AQA GCSE Chemistry 8462: Using resources – Study Guide

This study guide teaches Topic 10, Using resources (sections 4.10.1 to 4.10.4), of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 with GCSE exams from June 2018 (version 1.1). The topic is assessed on Paper 2, which can also draw on sections 4.1 to 4.3. Both tiers sit this topic. Two parts are Higher tier only: alternative methods of extracting metals (4.10.1.4) and the equilibrium and conditions work on the Haber process (part of 4.10.4.1). They are labelled below.

Use it with the Using resources revision notes and the Using resources practice questions. The course hub is AQA GCSE Chemistry and the printable checklist lists every statement. The previous topic is Chemistry of the atmosphere.

What this topic covers

Spec	What you must be able to do	Tier
4.10.1.1	Define sustainable development; distinguish finite and renewable resources; interpret resource data	Both
4.10.1.2	Distinguish potable and pure water; describe treatment of fresh and salty water; Required practical 8	Both
4.10.1.3	Describe sewage treatment; compare obtaining potable water from waste, ground and salt water	Both
4.10.1.4	Phytomining and bioleaching; evaluate biological extraction	Higher tier only
4.10.2.1–4.10.2.2	Interpret and carry out simple LCAs; evaluate reuse and recycling	Both
4.10.3.1–4.10.3.3	Corrosion and its prevention; alloys; glass, ceramics, polymers, composites	Both
4.10.4.1	Haber process: raw materials, conditions, recycling	Both
4.10.4.1	Equilibrium, rate and cost trade-offs in the Haber process	Higher tier only
4.10.4.2	NPK fertilisers; salts from phosphate rock; industrial vs laboratory production	Both

4.10.1 Earth’s resources and water

Sustainable development (4.10.1.1)

Humans use the Earth’s resources for warmth, shelter, food and transport. Finite resources (crude oil, metal ores) will run out. Renewable resources (timber, crops) can be replaced as fast as they are used. Natural products are often supplemented or replaced by agricultural and synthetic ones: synthetic fibres such as polyester alongside wool and cotton, synthetic rubber alongside natural rubber, and fertilisers boosting natural soil nutrients.

Sustainable development meets the needs of current generations without compromising the ability of future generations to meet their own needs.

Potable water (4.10.1.2)

Potable water is safe to drink: it has low levels of dissolved salts and microbes. It is not pure in the chemical sense, because it contains dissolved substances. Pure water contains only H₂O.

In the UK, rain provides fresh water in the ground, lakes and rivers. Most potable water is produced by:

1. choosing an appropriate source of fresh water
2. passing it through filter beds to remove solids
3. sterilising it with chlorine, ozone or ultraviolet light to kill microbes.

Where fresh water is scarce, desalination of salty water or sea water is used: distillation or membrane processes such as reverse osmosis. Both need large amounts of energy, so they are expensive.

Required practical 8 analyses and purifies water samples. You test pH (universal indicator or a pH meter), measure dissolved solids by evaporating a known volume and weighing the residue, and distil a sample. Pure distilled water has pH 7 and boils at 100 °C.

Worked example 1. 20.0 cm³ of tap water is evaporated to dryness. The residue has a mass of 0.012 g. Calculate the mass of dissolved solids per dm³.

20.0 cm³ = 0.0200 dm³
dissolved solids = 0.012 g ÷ 0.0200 dm³ = 0.60 g/dm³

Waste water treatment (4.10.1.3)

Sewage and agricultural waste water need organic matter and harmful microbes removed. Industrial waste water may also need harmful chemicals removed. Sewage treatment:

1. screening and grit removal
2. sedimentation, producing sewage sludge and effluent
3. anaerobic digestion of the sludge
4. aerobic biological treatment of the effluent.

Relative ease: ground water is easiest (filter and sterilise). Waste water needs more stages. Salt water needs desalination, which uses the most energy.

