Alkanes Exam Questions

(a) CnH2n+2 [1]

(b)

• Methane: CH4 [1]
• Ethane: C2H6 [1]
• Propane: C3H8 and Butane: C4H10 [1]

Examiner note: Each successive alkane differs by CH2. Candidates sometimes write “methene” or “ethine” — these are not real names. Alkane names always end in -ane. The general formula must include the subscript notation: CnH2n+2, not CnH2n (that is alkenes).

• A hydrocarbon is a compound containing only carbon and hydrogen [1]
• Saturated means it contains only single covalent bonds between carbon atoms / no C=C double bonds [1]
• Methane (CH4) has only single bonds: four C-H single bonds and no double bonds [1]

Examiner note: “Saturated” refers specifically to having no carbon-carbon double bonds, meaning the carbon atoms are bonded to the maximum number of hydrogen atoms possible. A compound with a C=O bond but no C=C bond is still considered saturated in organic chemistry terms at IGCSE level.

(a) CH4 + 2O2 → CO2 + 2H2O [1]

(b) Carbon dioxide: pass the gas through limewater — it turns milky/cloudy [1] Water: use anhydrous cobalt chloride paper — it turns from blue to pink (or anhydrous copper sulfate powder — it turns from white to blue) [1]

Examiner note: These product tests are essential knowledge. Limewater (calcium hydroxide solution) turns milky due to formation of insoluble calcium carbonate: Ca(OH)2 + CO2 → CaCO3 + H2O. The cobalt chloride paper test detects water specifically — not just any liquid.

(a) Ultraviolet (UV) light [1]

(b) CH4 + Cl2 → CH3Cl + HCl [1]

(c) A hydrogen atom in the methane molecule is replaced by a chlorine atom [1]

(d) Hydrogen chloride (HCl) [1]

Examiner note: This is a free-radical substitution reaction, but at IGCSE you only need to know it as a substitution. UV light provides the energy to break the Cl-Cl bond. Further substitution can occur: CH3Cl can react with more Cl2 to form CH2Cl2, CHCl3, and CCl4. The reaction does not occur in the dark.

(a) Boiling point increases as the number of carbon atoms / chain length increases [1]. Larger molecules have stronger intermolecular forces (van der Waals / London forces) between them, requiring more energy to separate [1]

(b) Liquid [1] — following the trend, pentane would have a boiling point above 25 degrees C (approximately 36 degrees C), so it would be a liquid at room temperature

Examiner note: The first four alkanes (methane to butane) are gases at room temperature. Pentane onwards are liquids. The trend is caused by increasing intermolecular forces, not stronger covalent bonds within molecules. This distinction is a common source of lost marks.

• Alkanes contain only strong C-C and C-H single covalent bonds [1]
• They have no functional group / no reactive site such as a double bond [1]
• Combustion: alkanes burn in air/oxygen to produce carbon dioxide and water (if complete combustion) [1]
• Substitution: with halogens (e.g., Cl2) in the presence of UV light, a hydrogen atom is replaced by a halogen atom [1]

Examiner note: Alkanes do NOT undergo addition reactions — that is alkenes. In substitution, one atom replaces another; in addition, atoms add across a double bond without anything being removed. This distinction is a high-value exam point.

(a) Incomplete combustion [1]

(b) Carbon monoxide (CO) and carbon/soot (C) [1]

(c) Carbon monoxide is toxic — it is colourless and odourless, so it cannot be detected without a carbon monoxide alarm / it binds to haemoglobin and prevents oxygen transport in the blood, which can cause death [1]

Examiner note: Incomplete combustion always also produces water. The carbon-containing products depend on how limited the oxygen supply is: some CO2 may still form alongside CO and C. In exam answers, the most important point is the danger of CO production in poorly ventilated spaces.