ZIMSEC O Level · Chemistry · Form 4
Organic compounds contain carbon. A homologous series has the same general formula, similar chemical properties and a gradual change in physical properties.
Fuels
Crude oil is separated by fractional distillation into fractions (refinery gas, petrol, kerosene, diesel, bitumen). Larger molecules have higher boiling points.
Alkanes, CₙH₂ₙ₊₂
Methane, ethane, propane. Saturated (single bonds only). Burn to give CO₂ and H₂O; react with chlorine in sunlight (substitution).
Alkenes, CₙH₂ₙ
Ethene, propene. Unsaturated (contain C=C). Test: decolourise bromine water (orange to colourless). Made by cracking large alkanes. Ethene forms poly(ethene) by addition polymerisation.
Alcohols
Ethanol, C₂H₅OH, made by fermentation of sugar with yeast (anaerobic, about 30 °C) – sugar cane ethanol is blended with petrol in Zimbabwe – or by adding steam to ethene. Used as a fuel and solvent.
Carboxylic acids
Ethanoic acid (vinegar) is a weak acid. Acid + alcohol → ester (sweet-smelling) + water.
Polymers
Addition polymers (poly(ethene), PVC) and condensation polymers (nylon, polyesters). Plastic waste is non-biodegradable and pollutes land and water.
Check yourself
- Give the general formula of the alkenes.
- How would you distinguish ethane from ethene?
- Name the process that makes ethanol from sugar.
Show answers
- CₙH₂ₙ
- Add bromine water: ethene decolourises it; ethane does not.
- Fermentation (by yeast).
Questions about this topic (0)
No questions about this topic yet.
Sign in or create a free account to join in.