Alternative methods of extracting metals (4.10.1.4) – Higher tier only

Copper ores are becoming scarce. Low-grade ores can be used by:

• phytomining: plants absorb metal compounds; the plants are harvested and burned; the ash contains metal compounds
• bioleaching: bacteria produce leachate solutions containing metal compounds.

Copper is then obtained by displacement using scrap iron (Fe + CuSO₄ → FeSO₄ + Cu) or by electrolysis. These methods avoid digging, moving and disposing of large amounts of rock. They are slow, though, and phytomining needs land.

4.10.2 Life cycle assessment and recycling

Life cycle assessment (4.10.2.1)

An LCA assesses the environmental impact of a product at each stage:

1. extracting and processing raw materials
2. manufacturing and packaging
3. use and operation during its lifetime
4. disposal at the end of its useful life

including transport and distribution at each stage. Use of water, resources, energy and some wastes is fairly easy to quantify. Giving numbers to pollutant effects needs value judgements, so an LCA is not purely objective. Selective or shortened LCAs can be misused, for example to support advertising claims.

Worked example 2. A glass drinks bottle needs 7.5 MJ of energy over its life and is used 20 times. A plastic bottle needs 1.5 MJ and is used once. Compare the energy per use.

glass: 7.5 ÷ 20 = 0.375 MJ per use
plastic: 1.5 ÷ 1 = 1.5 MJ per use
glass must be used at least 7.5 ÷ 1.5 = 5 times to match plastic

So on energy alone, the glass bottle is better if it is reused more than 5 times. A full comparison also needs water use, wastes and pollution. You must also be able to compare plastic and paper shopping bags in the same way.

Reducing the use of resources (4.10.2.2)

Reduce, reuse and recycle cuts the use of limited resources, energy, waste and environmental impacts. Metals, glass, building materials, clay ceramics and most plastics come from limited raw materials, and quarrying and mining damage the environment.

• Reuse: glass bottles are washed and refilled.
• Recycle: glass is crushed and melted into new glass products; metals are melted and recast.
• The amount of separation needed depends on the material and the final product. Some scrap steel can be added to iron from a blast furnace, reducing the iron that must be extracted from ore.

4.10.3 Using materials

Corrosion and its prevention (4.10.3.1)

Corrosion is the destruction of materials by chemical reactions with substances in the environment. Rusting is the corrosion of iron, and needs both air and water.

Test it with three iron nails:

Tube	Conditions	Result
A	Air and water	Rusts
B	Boiled water (air removed) with a layer of oil on top	No rust – no air
C	Dry air with a drying agent such as calcium chloride	No rust – no water

Prevention:

• Barrier coatings: grease, paint, electroplating. Aluminium has a natural oxide layer that protects it.
• Sacrificial protection: a more reactive metal, such as zinc in galvanising, reacts with air and water instead of the iron. It protects the iron even if the coating is scratched.

Alloys (4.10.3.2)

Alloy	Made of	Use
Bronze	Copper and tin	Statues, medals
Brass	Copper and zinc	Door handles, musical instruments
Gold alloy	Gold with silver, copper, zinc	Jewellery
High carbon steel	Iron, more carbon	Strong but brittle: cutting tools
Low carbon steel	Iron, less carbon	Softer, easily shaped: car bodies
Stainless steel	Iron with chromium and nickel	Hard, corrosion-resistant: cutlery
Aluminium alloys	Aluminium with other metals	Low density: aircraft

Gold purity is measured in carats: 24 carat is 100% gold, 18 carat is 75%. So the percentage of gold = carat ÷ 24 × 100.

