Chemistry • Year 12 • Module 7 • Lesson 7
HSC Exam Practice
Reactions of Alkynes & Alkanes — Hydration, Substitution & Combustion
Short answer
1.Short answer — Band 3–4
Define free radical substitution and identify the role of UV light in this reaction.
Describe how the number of pi bonds in an alkyne explains why its halogenation reaction proceeds in two distinct steps.
Identify the organic product and its functional group class when but-1-yne (HC≡CCH2CH3) reacts with water in the presence of dilute H2SO4 and Hg2+. Write the equation for the overall reaction.
Outline the difference between a geminal dihalide and a vicinal dihalide, giving one example of a reaction that produces each type from an alkyne starting material.
Distinguish between complete and incomplete combustion of a hydrocarbon, with reference to oxygen supply, products formed, and one environmental or health impact of each.
Write the balanced equation for complete combustion of pentane (C5H12). Show your working using the C → H → O method.
Data response
2.Data response — Band 4–5
The table below lists four hydrocarbon fuels. For each, write the balanced equation for complete combustion in excess oxygen using the systematic C → H → O method, then identify the products. 6 marks (1.5 per row)
| Fuel (formula) | Balanced combustion equation | Products |
|---|---|---|
| Methane (CH4) | ||
| Ethyne (HC≡CH) | ||
| Propane (C3H8) | ||
| Butane (C4H10) |
A gas torch uses propyne (CH3C≡CH) as its fuel in an oxy-acetylene-type system.
(a) Write the balanced equation for complete combustion of propyne using the C → H → O method. Show your working for each step. 3 marks
(b) The torch is operated in a poorly ventilated workshop. State the products formed under limited oxygen supply and identify one health hazard of each product. 2 marks
(c) Propyne also reacts with bromine. Write the equation for Step 1 of the bromination of propyne (one equivalent of Br2) and name the product. State whether the product decolourises bromine water. 1 mark
Extended response
3.Extended response — Band 5–6
“Adding HBr to propyne produces 1,2-dibromopropane because bromine atoms always add to adjacent (vicinal) carbons, regardless of whether you start from an alkyne or an alkene.”
— Chemistry revision website, accessed 2024.
Identify the scientific error in the claim above. Explain the correct chemistry, including the full two-step mechanism of HBr addition to propyne, and use Markovnikov’s rule to identify the actual product of each step. 5 marks
Evaluate the statement: “The two reactions available to alkanes — halogenation and combustion — both have significant industrial and environmental relevance in Australia.” In your response, discuss the conditions, products, and at least one named Australian industrial or environmental context for each reaction type. 6 marks
Q1.1 — Free radical substitution (3 marks)
Free radical substitution is a chain reaction in which a halogen molecule (X2) reacts with an alkane, with one H atom replaced by a halogen atom and HX produced as a co-product [1]. UV light provides the energy (it is an energy source, not a catalyst) to break the X—X bond by homolysis, producing two halogen radicals (X•) in the initiation step [1]. UV light is electromagnetic radiation, not matter, so it cannot be a catalyst; it is absorbed in initiation and is not regenerated [1].
Q1.2 — Two pi bonds (2 marks)
A C≡C triple bond has two pi bonds [1]. Each addition step opens one pi bond; the first equivalent of X2 opens the first pi bond to give a dihaloalkene (which still has one pi bond), and the second equivalent opens the second pi bond to give the fully saturated tetrahaloalkane [1].
Q1.3 — But-1-yne hydration (3 marks)
HC≡CCH2CH3 + H2O → CH3COCH2CH3 [1]. Product: butan-2-one [1]. Functional group class: ketone (not alcohol — the enol intermediate tautomerises to the keto form) [1].
Q1.4 — Geminal vs vicinal dihalide (3 marks)
A geminal dihalide has both halogen atoms on the same carbon (e.g. CH3CHBr2); a vicinal dihalide has halogen atoms on adjacent carbons (e.g. CHBrCHBr) [1]. Geminal: full hydrohalogenation of an alkyne (e.g. HC≡CH + 2HBr → CH3CHBr2) [1]. Vicinal: halogenation of an alkyne (step 1, 1 eq Br2; e.g. HC≡CH + Br2 → CHBr=CHBr) [1].
Q1.5 — Complete vs incomplete combustion (4 marks)
Complete combustion occurs in excess O2; products are CO2 + H2O only; clean blue flame [1]. Environmental impact: CO2 is a greenhouse gas contributing to the enhanced greenhouse effect and climate change [1]. Incomplete combustion occurs in limited O2; products include CO and/or soot (C) + H2O; yellow/sooty flame [1]. Health/environmental impact: CO binds haemoglobin ≈250× more strongly than O2, blocking O2 transport; soot (PM2.5) causes respiratory and cardiovascular disease [1].
