HSC Exam Practice

Sample response. An addition reaction is a reaction in which two reactants combine to form a single product, with no atoms lost. For alkenes, the C=C pi bond opens and the atoms of the incoming reagent bond to each of the two carbons of the former double bond, converting it to a single bond.

Marking notes. 1 mark for “two reactants combine to form one product / nothing lost”; 1 mark for “the pi bond (C=C) opens / the reagent adds across the double bond”. Both required for 2 marks.

Sample response. (1) Hydrogenation: H₂ added; product is a saturated alkane. (2) Halogenation: X₂ (Br₂ or Cl₂) added; product is a dihaloalkane / vicinal dihalide. (3) Hydrohalogenation: HX (HCl, HBr, or HI) added; product is a monohalogenated alkane. (4) Hydration: H₂O (steam) added; product is an alcohol.

Marking notes. 1 mark per correctly matched reaction type + reagent + product type. 4 marks for all four correct.

Sample response. CH₂=CH₂ + H₂O → CH₃CH₂OH. Conditions: (1) H₃PO₄ (or H₂SO₄) catalyst; (2) ~300°C; (3) high pressure (~65 atm).

Marking notes. 1 mark for balanced equation with correct structural/molecular formulae for ethene, water, and ethanol. 1 mark for catalyst (H₃PO₄ or H₂SO₄). 1 mark for BOTH temperature (~300°C) AND high pressure (~65 atm) — both must be stated to earn the third mark. Common error: stating only the catalyst or only the temperature. Note: H₂SO₄ is also acceptable as catalyst.

Sample response. Markovnikov’s rule: when HX adds to an unsymmetrical alkene, H adds to the carbon of the C=C that already has more H atoms (less substituted carbon) and X adds to the carbon with fewer H atoms (more substituted carbon). [1] For propene + HCl: C1 (=CH₂) has 2 H atoms; C2 (=CH–) has 1 H atom. Applying Markovnikov’s rule: H adds to C1; Cl adds to C2. [1] Major product: 2-chloropropane (CH₃CHClCH₃). [1] Balanced equation: CH₃CH=CH₂ + HCl → CH₃CHClCH₃. [1]

Marking notes. 1 mark — states Markovnikov’s rule accurately (H to more-H carbon; X to fewer-H / more-substituted carbon). 1 mark — correctly identifies which carbon of propene’s C=C has more H (C1 = CH₂) and applies the rule. 1 mark — names major product as 2-chloropropane. 1 mark — balanced equation.

Sample response. Reagent: aqueous bromine solution (Br₂(aq)), orange/brown in colour. Positive result: the solution decolourises (becomes colourless), indicating that Br₂ has reacted by addition across a multiple bond. [1 mark] Limitation: the test confirms the presence of unsaturation (C=C or C≡C) but does not specifically confirm the compound is an alkene — alkynes also decolourise bromine water. Additional evidence (e.g. hydrogenation stoichiometry, IR spectroscopy, or degree-of-unsaturation calculation) is required to distinguish alkenes from alkynes. [2 marks: 1 for the reagent + observation; 1 for identifying the limitation; 1 for the correct limitation statement including alkynes.]

Marking notes. 1 mark for reagent (Br₂(aq)) and observation (orange/brown → colourless). 1 mark for identifying that the test confirms unsaturation, not specifically alkenes. 1 mark for stating that alkynes also give a positive result (or equivalent explanation of the limitation).

Sample response. (i) Reagent: hydrogenation uses H₂ gas; halogenation uses X₂ (Br₂ or Cl₂). (ii) Catalyst: hydrogenation requires a Ni, Pd, or Pt heterogeneous catalyst; halogenation requires no catalyst. (iii) Product: hydrogenation produces a saturated alkane; halogenation produces a vicinal dihaloalkane with one halogen on each carbon of the former double bond.

Marking notes. 1 mark per correctly contrasted criterion (reagent, catalyst, product). Any equivalent phrasing accepted.

Sample response. C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂. Conditions: ~35°C, anaerobic conditions, yeast enzymes (zymase).

Marking notes. 1 mark for balanced equation with glucose, ethanol, and CO₂ correctly shown. 1 mark for stating conditions: temperature (~35°C), anaerobic/yeast/enzyme. Accept C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.

