Alkenes

O Level / IGCSE Chemistry · Organic chemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Alkenes are hydrocarbons that contain a carbon–carbon double bond, C=C. Because of this bond they are called unsaturated. Their general formula is CnH2n.

Alkenes are made by cracking. Large alkane molecules are passed over a catalyst at a high temperature and break into smaller alkanes, alkenes and sometimes hydrogen. Cracking is done because there is more demand for short-chain fuels such as petrol than for long-chain fractions, and because alkenes are needed to make polymers and other chemicals.

The double bond lets alkenes do addition reactions. The C=C opens, the two new atoms join on, and only one product is formed.

Rules to remember

Common mistake

Students write that bromine water goes 'clear' with an alkene. The correct word is colourless: the change is orange to colourless.

Worked example

One molecule of dodecane, C12H26, is cracked to give one molecule of hexane, C6H14, and molecules of ethene only. How many ethene molecules form?

  1. Carbon atoms left after hexane = 12 − 6 = 6.
  2. Each ethene, C2H4, has 2 carbon atoms, so number of ethene molecules = 6 ÷ 2 = 3.
  3. Check hydrogen: 14 + (3 × 4) = 26, which matches C12H26.
  4. Equation: C12H26 → C6H14 + 3C2H4.

Answer: 3 molecules of ethene

Practice questions

Try each one, then open the answer.

1. Which conditions are used to crack large alkane molecules?

  1. A
    yeast at 30 °C
  2. B
    a high temperature and a catalyst
  3. C
    room temperature and a nickel catalyst
  4. D
    a low temperature and ultraviolet light
Show answer

Answer: B. Cracking breaks long-chain alkanes into shorter alkanes and alkenes (and sometimes hydrogen) using heat and a catalyst.

2. Why are alkenes described as unsaturated?

  1. A
    their molecules contain a C=C double bond
  2. B
    their molecules contain only single bonds
  3. C
    they dissolve in water
  4. D
    their molecules contain oxygen
Show answer

Answer: A. Unsaturated compounds contain at least one carbon–carbon bond that is not single, so more atoms can be added across it.

3. Propene reacts with steam in the presence of an acid catalyst. What is the molecular formula of the product?

  1. A
    C3H6O2
  2. B
    C3H6Br2
  3. C
    C3H8
  4. D
    C3H7OH
Show answer

Answer: D. H2O adds across the double bond: C3H6 + H2O → C3H7OH, an alcohol (propanol).

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