paper, glass, metals and many plastics — into new products.
  3. Both reduce the volume sent to landfills and recover useful materials and energy.

Answer: Segregating and composting biodegradable waste, and recycling non-biodegradable waste, both cut pollution and recover resources.

6
Worked Example
Give one green-chemistry alternative for each: (i) a volatile organic solvent in a reaction, (ii) chlorine for bleaching paper.
Solution
  1. For volatile organic solvents, use water or supercritical/liquid carbon dioxide as a safer reaction medium.
  2. For chlorine bleaching of paper, use hydrogen peroxide, which decomposes to water and oxygen.
  3. Both substitutions avoid toxic emissions and hazardous by-products.

Answer: (i) Replace volatile organic solvents with water or liquid CO2; (ii) replace chlorine with H2O2 for bleaching.

Key Points

  • Pollution control follows reduce–reuse–recycle; segregate waste, compost biodegradable matter (biogas), and treat sewage and industrial gases before release.
  • Green chemistry designs processes that prevent pollution at source by using safer reagents, solvents and reactions, rather than cleaning up afterwards.
  • Atom economy measures the fraction of reactant atoms ending up in the product; a high yield can still mean poor atom economy if many atoms become by-products.
  • Green examples: bleaching with H2O2 instead of chlorine, replacing organic solvents with water/liquid CO2, and dry cleaning with liquid CO2 instead of PERC.
  • Sustainable development meets present needs without harming future generations, using clean-by-design processes, renewable resources and conservation.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Green chemistry mainly aims to:
Explanation: Green chemistry designs processes that avoid generating hazardous substances in the first place.
Q2.Atom economy of a reaction measures:
Explanation: Atom economy is the proportion of reactant atoms incorporated into the desired product; higher is greener.
Q3.Which of the following is a green-chemistry bleaching agent that breaks down into water and oxygen?
Explanation: H2O2 decomposes to harmless water and oxygen, avoiding toxic chlorinated by-products.
Q4.Liquefied carbon dioxide is used in green dry cleaning to replace:
Explanation: Liquid CO2 with a detergent replaces the suspected carcinogen PERC, a safer and recyclable medium.
Practice more MCQs → 📝 Assignment · 30 marks