Sample response. Direct hydration requires high temperature (~300°C) to provide sufficient activation energy for the H₃PO₄-catalysed addition of steam across the ethene C=C bond; without this energy, the reaction rate is negligible. [1] High pressure (~65 atm) is required because the equilibrium CH₂=CH₂(g) + H₂O(g) ⇌ CH₃CH₂OH(g) involves 2 mol gas on the left and 1 mol gas on the right; by Le Chatelier’s principle, high pressure shifts equilibrium toward the product (fewer moles of gas), increasing ethanol yield. [1] Fermentation uses enzymes (biological catalysts at body temperature ~35°C) that lower activation energy without requiring high temperatures or pressures; the biochemical pathway does not involve a high-pressure equilibrium. [1]

Marking notes. 1 mark for temperature role (activation energy / rate of catalysed reaction). 1 mark for pressure role (Le Chatelier, 2 mol gas → 1 mol, equilibrium shift). 1 mark for why fermentation needs neither (enzyme catalysis at low temperature, no gas-phase equilibrium under pressure).

Sample response. Advantage: the Nowra-based company can use locally grown wheat starch as a renewable feedstock, reducing dependence on crude-oil-derived ethene and lowering the carbon footprint of the process; this is advantageous in a region with substantial grain agriculture. [1] Disadvantage: the fermentation product is only ~15% ethanol and requires energy-intensive distillation to reach commercial purity; also the batch process (vs continuous hydration) limits throughput and increases labour and reactor turnaround costs. [1] One additional mark for clear scientific language, specific connection to the NSW / Manildra context, or an additional valid point beyond the above. [1]

Marking notes. 1 mark — scientifically valid advantage (renewable feedstock / lower fossil fuel dependence / agricultural region context). 1 mark — scientifically valid disadvantage (low purity / distillation required / batch vs continuous). 1 mark — additional scientific detail or quality of explanation across both points.

Sample response. The bromine water test uses aqueous bromine solution (Br₂(aq)), which is orange/brown. When added to an unsaturated organic compound, Br₂ reacts by addition across the C=C or C≡C bond, consuming the Br₂ and producing a colourless halogenated product. A positive result (decolourisation) indicates the presence of unsaturation; a negative result (solution remains orange/brown) indicates the absence of C=C and C≡C bonds. The chemistry is halogenation: alkene + Br₂ → dihaloalkane (e.g. CH₂=CH₂ + Br₂ → CH₂BrCH₂Br). At least two compound types give a positive result: (1) alkenes (e.g. but-1-ene) — C=C reacts by addition halogenation; (2) alkynes (e.g. but-2-yne) — C≡C reacts by addition (the triple bond can react with 1 or 2 mol Br₂). Both decolourise bromine water immediately at room temperature. The critical limitation is that the test is not specific to alkenes: it confirms only the presence of unsaturation (C=C or C≡C) and cannot distinguish between alkene and alkyne. A student who decolourises bromine water and concludes “the compound is an alkene” has overstated the evidence. To overcome this limitation: (1) hydrogenation stoichiometry — alkenes consume 1 mol H₂ per C=C; alkynes consume 2 mol H₂ per C≡C (if internal) or can be partially hydrogenated to an alkene; (2) infrared (IR) spectroscopy — terminal alkynes show a C–H stretch at ~3300 cm^¯1 and a C≡C stretch at ~2100 cm^¯1, absent in alkenes; (3) the degree of unsaturation from the molecular formula (C_nH_2n = alkene or cycloalkane; C_nH_2n–2 = alkyne or diene). An additional important limitation is that some aldehydes and other oxidisable species may bleach or react with Br₂(aq) under some conditions, further reducing specificity. Overall, the bromine water test is a rapid, inexpensive, and practical first-line qualitative test suitable for the HSC lab context, but its result must be interpreted as “unsaturation present or absent” rather than “alkene confirmed,” and complementary tests are always needed for definitive compound identification.

Marking criteria (max 8 marks).

• 1 mark — Identifies reagent (Br₂(aq)) and its colour (orange/brown), and states the positive result (decolourises to colourless).
• 1 mark — States the underlying chemistry: addition of Br₂ across C=C (or C≡C); writes or describes the halogenation equation.
• 1 mark — Names at least two compound types that give a positive result (alkenes and alkynes, with examples).
• 1 mark — States the specific limitation: the test confirms unsaturation (C=C or C≡C) but does not distinguish alkenes from alkynes.
• 1 mark — Explains why this limitation matters: a positive result can arise from either alkene or alkyne, making a definitive alkene identification impossible from this test alone.
• 1 mark — Proposes a valid experimental method to distinguish alkenes from alkynes (e.g. hydrogenation stoichiometry, IR spectroscopy, degree of unsaturation).
• 1 mark — Provides a scientifically accurate overall assessment of the test (rapid, inexpensive, first-line qualitative tool; not specific; must be paired with additional tests).
• 1 mark — Quality of scientific language and reasoning throughout: precise use of terms (unsaturation, addition, halogenation, positive/negative result), clear structure, and correct chemistry in all equations or descriptions.