This question is about hydrocarbons.
(a) Table 8.1 shows the names, formulae and boiling points of methane, ethane, propane and butane.
Use the information in Table 8.1 to answer these questions.
(i) Name the homologous series that includes methane, ethane, propane and butane.
(ii) State the trend in the boiling point of this homologous series as the number of carbon atoms increases.
(iii) Deduce the general formula of this homologous series.
(b) Complete the word equation for the complete combustion of ethane.
(c) Long-chain hydrocarbons can be cracked to produce alkenes and hydrogen.
(i) State two conditions for cracking.
(ii) The diesel oil fraction from the fractional distillation of petroleum can be used for cracking. Give one other use of the diesel oil fraction.
(d) Alkene molecules can react together to produce polymers. Define the term polymer.
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 11.4 — Alkanes (Parts (a)(i), (a)(ii), (a)(iii))
• Topic 11.3 — Fuels / Combustion (Part (b))
• Topic 11.5 — Alkenes / Cracking (Parts (c)(i), (c)(ii))
• Topic 11.8 — Polymers (Part (d))
(a)(i) Alkanes.
Methane, ethane, propane, and butane are the first four members of the alkane homologous series, which are saturated hydrocarbons containing only single carbon-carbon bonds.
(a)(ii) The boiling point increases as the number of carbon atoms increases.
As the carbon chain length increases, the molecules have larger surface areas, leading to stronger intermolecular forces (London dispersion forces) that require more thermal energy to overcome.
(a)(iii) CₙH₂ₙ₊₂
This is the general formula for alkanes. For n=1 (methane), we get CH₄; for n=2 (ethane), C₂H₆; for n=4 (butane), C₄H₁₀, which matches the table data.
(b) ethane + oxygen → carbon dioxide + water.
Complete combustion of a hydrocarbon (containing only carbon and hydrogen) in excess oxygen always produces only two products: carbon dioxide and water vapour.
(c)(i) 1. High temperature. 2. A catalyst (e.g., zeolite or aluminium oxide).
Cracking is a thermal decomposition process that breaks long-chain alkanes into smaller, more useful alkenes using high temperatures (around 500-700°C) and a catalyst to speed up the reaction.
(c)(ii) Fuel for diesel engines (or fuel for buses, trucks, trains).
The diesel oil/gas oil fraction is primarily used as a fuel in compression-ignition (diesel) engines for vehicles, ships, and home heating systems.
(d) A polymer is a large molecule built up from many smaller, repeating units called monomers.
In the context of alkenes, the double bond opens up to allow thousands of alkene molecules (monomers) to join together in a long chain (addition polymerisation) to form a polymer like poly(ethene) or poly(propene).
