Alkanes

The first six alkanes (memorise these):

n	Name	Formula	State at room temp	Common source / use
1	Methane	CH₄	gas	natural gas; main greenhouse gas
2	Ethane	C₂H₆	gas	in natural gas; feedstock
3	Propane	C₃H₈	gas	bottled gas (LPG)
4	Butane	C₄H₁₀	gas (compressed to liquid)	cigarette lighters, camping stoves
5	Pentane	C₅H₁₂	liquid	solvent
6	Hexane	C₆H₁₄	liquid	solvent

All follow the CₙH₂ₙ₊₂ rule:

• 1 C → 2(1)+2 = 4 H → CH₄.
• 4 C → 2(4)+2 = 10 H → C₄H₁₀.
• 6 C → 2(6)+2 = 14 H → C₆H₁₄.

Properties change gradually:

• BP increases with chain length (CH₄ −162 °C; C₆H₁₄ +69 °C).
• Viscosity (thickness) increases.
• Volatility (how easily it evaporates) decreases.
• Flammability decreases (longer chains burn less readily).

Alkanes have very stable C-C and C-H bonds. Most reagents that easily attack alcohols, alkenes or carboxylic acids do nothing to alkanes at room temperature. So alkanes don't react with acids, bases, water, oxidising agents (in cold conditions), or bromine water.

The big EXCEPTION is combustion — burning in oxygen, an exothermic redox reaction.

Complete combustion (plenty of oxygen): $\text{C}_x\text{H}_y + (x + y/4)\text{O}_2 \rightarrow x\text{CO}_2 + (y/2)\text{H}_2\text{O}$

For methane: CH₄ + 2 O₂ → CO₂ + 2 H₂O. For propane: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O.

Incomplete combustion (limited oxygen) — produces CO (toxic carbon monoxide) AND particulates (soot). E.g. 2 CH₄ + 3 O₂ → 2 CO + 4 H₂O.

You can SEE this in a Bunsen burner:

• Air hole OPEN → blue flame (complete combustion, CO₂ only). HOT but no soot.
• Air hole CLOSED → yellow flame (incomplete combustion). COOLER, sooty.

This is why blocked chimneys and faulty gas appliances are so dangerous — they produce CO. A CO alarm is essential in any home with gas heating.

Alkanes are the world's biggest source of energy. Practical uses depend on chain length:

• CH₄ (natural gas): piped to homes and power stations. Burns cleanly.
• LPG (C₃-C₄, propane and butane): bottled gas for camping, off-grid heating.
• Petrol (C₆-C₁₀): car engines.
• Kerosene (C₁₀-C₁₆): jet fuel and lamp oil.
• Diesel (C₁₂-C₂₀): trucks, ships, trains.
• Fuel oil and bitumen (C₂₀+): ship fuel, road tar.

Why they're so widely used:

• Energy density: high energy per kg.
• Easy to store: liquids at modest pressure or temperature.
• Easy to transport: pipelines, tankers, bottles.
• Established infrastructure: filling stations everywhere.

The downside is the inevitable CO₂ from every combustion event. Burning alkanes for energy is the single biggest contributor to climate change. Modern policy is shifting toward renewable electricity, hydrogen and biofuels — but alkanes still dominate today.

MYP criterion D classic: 'Evaluate the use of alkanes as fuels.' Strong answers cover energy density (positive), CO₂ emissions (negative), comparative ease of storage vs hydrogen (positive), and air quality at point of use (incomplete combustion → CO + PM).