Fuels, combustion and energy content
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Question
What is a fuel?
Answer
A substance that releases useful **energy** when it is **burned** (combusted) in oxygen.
Question
What is combustion?
Answer
The reaction of a fuel with **oxygen** that releases energy as heat; it is always **exothermic** (ΔH < 0).
Question
Products of complete combustion of a hydrocarbon?
Answer
**Carbon dioxide (CO_{2}) and water (H_{2}O)** only — with maximum energy released.
Question
Products of incomplete combustion?
Answer
**Carbon monoxide (CO) and/or carbon (soot)** plus water — less energy is released.
Question
When does incomplete combustion happen?
Answer
When there is a **limited supply of oxygen**, so the carbon is not fully oxidised.
Question
Why is carbon monoxide dangerous?
Answer
CO is a **toxic** gas that binds to haemoglobin, stopping the blood from carrying oxygen.
Question
What is specific energy?
Answer
Energy released **per unit mass** of fuel (e.g. **kJ g⁻¹**) — matters when weight is important.
Question
What is energy density?
Answer
Energy released **per unit volume** of fuel (e.g. **kJ cm⁻³**) — matters when storage space is important.
Question
Fossil fuels vs biofuels — renewable?
Answer
Fossil fuels (coal, oil, gas) are **non-renewable**; biofuels (e.g. ethanol, biodiesel) are **renewable**.
Question
Why are biofuels near carbon-neutral?
Answer
The crop **absorbs CO_{2}** as it grows, roughly balancing the CO_{2} released when the fuel is burned.
Question
Why do fossil fuels raise net CO_{2}?
Answer
They release carbon that was **locked away for millions of years**, adding **new** CO_{2} to the atmosphere.
Question
Give an example of a biofuel.
Answer
**Ethanol** (from fermented sugar cane/corn) or **biodiesel** (from plant oils).
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Topic 4.3 hub
Energy from fuels
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