Guiding Spaceship Earth: Resources of Materials and Energy — AQA Combined Science: Synergy
This topic covers metal extraction, energy resources, energy conservation and efficiency, life cycle assessment and recycling.
Metal extraction by reduction of oxides
Metals less reactive than carbon can be extracted from their oxides by reduction with carbon (carbon removes the oxygen). For example: iron oxide + carbon → iron + carbon dioxide. This is oxidation and reduction in terms of oxygen.
Metal extraction by electrolysis
Metals more reactive than carbon (e.g. aluminium) are extracted by electrolysis of the molten ore (aluminium oxide mixed with cryolite to lower the melting point). Electrolysis uses a lot of electrical energy, making it expensive.
Metal extraction by biological methods (Higher Tier)
As high-grade ores run out, low-grade ores are used:
- Phytomining — plants absorb metal compounds; burning them gives an ash rich in the metal.
- Bioleaching — bacteria produce solutions containing metal compounds. These have a lower environmental impact than traditional mining but are slow. The metal is obtained from solution by displacement or electrolysis.
Energy resources
- Non-renewable: fossil fuels (coal, oil, gas) and nuclear — reliable, high output, but finite and (fossil fuels) polluting.
- Renewable: solar, wind, hydroelectric, tidal, wave, geothermal, bio-fuel — will not run out and are cleaner, but many are unreliable or have high set-up costs.
You should be able to evaluate resources considering reliability, cost and environmental impact.
Energy conservation and dissipation
Energy is always conserved but, in every transfer, some is dissipated ("wasted"), usually as thermal energy to the surroundings (often through friction), where it spreads out and becomes less useful.
Preventing unwanted energy transfers
- Lubrication reduces friction.
- Thermal insulation reduces heat loss; the rate of heat transfer through walls depends on their thickness and thermal conductivity — thicker walls and lower conductivity reduce heat loss.
Energy efficiency
$$\text{efficiency} = \frac{\text{useful output energy transfer}}{\text{total input energy transfer}}$$ Efficiency is always less than 100% because some energy is dissipated. It can be given as a decimal or percentage.
Life cycle assessment
A life cycle assessment (LCA) evaluates the environmental impact of a product over its whole life: raw materials → manufacture → use → disposal (including transport). Some impacts are easy to quantify (e.g. CO₂); others need value judgements, so LCAs can be biased, e.g. in advertising.
Recycling
Reducing, reusing and recycling conserve finite resources and energy and reduce waste. Recycling metals saves the energy and ore needed to extract new metal; glass can be reused or remelted.
Exam tips
- Match the extraction method to the metal's position relative to carbon in the reactivity series.
- Describe phytomining and bioleaching with advantages and disadvantages.
- Identify where energy is dissipated and how to reduce unwanted transfers.
- Work through the four LCA stages and recognise where value judgements make them subjective.