C26: Sustainable Chemistry
Sustainability, corrosion, life cycle assessment and materials
Sustainability
Sustainability means meeting the needs of the present without compromising the ability of future generations to meet their own needs. In chemistry, this means using resources responsibly and minimising environmental impact.
- Many resources are finite — they will run out if we continue using them at current rates.
- Sustainable development requires us to find ways to use resources more efficiently and find alternatives.
Reducing Use of Resources
The reduce, reuse, recycle principles help conserve finite resources and reduce waste and environmental impact.
| Principle | Meaning | Example |
|---|---|---|
| Reduce | Use less of a resource in the first place | Use less packaging; reduce energy consumption |
| Reuse | Use a product again for the same or a different purpose | Reuse glass bottles; repurpose containers |
| Recycle | Process waste materials to make new products | Recycle aluminium cans, glass, paper, plastics |
Worked Example 1: Benefits of recycling
Question: Explain the environmental benefits of recycling aluminium rather than extracting it from its ore.
Answer: Recycling aluminium: (1) Conserves the finite resource (bauxite ore). (2) Uses only 5% of the energy needed to extract aluminium from its ore, so fewer fossil fuels are burned and less CO₂ is released. (3) Reduces waste sent to landfill. (4) Reduces the environmental impact of mining (habitat destruction, pollution).
Corrosion and Its Prevention
Corrosion is the deterioration of metals by reaction with substances in the environment. Rusting is the corrosion of iron, which requires both oxygen and water. Corrosion weakens metals and can be dangerous and expensive.
Iron + Oxygen + Water → Hydrated iron(III) oxide (rust)
Both oxygen and water must be present for iron to rust. If either is absent, rusting does not occur.
| Prevention Method | How It Works | Example |
|---|---|---|
| Painting | Creates a barrier between the metal and air/water | Car bodies, bridges, garden gates |
| Oiling / greasing | Creates a water-repellent barrier | Bicycle chains, machine parts |
| Galvanising | Coating iron/steel with a layer of zinc. Zinc acts as a barrier and also provides sacrificial protection | Galvanised buckets, fencing, roofing |
| Sacrificial protection | Attaching a more reactive metal (e.g. zinc or magnesium) that corrodes instead of the iron | Ship hulls, underground pipes, bridge supports |
Sacrificial protection works because a more reactive metal (e.g. zinc, magnesium) will oxidise in preference to iron. The more reactive metal sacrifices itself to protect the iron.
Worked Example 2: Explaining corrosion prevention
Question: Explain how galvanising prevents iron from rusting, even if the zinc layer is scratched.
Answer: Galvanising coats iron with zinc. The zinc acts as a barrier, preventing oxygen and water from reaching the iron. If the zinc is scratched, the exposed iron is still protected because zinc is more reactive than iron. The zinc oxidises (corrodes) in preference to the iron — this is sacrificial protection. The zinc sacrifices itself, so the iron does not rust.
Worked Example 3: Conditions for rusting
Question: A student places iron nails in three test tubes: (A) with only boiled water (no oxygen), (B) with only dry air (no water), (C) with air and water. Which nail rusts and why?
Answer: Only nail C rusts. Rusting requires both oxygen and water. Tube A has water but no oxygen (boiled water removes dissolved oxygen, plus a layer of oil prevents air dissolving). Tube B has oxygen but no water. Only tube C has both, so only nail C rusts.
Life Cycle Assessment (LCA)
A life cycle assessment (LCA) assesses the environmental impact of a product at each stage of its life: raw materials, manufacturing, using, and disposing.
| LCA Stage | What Is Assessed | Example (Plastic Bag) |
|---|---|---|
| Raw materials | Extraction, processing, transport; energy use and pollution | Crude oil extracted and refined (finite resource, energy, CO₂) |
| Manufacturing | Energy, waste, emissions during production | Polymerisation, shaping into bags (energy, waste) |
| Using | Environmental impact during the product lifetime | Lightweight, used once — low energy in use |
| Disposing | Landfill, incineration or recycling; pollution | Non-biodegradable → landfill; or recycled (energy) |
Worked Example 4: Comparing products using LCA
Question: Use LCA to compare the environmental impact of paper bags and plastic bags.
Answer: Paper bags: Raw materials — trees (renewable but deforestation risk); manufacturing — uses lots of energy, water and chemicals; using — reusable but less durable; disposing — biodegradable and recyclable. Plastic bags: Raw materials — crude oil (finite resource); manufacturing — less energy than paper; using — lightweight, reusable, very durable; disposing — non-biodegradable, persists in landfill for hundreds of years, can be recycled. Overall: plastic bags use less energy to make and are more durable, but cause more long-term waste problems. Paper bags are from renewable sources and biodegradable, but require more energy and water to produce.
Alloys
An alloy is a mixture of a metal with one or more other elements. Alloys are harder than pure metals because the different-sized atoms disrupt the regular structure, preventing layers from sliding over each other.
- Steel = iron + carbon — harder and stronger than pure iron; used for construction, tools, cars.
- Stainless steel = iron + carbon + chromium — resistant to corrosion; used for cutlery, surgical instruments.