Q1.6 — Pentane combustion (3 marks)
C: 5C → 5CO2 [1]. H: 12H → 6H2O [1]. O right: 10 + 6 = 16 → O2 = 8. Balanced equation: C5H12 + 8O2 → 5CO2 + 6H2O [1].
Q2.1 — Combustion equations table (6 marks)
Methane: C: 1C → 1CO2. H: 4H → 2H2O. O: 2+2=4 → O2=2. CH4 + 2O2 → CO2 + 2H2O. Products: CO2 + H2O (complete combustion only).
Ethyne: C: 2C → 2CO2. H: 2H → 1H2O. O: 4+1=5 → O2=5/2. Multiply by 2: 2HC≡CH + 5O2 → 4CO2 + 2H2O. Products: CO2 + H2O.
Propane: C: 3C → 3CO2. H: 8H → 4H2O. O: 6+4=10 → O2=5. C3H8 + 5O2 → 3CO2 + 4H2O. Products: CO2 + H2O.
Butane: C: 4C → 4CO2. H: 10H → 5H2O. O: 8+5=13 → O2=13/2. Multiply by 2: 2C4H10 + 13O2 → 8CO2 + 10H2O. Products: CO2 + H2O.
Marking notes: 1 mark per row for correct balanced equation; 0.5 mark per row for correct products if equation is unbalanced but products are correct. Accept fractional coefficients for ethyne row.
Q2.2 — Propyne combustion and reactions (6 marks)
(a) Balanced equation for complete combustion of propyne (3 marks):
C: 3C → 3CO2 [1]. H: 4H → 2H2O [1]. O right: 6 + 2 = 8 → O2 = 4 [1]. CH3C≡CH + 4O2 → 3CO2 + 2H2O.
(b) Limited oxygen (2 marks): Incomplete combustion produces CO (carbon monoxide) and/or C (soot/carbon) [1]. CO binds haemoglobin ~250× more strongly than O2, blocking oxygen transport — toxic. Soot (C, PM2.5) causes respiratory and cardiovascular disease [1].
(c) Step 1 bromination (1 mark): CH3C≡CH + Br2 → CH3CBr=CHBr (1,2-dibromopropene, a dihaloalkene). The product still has one C=C pi bond, so it does decolourise bromine water in a further addition (step 2) [1].
Q3.1 — Source critique: HBr addition to propyne (5 marks)
Error identified (1 mark): The claim that HBr addition always gives a vicinal (1,2-) dihalide is incorrect for hydrohalogenation. Unlike halogenation (Br2 addition, which gives a vicinal dihalide), HBr addition follows Markovnikov’s rule, which places both Br atoms on the same (more substituted) carbon in full addition, giving a geminal dihalide.
Step 1 with Markovnikov (2 marks): CH3C≡CH + HBr → CH3CBr=CH2 (2-bromopropene). H adds to C3 (terminal, more H), Br adds to C2 (more substituted, fewer H) per Markovnikov [1 for equation, 1 for Markovnikov reasoning].
Step 2 with Markovnikov (2 marks): CH3CBr=CH2 + HBr → CH3CBr2CH3 (2,2-dibromopropane — a geminal dihalide). H adds to C3 (=CH2, more H), Br adds to C2 (already has Br) [1 for equation, 1 for identifying geminal product & distinction from vicinal].
Q3.2 — Alkane reactions: marking criteria (6 marks)
Halogenation — conditions and products (2 marks): Alkane + X2 + UV light (energy source, not catalyst) → haloalkane + HX. UV light initiates radical chain; HX is a co-product. Example: CH4 + Cl2 → CH3Cl + HCl. [1 for conditions including UV as energy source; 1 for products including HX].
Halogenation — Australian industrial context (1 mark): NSW Health uses chlorination (Cl2 in water) for drinking water disinfection; chlorination of organic matter can produce chlorinated by-products. PVC synthesis via Qenos (Altona, VIC) involves chloroethene (vinyl chloride) production, which requires halogenation chemistry.
Combustion — conditions and products (2 marks): Complete combustion (excess O2): alkane + O2 → CO2 + H2O; incomplete combustion (limited O2): produces CO and/or soot + H2O [1 for each combustion type with products].
Combustion — Australian environmental context (1 mark): Natural gas (predominantly methane) combustion in Australian energy generation and vehicles contributes CO2 to the enhanced greenhouse effect; bushfire smoke (incomplete combustion of biomass) produces CO and PM2.5 which are health hazards for Australian communities (e.g. 2019–20 Black Summer fires).