HSC Chemistry — Module 7
Hydrocarbons — HSC Chemistry — Module 7 Flashcards & Quiz
Hydrocarbons are the simplest organic molecules — compounds of carbon and hydrogen only — and they form the basis of the HSC Chemistry Module 7 organic pathway. You need to distinguish alkanes (saturated, substitution), alkenes (C=C, addition), alkynes (C≡C) and cyclic forms, predict combustion products, balance combustion equations, and explain structural and geometric isomerism. The trend in boiling point and solubility with chain length is exam-standard.
Key Points
- Alkanes (CₙH₂ₙ₊₂) are saturated hydrocarbons; only C-C single bonds; relatively unreactive except with halogens (substitution) and O₂ (combustion).
- Alkenes (CₙH₂ₙ) have a C=C double bond; react readily by addition (Br₂, H₂, H₂O, HX) and show geometric isomerism (cis/trans).
- Alkynes (CₙH₂ₙ₋₂) have a C≡C triple bond; also add Br₂ and H₂, but give different products depending on how much reagent is added.
- Cyclic hydrocarbons (e.g. cyclohexane, benzene) have closed carbon rings; benzene shows aromaticity and unusual stability.
- Combustion: complete combustion in excess O₂ → CO₂ + H₂O; incomplete combustion → CO or C (soot) + H₂O.
- Trends: boiling point increases with chain length (more dispersion forces); straight-chain > branched (more contact surface).
Common Mistakes to Avoid
- Confusing alkanes (saturated, substitution) with alkenes (unsaturated, addition).
- Writing incomplete combustion products as CO₂ only — incomplete combustion gives CO and/or soot.
- Forgetting that alkenes react by ADDITION (breaking the π bond), not substitution.
- Missing cis/trans isomerism in alkenes with 1,2-disubstituted carbons.
- Using "saturated" to mean "full of carbon" — it means "full of hydrogen" (all single bonds).
Exam Strategy
HSC Module 7 hydrocarbon questions ask you to write reaction equations, distinguish types, or predict products. Method: identify the hydrocarbon type (alkane/alkene/alkyne/aromatic), match to the typical reaction (combustion, substitution, addition), balance the equation, include catalysts and conditions where relevant.
Sample Flashcards
Q1: What are alkanes, alkenes and alkynes?
Alkanes: saturated hydrocarbons with only single C-C bonds. General formula CₙH₂ₙ₊₂. Alkenes: unsaturated with at least one C=C double bond. General formula CₙH₂ₙ. Alkynes: unsaturated with at least one C≡C triple bond. General formula CₙH₂ₙ₋₂.
Q2: How do boiling points change in a homologous series?
Within a homologous series, boiling point INCREASES as chain length increases. Longer chains = more electrons = stronger dispersion forces = more energy needed to separate molecules. Branching DECREASES boiling point because the molecule is more compact with less surface area.
Sample Quiz Questions
Q1: Alkanes are unsaturated hydrocarbons.
Answer: FALSE
Alkanes are SATURATED — they contain only single C-C bonds. Alkenes and alkynes are unsaturated (contain double or triple bonds).
Q2: The general formula for alkenes is CₙH₂ₙ.
Answer: TRUE
Alkenes follow the general formula CₙH₂ₙ (e.g. ethene C₂H₄, propene C₃H₆). This is due to the one C=C double bond.
Q3: Boiling point decreases as carbon chain length increases in a homologous series.
Answer: FALSE
Boiling point INCREASES with chain length because longer chains have more electrons and stronger dispersion forces.
Revision Tip
Hydrocarbon reactions are pattern-based — build a Revizi flashcard deck with each reaction type (combustion, substitution, addition) and 2-3 examples.
Related Concepts
Last updated: 9 October 2026 · 2 sample flashcards · 4 sample quiz questions