- Brass = copper + zinc — used for musical instruments, fittings.
- Bronze = copper + tin — used for statues, bearings.
Worked Example 5: Why alloys are harder
Question: Explain why steel is harder than pure iron.
Answer: In pure iron, all atoms are the same size and arranged in regular layers that can slide over each other easily, making the metal soft and malleable. In steel, carbon atoms are added which are a different size. These disrupt the regular arrangement of iron atoms, preventing the layers from sliding, so the alloy is harder and stronger.
Ceramics, Composites and Polymers
| Material | Description | Properties | Examples |
|---|---|---|---|
| Ceramics | Non-metal solids made by heating substances (e.g. clay) to high temperatures | Hard, brittle, high melting points, poor conductors | Pottery, bricks, glass, porcelain |
| Composites | Mix of two materials: a matrix (binder) + reinforcement (fibres/particles) | Strong, lightweight; properties depend on components | Fibreglass (glass fibres in polymer), carbon fibre, concrete |
| Polymers | Long-chain molecules made from monomers | Lightweight, flexible, range of properties | Polyethene, PVC, PET |
Phosphate Rock and Fertilisers
Phosphate rock is a finite resource used to make phosphate fertilisers. It is mined and treated with acid to make it soluble so plants can absorb the nutrients.
- Phosphate rock is insoluble, so it is treated with sulfuric acid to produce calcium phosphate (single superphosphate) — which is soluble.
- Treating with nitric acid produces ammonium nitrate — a nitrogen-rich fertiliser.
- Since phosphate rock is finite, sustainable alternatives must be found.
Alternative Industrial Methods and Green Chemistry
Green chemistry aims to reduce the environmental impact of industrial processes. This includes using renewable raw materials, reducing energy use, minimising waste, and finding cleaner reactions.
- Using renewable raw materials instead of fossil fuels (e.g. plant-based feedstocks).
- Haber process: producing hydrogen from renewable electricity (electrolysis of water) instead of from natural gas, making ammonia production more sustainable.
- Using catalysts to reduce energy requirements and increase yields.
- Designing processes with atom economy in mind — maximising the proportion of reactant atoms that end up in useful products.
Worked Example 6: Sustainable Haber process
Question: The Haber process uses hydrogen from natural gas. Explain how using hydrogen from renewable electricity could make the process more sustainable.
Answer: Currently, hydrogen for the Haber process comes from natural gas (methane), a finite fossil fuel, and the process releases CO₂. Using electrolysis of water powered by renewable electricity (wind, solar) would produce hydrogen without burning fossil fuels or releasing CO₂. This would make the Haber process more sustainable because: (1) it uses a renewable energy source instead of a finite one, and (2) it reduces greenhouse gas emissions.
Practice Questions
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Q1. Define sustainability. Explain how the principles of reduce, reuse and recycle help achieve sustainable development.
Show Answer
Sustainability means meeting current needs without compromising the ability of future generations to meet their needs. Reduce means using less resources (e.g. less packaging), reuse means using products again (e.g. refilling bottles), and recycle means processing waste into new products (e.g. recycling aluminium). Together, they conserve finite resources, reduce waste, and lower energy use and pollution, making development more sustainable.
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Q2. What conditions are needed for iron to rust? Describe three methods of preventing corrosion and explain how each works.
Show Answer
Iron needs both oxygen and water to rust. Prevention methods: (1) Painting — creates a barrier preventing oxygen and water reaching the metal. (2) Galvanising — coating with zinc, which acts as a barrier and provides sacrificial protection. (3) Sacrificial protection — attaching a more reactive metal (e.g. zinc, magnesium) which corrodes instead of the iron.
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Q3. Explain why a more reactive metal such as zinc can be used for sacrificial protection of iron.
Show Answer
Zinc is more reactive than iron, meaning it loses electrons (oxidises) more readily. When both metals are in contact with oxygen and water, the zinc corrodes in preference to the iron. The zinc sacrifices itself — it is oxidised instead of the iron, protecting the iron from rusting. As long as some zinc remains in electrical contact with the iron, the iron will not rust.
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Q4. What are the four stages of a life cycle assessment? For each stage, give one environmental impact of a plastic drink bottle.
Show Answer
(1) Raw materials — crude oil extraction (finite resource, habitat damage, CO₂). (2) Manufacturing — energy for polymerisation and moulding (CO₂, waste). (3) Using — transporting filled bottles (fuel, CO₂). (4) Disposing — landfill (non-biodegradable, persists for hundreds of years) or recycling (energy use).
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Q5. Explain why steel is harder than pure iron. Name two other alloys and state what they are made from.
Show Answer
Steel is harder because carbon atoms (different size) disrupt the regular layers of iron atoms. This prevents the layers from sliding over each other easily, making the alloy harder. Two other alloys: brass (copper + zinc) and bronze (copper + tin). Also acceptable: stainless steel (iron + carbon + chromium).
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Q6. Explain how the Haber process could be made more sustainable by using renewable energy. Suggest one other way to make an industrial process more sustainable.
