Energy resources and sustainabilityIB MYP Chemistry: Revision notes

Fossil fuels (coal, crude oil and natural gas) formed from the remains of living things over millions of years. They are non-renewable: we use them far faster than they form.

Burning a hydrocarbon fuel in plenty of oxygen releases energy and makes carbon dioxide and water:

methane + oxygen → carbon dioxide + water

CH₄ + 2O₂ → CO₂ + 2H₂O

• Advantages: cheap, easy to transport and store, and they release a lot of energy per gram.
• Disadvantages: burning them adds carbon dioxide to the atmosphere (enhanced global warming); incomplete burning gives carbon monoxide and soot; some fuels make sulfur dioxide, which causes acid rain.

In a nuclear power station, atoms of uranium are split in a process called nuclear fission. This releases heat, which makes steam to turn turbines and generate electricity.

• Advantages: no carbon dioxide released while generating electricity; a tiny mass of fuel gives a huge amount of energy; a reliable supply day and night.
• Disadvantages: radioactive waste stays dangerous for thousands of years; an accident can release radiation; building and closing the station is very expensive; uranium is a non-renewable resource.

Biofuels are fuels made from plants or animal waste that lived recently. They are renewable because the crops can be regrown.

• Ethanol is made by fermenting sugar from crops such as sugar cane. It is often mixed with petrol.
• Biodiesel is made from vegetable oils (such as palm or rapeseed) or waste cooking oil, and can be used in diesel engines.

As the crops grow they absorb carbon dioxide by photosynthesis, so the net emissions can be lower than from fossil fuels (carbon neutral in the best case). Drawbacks: land and water used for fuel crops cannot grow food, so food prices may rise; clearing forest for plantations releases carbon and destroys habitats.

Hydrogen burns with oxygen (or reacts in a fuel cell) to make only water:

2H₂ + O₂ → 2H₂O

It releases more energy per gram than any other common fuel. But it is a gas of very low density, so it is hard to store; it is flammable; and most hydrogen is made from natural gas, which releases carbon dioxide. Hydrogen made by electrolysis of water using renewable electricity is much cleaner.

Renewable resources such as solar, wind, hydroelectric and geothermal will not run out and release no carbon dioxide when generating electricity. Drawbacks: output depends on weather or location, building them costs a lot, and they can affect land and wildlife.

To compare fuels fairly, find the energy released per gram.

energy per gram = energy per mole ÷ relative formula mass

Worked example: burning 1 mole of ethanol (C₂H₅OH, Mr = 46) releases 1367 kJ.

Energy per gram = 1367 ÷ 46 = 29.7 kJ g⁻¹

Approximate values:

• hydrogen: 286 ÷ 2 = 143 kJ g⁻¹
• methane: 890 ÷ 16 = 55.6 kJ g⁻¹
• octane (petrol): 5470 ÷ 114 = 48.0 kJ g⁻¹
• ethanol: 29.7 kJ g⁻¹

A higher value means less fuel is needed for the same energy, but cost, safety, storage and pollution matter too.

Criterion D questions ask you to weigh social, economic and environmental impacts, then reach a justified conclusion.

• Environmental: carbon dioxide emissions, habitat loss, waste.
• Economic: cost to build, run and clean up; jobs.
• Social: safety, health, and fair access: everyone should be able to afford reliable energy, and fuel crops should not make food too expensive.

Green chemistry (outline) means designing chemical processes to cause less harm: use renewable raw materials, make less waste, use less energy, avoid hazardous substances and recycle materials. The chemical industry supplies fuels, fertilisers and plastics but also causes pollution, so its benefits must be weighed against its harm.