Worked example 3. A 14 carat gold chain has a mass of 6.0 g. Calculate the mass of gold.

fraction of gold = 14/24
mass of gold = 14/24 × 6.0 = 3.5 g

Ceramics, polymers and composites (4.10.3.3)

• Soda-lime glass: heat sand, sodium carbonate and limestone. Borosilicate glass: sand and boron trioxide; melts at a higher temperature.
• Clay ceramics (pottery, bricks): shape wet clay, then heat in a furnace.
• LD and HD poly(ethene) are both made from ethene, under different conditions. Low density forms at very high pressure with a trace of oxygen; high density forms at lower temperature and pressure with a catalyst. The chains differ, so the properties differ.
• Thermosoftening polymers have separate chains held by weak intermolecular forces, so they melt when heated. Thermosetting polymers have cross-links between chains, so they do not melt.
• Composites: a matrix (binder) surrounds reinforcement fibres or fragments. Examples: reinforced concrete, fibreglass, carbon fibre composites, wood.

You may be given data to compare glass, ceramics, polymers, composites and metals, and asked to select a material. Match each property to the job: density, strength, brittleness, melting point, cost.

4.10.4 The Haber process and NPK fertilisers

The Haber process (4.10.4.1)

Ammonia is made from nitrogen (from the air) and hydrogen (from natural gas). The purified gases pass over an iron catalyst at about 450 °C and about 200 atmospheres.

nitrogen + hydrogen ⇌ ammonia
N₂ + 3H₂ ⇌ 2NH₃

The reaction is reversible, so only some of the gases react. On cooling, ammonia liquefies and is removed. Unreacted nitrogen and hydrogen are recycled.

Higher tier only – conditions and equilibrium

• The forward reaction is exothermic. A lower temperature moves the equilibrium right, giving a higher yield, but the rate is too slow. About 450 °C is a compromise between yield and rate.
• There are 4 molecules of gas on the left and 2 on the right. Higher pressure moves the equilibrium right and also raises the rate. But high pressure needs strong equipment and lots of energy, so it is expensive. About 200 atmospheres balances yield against cost and safety.
• The iron catalyst increases the rate. It does not change the position of equilibrium.
• Recycling unreacted gases means the raw materials are not wasted.

You may be asked to read graphs of percentage yield or rate against temperature and pressure.

NPK fertilisers (4.10.4.2)

Compounds of nitrogen, phosphorus and potassium improve agricultural productivity. NPK fertilisers are formulations of salts that contain all three elements in suitable percentages.

• Ammonia makes ammonium salts and nitric acid. For example, ammonia + nitric acid → ammonium nitrate.
• Potassium chloride, potassium sulfate and phosphate rock are mined. Phosphate rock is insoluble, so it cannot be used directly.
• Phosphate rock + nitric acid → phosphoric acid and calcium nitrate.
• Phosphate rock + sulfuric acid → single superphosphate (calcium phosphate and calcium sulfate).
• Phosphate rock + phosphoric acid → triple superphosphate (calcium phosphate).

Worked example 4. Calculate the percentage by mass of nitrogen in ammonium sulfate, (NH₄)₂SO₄. (Aᵣ: N = 14, H = 1, S = 32, O = 16)

Mr = 2(14 + 4) + 32 + 4(16) = 132
mass of N = 2 × 14 = 28
% N = 28 ÷ 132 × 100 = 21.2%

Industrial vs laboratory production. In the lab, ammonium sulfate is made in small batches by titrating ammonia solution with dilute sulfuric acid, then crystallising. Industry works continuously on a huge scale, makes its ammonia and sulfuric acid on site in integrated processes, and uses more concentrated reactants.

Common errors

• Calling potable water “pure”. It contains dissolved substances.
• Listing “boiling” as a UK treatment step. The steps are source, filter beds, sterilising.
• Saying rust needs only water. It needs air and water.
• Explaining galvanising as “a coating” only. Sacrificial protection depends on zinc being more reactive than iron.
• Saying thermosetting polymers have “stronger bonds”. The key is cross-links between chains.
• Stating that the catalyst increases the yield of ammonia. It only increases rate.
• Forgetting that unreacted gases are recycled.

Next steps

Go to the revision notes for a condensed version and self-test, then try the practice questions. For a course-wide check, take a free 10-minute diagnostic.

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016, GCSE exams June 2018 onwards, version 1.1, published by AQA – section 4.10 Using resources.