Show Answer
The Haber process could be made more sustainable by using hydrogen from electrolysis powered by renewable electricity (wind, solar) instead of from natural gas. This would eliminate CO₂ emissions from hydrogen production. Another way to make an industrial process more sustainable: use catalysts to reduce energy requirements; use renewable raw materials instead of fossil fuels; improve atom economy to reduce waste; or use carbon capture and storage to reduce emissions.
Exam tip: For corrosion questions, always state that both oxygen AND water are needed for rusting. When explaining sacrificial protection, emphasise that the protecting metal must be more reactive than iron. For LCA questions, go through all four stages systematically. When comparing materials (alloys, ceramics, composites, polymers), link properties to uses — e.g. ceramics are hard and brittle so used for tiles; composites are strong and lightweight so used for aircraft.
🔢 Maths Skills
Mathematical Skills
You need to calculate atom economy, interpret data on energy use for recycling vs extraction, and compare environmental impacts quantitatively.
Maths Example
Extracting 1 kg of aluminium from ore requires 200 MJ of energy. Recycling 1 kg of aluminium requires only 5% of this energy. Calculate the energy saved by recycling 1000 kg of aluminium.
Energy for extraction = 200 × 1000 = 200,000 MJ. Energy for recycling = 200 × 0.05 × 1000 = 10,000 MJ. Energy saved = 200,000 − 10,000 = 190,000 MJ
Maths Example 2
In the reaction: N₂ + 3H₂ → 2NH₃, calculate the atom economy if NH₃ is the desired product. (Aᵣ: N=14, H=1)
Mᵣ of NH₃ = 14 + 3 = 17. Total Mᵣ of products = 2 × 17 = 34. Atom economy = (34 ÷ 34) × 100 = 100% (no waste products — all atoms end up in the desired product).
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Rusting only needs oxygen Correct: Rusting requires BOTH oxygen AND water — if either is absent, iron will not rust
2. Wrong: Galvanising only works as a barrier Correct: Galvanising provides BOTH a barrier AND sacrificial protection — even if the zinc layer is scratched, the zinc corrodes in preference to the iron
3. Wrong: Recycling always uses less energy than extracting from raw materials Correct: Recycling almost always uses less energy, but it still requires energy for collection, sorting, cleaning and reprocessing — it is not free
4. Wrong: Alloys are compounds Correct: Alloys are MIXTURES, not compounds — the different elements are mixed together but not chemically bonded in fixed ratios
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how rusting occurs, describe three methods of preventing corrosion, and evaluate which method is best for protecting a ship's hull.
Rusting is the corrosion of iron, which requires both oxygen and water. The iron reacts with oxygen and water to form hydrated iron(III) oxide (rust), which is flaky and weakens the metal. Three methods of preventing corrosion are: painting, which creates a barrier between the metal and air/water; galvanising, which coats the iron with zinc providing both a barrier and sacrificial protection; and sacrificial protection, where a more reactive metal (zinc or magnesium) is attached to the iron and corrodes instead. For a ship's hull, sacrificial protection is the best method. Painting is impractical because it would need constant repainting on the underwater surface and any scratches would expose the iron. Galvanising is difficult to apply to large steel structures. Sacrificial protection using zinc or magnesium blocks bolted to the hull works because these metals are more reactive than iron and will corrode in its place. When the blocks wear away, they can be replaced, making this a practical and effective long-term solution.
Mark scheme: 1 mark for rusting needs oxygen AND water; 1 mark for painting as barrier; 1 mark for galvanising (barrier + sacrificial); 1 mark for sacrificial protection explained; 1 mark for evaluating why sacrificial is best for ships; 1 mark for practical reason (replaceable blocks, hard to paint underwater)
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A company is choosing between two types of water pipe: copper and plastic (PVC).
- Copper pipe: made from copper ore (finite resource); manufacturing uses 50 MJ/m; lasts 80 years; can be recycled; costs £15/m
- PVC pipe: made from crude oil (finite resource); manufacturing uses 8 MJ/m; lasts 50 years; cannot be easily recycled; costs £4/m
(a) Compare the environmental impact of each pipe using the principles of life cycle assessment.
(b) A builder claims that PVC pipe is the more sustainable choice. Evaluate this claim.
(c) Suggest how the sustainability of copper piping could be improved.
Answers: (a) Raw materials: both use finite resources (copper ore vs crude oil). Manufacturing: PVC uses much less energy (8 vs 50 MJ/m), so less CO₂ from energy. Using: both work well but copper lasts longer (80 vs 50 years). Disposing: copper can be recycled, reducing future resource need; PVC cannot be easily recycled and may end up in landfill. (b) The builder's claim is partially correct — PVC uses less energy to make and costs less. However, sustainability considers the whole life cycle. Copper lasts longer (fewer replacements needed) and is recyclable, meaning less waste and less need for new raw materials over time. PVC ends up in landfill after 50 years. Over a long period, copper may actually be more sustainable despite higher initial energy and cost. (c) Copper piping sustainability could be improved by: using recycled copper (uses only 15% of the energy of extraction); improving manufacturing efficiency; designing pipes to be easily removed and recycled at end of life.
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