HSC Chemistry — Module 7
Hydrocarbons — 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.
Q3: Why is carbon able to form such a large variety of compounds?
Carbon has 4 valence electrons and can form 4 stable covalent bonds. It can bond to other carbon atoms to form chains, branches and rings of virtually unlimited length. It bonds to many other elements (H, O, N, S, halogens). This versatility enables millions of organic compounds.
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: March 2026 · 3 flashcards · 5 quiz questions