CIE iGCSE Co-Ordinated Science C11.4 Alkanes Exam Style Questions Paper 4
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C11.2 — Naming organic compounds (Part (a)(i), (a)(iii))
• Topic C11.4 — Alkanes (Part (a)(ii))
• Topic C11.5 — Alkenes (Part (a)(ii), (b), (c))
• Topic C11.7 — Polymers (Part (d))
▶️ Answer/Explanation
(a)(i) \( C_4H_{10} \)
Butane is an alkane with four carbon atoms. The general formula for alkanes is \( C_nH_{2n+2} \). For n = 4, the molecular formula is \( C_4H_{10} \).
(a)(ii) Unsaturated: Propene contains a double carbon-carbon covalent bond / one of the carbon-carbon bonds is not a single bond.
Hydrocarbon: Propene contains only hydrogen and carbon atoms.
An unsaturated hydrocarbon contains at least one carbon-carbon double or triple bond. Propene contains a C=C double bond, so it is unsaturated.
A hydrocarbon is a compound that contains only hydrogen and carbon atoms. Propene has the formula \( C_3H_6 \), containing only carbon and hydrogen.
(a)(iii)
But-2-ene is an alkene with four carbon atoms. The double bond is between carbon atoms 2 and 3 (numbering from the end nearest the double bond). Each carbon atom must have four bonds in total. The structure shows the correct arrangement with the C=C double bond in the middle of the chain.
(b) Propene: turns colourless / decolourised.
Alkenes decolourise aqueous bromine (orange → colourless) because the bromine adds across the carbon-carbon double bond in an addition reaction. Alkanes do not react with bromine under normal conditions because they only contain single bonds, so they are saturated and the bromine remains orange.
(c) Propene is made by cracking large alkane molecules. The process involves:
- Breaking down large alkane molecules into smaller molecules.
- Producing alkene molecules (such as propene).
Conditions:
- High temperature.
- Catalyst.
Cracking is a process used in the petroleum industry to break down large, less useful alkane molecules into smaller, more useful molecules such as alkenes. Long-chain alkanes are heated to a high temperature (about 500-700°C) in the presence of a catalyst (such as zeolite or aluminium oxide). The large molecules break apart at their carbon-carbon bonds to form smaller alkanes and alkenes, including propene.
(d)
Addition polymerisation occurs when many alkene monomers join together. The double bond in each propene molecule breaks, and the molecules join end-to-end to form a long chain (poly(propene)). The repeat unit is —CH₂—CH(CH₃)—, and the equation shows n monomers forming a polymer chain with single bonds between all carbon atoms.
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C11.3 — Fuels
• Topic C11.4 — Alkanes
• Topic C11.5 — Alkenes
• Topic C3.3 — The mole and Avogadro constant
▶️ Answer/Explanation
(a) refinery gas
In fractional distillation, the fraction with the lowest boiling point condenses at the top of the column where it is coolest. Refinery gas has the lowest boiling point range (< 25°C).
(b) C₇H₁₄ and C₈H₁₆
Alkenes are unsaturated hydrocarbons containing a C=C double bond. They turn aqueous bromine colourless through addition reactions. C₇H₁₄ and C₈H₁₆ have the general formula CₙH₂ₙ, characteristic of alkenes.
(c)(i) single ; saturated
Alkanes contain only single C–C bonds, making them saturated hydrocarbons. This means they can hold the maximum number of hydrogen atoms.
(c)(ii) 10,800 dm³
Molar mass of C₉H₂₀ = (9 × 12) + (20 × 1) = 128 g/mol
Moles of C₉H₂₀ = 6400 ÷ 128 = 50 mol
From the equation, 1 mol C₉H₂₀ produces 9 mol CO₂
Moles of CO₂ = 50 × 9 = 450 mol
Volume of CO₂ = 450 × 24 = 10,800 dm³
(d)
Propane has three carbon atoms connected by single bonds, with each carbon bonded to enough hydrogen atoms to complete its valence of four. Each line represents a covalent bond.