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Flashcards · 7405 · AS Topic 20
Alkanes (AS Level) flashcards
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Topic 20: Alkanes
- How does fractional distillation of crude oil separate it?
- The vapour rises through a column that is hot at the bottom and cool at the top. Each hydrocarbon condenses where the temperature falls below its boiling point; short chains collect near the top.
- Why do longer-chain alkanes have higher boiling points?
- More electrons and more surface contact, so stronger van der Waals forces between molecules, which need more energy to overcome.
- Thermal vs catalytic cracking: conditions and main products
- Thermal: high pressure, high temperature; high percentage of alkenes. Catalytic: slight pressure, high temperature, zeolite catalyst; motor fuels and aromatic hydrocarbons.
- Economic reasons for cracking
- Distillation gives a surplus of long-chain fractions; demand is higher for short-chain fractions such as petrol, which are worth more. Cracking also makes alkenes for polymers and other chemicals.
- Where do NO and SO₂ from engines and power stations come from?
- NO: nitrogen and oxygen from the air reacting at high engine temperature (N₂ + O₂ → 2NO). SO₂: sulfur impurities in the fuel.
- What does a catalytic converter do? Give one equation.
- Converts CO, NOₓ and unburned hydrocarbons into CO₂, N₂ and H₂O. For example 2CO + 2NO → 2CO₂ + N₂.
- Chlorination of methane: conditions and the two propagation steps
- UV light; free-radical substitution. Cl• + CH₄ → •CH₃ + HCl •CH₃ + Cl₂ → CH₃Cl + Cl•
- Initiation and termination steps in the chlorination of methane
- Initiation: Cl₂ → 2Cl• Termination (any two radicals): Cl• + Cl• → Cl₂; •CH₃ + Cl• → CH₃Cl; •CH₃ + •CH₃ → C₂H₆