Alkane Properties and Combustion

Overview

This lecture covers the key physical properties of alkanes, how they change with chain length, and how to write and balance combustion equations for alkanes.

Properties of Alkanes

Alkanes are hydrocarbons containing only single bonds between carbon atoms.
Boiling point increases as the carbon chain length increases.
Short-chain alkanes (four carbons or fewer) are gases at room temperature; longer chains are liquids or solids.
Short-chain alkanes are more volatile, meaning they evaporate easily.
Longer-chain alkanes are more viscous, making them thick and sticky.
Shorter alkanes are more flammable and easier to ignite.

Combustion of Alkanes

Alkanes are used as fuels because they release energy when burned with oxygen.
Complete combustion occurs when there is enough oxygen, producing carbon dioxide and water.
Combustion reactions are exothermic, releasing energy.
Carbon and hydrogen in the hydrocarbon are oxidized during combustion.

Writing and Balancing Combustion Equations

To write the combustion equation, react the alkane with O₂ to form CO₂ and H₂O.
Balance carbons first (set CO₂), then hydrogens (set H₂O), then oxygens (set O₂).
Example: Complete combustion of propane (C₃H₈):
C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
Example: Complete combustion of nonane (C₉H₂₀):
C₉H₂₀ + 14 O₂ → 9 CO₂ + 10 H₂O

Key Terms & Definitions

Alkane — a hydrocarbon with only single bonds between carbon atoms.
Volatile — easily evaporated at normal temperatures.
Viscous — thick and sticky, resists flow.
Flammable — easily set on fire or ignited.
Combustion — a chemical reaction with oxygen that releases energy, usually as heat and light.
Complete Combustion — occurs when there is enough oxygen for all carbon to form CO₂ and all hydrogen to form H₂O.
Exothermic Reaction — a reaction that releases energy to the surroundings.

Action Items / Next Steps

Practice writing and balancing combustion equations for other alkanes.
Review the properties and trends of alkanes with different chain lengths.