CIE iGCSE Co-Ordinated Science C11.2 Naming organic compounds Exam Style Questions Paper 4
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C11.1 — Formulas and terminology / Hydrocarbons (Part (a))
• Topic C11.3 — Fuels / Natural gas (Part (b))
• Topic C11.2 — Naming organic compounds (Part (c))
• Topic C11.7 — Polymers / Addition polymerisation (Part (d)(i) & (d)(ii))
• Topic C3.1 — Formulas / Balancing equations (Part (e))
▶️ Answer/Explanation
(a) Compound C is not a hydrocarbon because it contains oxygen; hydrocarbons contain only hydrogen and carbon.
Compound C has an -OH group (alcohol functional group), meaning it contains oxygen atoms in addition to carbon and hydrogen, so it is not a hydrocarbon.
(b) Natural gas
Compound D is methane (CH₄), which is the main constituent of natural gas, a fossil fuel formed from ancient organic matter.
(c) Propane
Compound B has the formula C₃H₈, which is the third member of the alkane homologous series, named propane.
(d)(i) The polymer structure is –[–CH₂–CH₂–]– with single bonds between carbon atoms:
Ethene monomers (compound A with C=C bond) join by breaking the double bond and forming single bonds between carbon atoms in a long chain.
(d)(ii) Difference 1: Addition polymerisation only occurs in unsaturated monomers (with C=C bonds), while condensation polymerisation involves different types of monomers.
Difference 2: Addition polymerisation only forms the polymer molecule, whereas condensation polymerisation forms the polymer and a small molecule (like water) per linkage.
Addition polymerisation uses monomers with double bonds; condensation polymerisation joins monomers with functional groups, releasing a by-product.
(e) \( \text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O} \)
Propane combustion requires 5 molecules of oxygen to produce 3 molecules of carbon dioxide and 4 molecules of water. The equation is balanced with 3 carbon, 8 hydrogen, and 10 oxygen atoms on both sides.
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.