Alkanes

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

Alkanes are hydrocarbons in which every bond is a single covalent bond. Because there are no double bonds they are called saturated. Their general formula is CnH2n+2.

Alkanes are generally unreactive. They do only two things you need to know. They burn, and they react with chlorine in ultraviolet light.

The reaction with chlorine is a substitution reaction: one hydrogen atom is replaced by one chlorine atom, and hydrogen chloride is the second product. It is a photochemical reaction, because the ultraviolet light provides the activation energy, Ea. It does not happen in the dark.

Rules to remember

Common mistake

Students write the product of methane and chlorine as CH4Cl2, as if chlorine were added. A hydrogen atom is swapped for a chlorine atom, so the products are CH3Cl and HCl.

Worked example

Propane burns completely: C3H8 + 5O2 → 3CO2 + 4H2O. What volume of carbon dioxide, at room temperature and pressure, forms from 11 g of propane? (H 1, C 12; molar gas volume 24 dm3)

  1. Mr of C3H8 = (3 × 12) + (8 × 1) = 44.
  2. Moles of propane = 11 ÷ 44 = 0.25 mol.
  3. 1 mol of propane gives 3 mol of CO2, so moles of CO2 = 0.25 × 3 = 0.75 mol.
  4. Volume of CO2 = 0.75 × 24 = 18 dm3.

Answer: 18 dm3 of carbon dioxide

Practice questions

Try each one, then open the answer.

1. Alkanes are generally unreactive. Which two types of reaction do they undergo?

  1. A
    neutralisation and esterification
  2. B
    combustion and substitution with chlorine
  3. C
    addition with bromine and addition with steam
  4. D
    fermentation and polymerisation
Show answer

Answer: B. Alkanes burn in oxygen and react with chlorine in ultraviolet light. Addition reactions need a C=C bond, which alkanes do not have.

2. Which formula represents an alkane?

  1. A
    C5H11OH
  2. B
    C5H8
  3. C
    C5H12
  4. D
    C5H10
Show answer

Answer: C. Alkanes fit CnH2n+2. With n = 5: C5H12.

3. Methane reacts with chlorine in a substitution reaction: CH4 + Cl2 → CH3Cl + HCl. What mass of hydrogen chloride is formed when 1.6 g of methane reacts completely? (H 1, C 12, Cl 35.5)

  1. A
    7.3 g
  2. B
    1.8 g
  3. C
    36.5 g
  4. D
    3.65 g
Show answer

Answer: D. 1.6 g of CH4 is 1.6 ÷ 16 = 0.10 mol. One mole of HCl forms per mole of CH4, so 0.10 mol × 36.5 = 3.65 g. In a substitution reaction one chlorine atom replaces one hydrogen atom, and that hydrogen leaves as HCl.

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