Hydrocarbons and Simple Alkanes
Australia's Lytton refinery in Brisbane processed 109,000 barrels of crude oil every single day in 2020, turning a thick black liquid into 50 different products.
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Q1 · Petrol, natural gas, candle wax, and butter all burn, what do you think these substances have in common at the molecular level that makes them flammable?
Q2 · Why do you think the length of a hydrocarbon molecule (e.g. short chain vs long chain) might change how it behaves, for instance whether it is a gas, liquid, or solid at room temperature?
● Know
- The general formula for alkanes: CnH(2n+2)
- The names and formulas of the first five alkanes (methane to pentane)
- How boiling point changes with chain length
● Understand
- Why alkanes are called 'saturated' hydrocarbons
- Why longer-chain alkanes have higher boiling points
- What makes alkanes a homologous series
● Can do
- Write molecular formulas for alkanes given the number of carbons
- Name alkanes from their formula
- Predict and explain the trend in boiling points
Strike a match near a natural gas stove: the flame appears instantly and burns with a clean blue colour, because methane, a single carbon bonded to four hydrogens, ignites easily and combusts completely. Alkanes are the simplest family of organic compounds, they contain only carbon and hydrogen, with all single bonds between carbon atoms. The general formula is $C_nH_{2n+2}$, where n is the number of carbon atoms. A molecule with all single bonds is called saturated because the carbons cannot accommodate any additional hydrogen atoms. Each carbon in an alkane is tetrahedral, with bond angles of approximately 109.5°. The chain can be straight (unbranched) or branched, both are alkanes.
Checking the formula for pentane (n=5): $C_nH_{2n+2}$ = C₅H₁₂. Count: 5 carbons, 12 hydrogens. Pentane is the main component of petroleum naphtha, used as a blowing agent in expanded polystyrene foam cups. At room temperature and pressure it is a volatile colourless liquid (boiling point 36 °C).
Natural gas piped to Australian homes (Origin Energy, AGL, Jemena networks across NSW and Victoria) is approximately 90% methane (CH₄), the smallest alkane. Households use it for heating and cooking. Australia exports about 80 million tonnes of LNG (liquefied natural gas = mostly methane) per year from the NW Shelf, the world's largest LNG export sector.
The first five alkanes are methane, ethane, propane, butane, and pentane, and every one of their names ends in the -ane suffix that marks the family. The simplest alkane is methane (CH₄, n=1): 1 carbon, 4 hydrogens. Then ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂). The formula $C_nH_{2n+2}$ predicts them all. Each successive member adds one CH₂ unit to the chain, this regular pattern defines a homologous series: a family of compounds with the same general formula, differing by CH₂, and showing a gradual trend in physical properties as chain length increases.
Checking butane (n=4) against the general formula: $C_nH_{2n+2}$ gives C₄H₁₀, so 4 carbons and 10 hydrogens. Butane is the gas compressed into cigarette lighters and camping gas canisters, it is a member of the same series as methane, just two carbons longer.
The first four alkanes show up right across Australia's gas supply: methane and ethane dominate the natural gas piped to homes by Jemena and AGL, while propane and butane are the main components of the LPG bottles sold at service stations for barbecues, caravans, and rural heating. The same homologous series spans the entire domestic fuel-gas market.
As chain length increases across the alkane homologous series, three physical properties change in a smooth, predictable trend. (1) Boiling point rises: methane (−162 °C), then ethane (−89 °C), then propane (−42 °C), then butane (−1 °C), then pentane (+36 °C). Longer chains have more surface area for van der Waals (intermolecular) forces, so more energy is required to separate the molecules. (2) Viscosity increases: short alkanes are thin, runny liquids; long alkanes are thick and oily; very long chains form waxes and solids. (3) Flammability decreases: short-chain alkanes (methane, ethane) are highly flammable gases; long-chain waxes are much harder to ignite.
These trends are not coincidences, they all follow from the single underlying cause: longer chains have stronger intermolecular forces. Understanding this cause-and-effect relationship is more powerful than memorising each individual trend, because it lets you predict properties for any alkane you have never encountered. An alkane with n=20 must have a higher boiling point than pentane, be more viscous, and be less flammable, no measurement needed.
Paraffin wax (C₂₀–C₄₀ alkanes) has a melting point of 46–68 °C and is solid at room temperature, the same family as methane gas. The only difference is chain length (1 carbon vs 20–40), which creates intermolecular forces strong enough to make the substance solid rather than gaseous at 25 °C.
Lubricating oils for Australian mining machinery (Caterpillar excavators, haul trucks) use long-chain alkane-based mineral oils (C₂₀–C₃₅) precisely because their high viscosity and high boiling points make them stable at the operating temperatures of heavy diesel engines in the Pilbara heat, where summer temperatures exceed 45 °C.
As alkane chain length increases, boiling point in a smooth, predictable trend. Longer chains have more surface area for van der Waals, or , forces. More energy is therefore needed to the molecules. also increases, so short alkanes are runny while long ones are thick and oily. Flammability , so long-chain waxes are much harder to ignite than methane.
At the start of this lesson, you heard that methane is an invisible gas while candle wax is a solid at room temperature, yet both are alkanes, differing only in how many carbon atoms they chain together. The general formula CₙH₂ₙ₊₂ predicts the structure of every alkane from a single equation.
Now that you've worked through the lesson, can you explain how increasing the chain length changes the boiling point and physical state of an alkane? Why is methane a gas, pentane a liquid, and paraffin wax a solid, even though all three belong to the same homologous series?
Q1. Write the molecular formula and state the number of hydrogen atoms in hexane (6 carbons). Use the general formula CnH(2n+2).
Q2. Explain the trend in boiling points for methane, propane, and octane. Why does natural gas (mainly methane) exist as a gas at room temperature while candle wax (long chains) is a solid?
Q3. Construct a table comparing the first four alkanes (methane to butane): name, formula, number of carbons, number of hydrogens, state at room temperature, and approximate boiling point.