12.1 Properties of Alkanes
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What are alkanes?
Homologous series of saturated hydrocarbons with general formula CnH2n+2
Describe the bonding within alkanes:
each C has 4 𝝈-bonds
What is a sigma bond?
End-to-end overlap of 2 orbitals 1 from each bonded atom. Consists of 2 electrons. Electron density centred about a line directly between the 2 nuclei.
Describe the shape of alkanes:
• bond angle = 109.5 • tetrahedral shape around each carbon • each carbon surrounded by 4 sigma bonded electron pairs + no unpaired electrons • Carbon chains can rotate about sigma bonds
What are the factors which affect the boiling point of alkanes?
• CHAIN LENGTH • SIDE CHAINS
Describe & explain the factors which affect the boiling point of alkanes:
• CHAIN LENGTH • Chain length ￪ = boiling point ￪ • larger surface area of contact • more London forces between molecules • ∴ require more energy to overcome • BRANCHING • Branching ￪ = boiling point ￬ • fewer contact points between molecules • less London forces • ∴ require less energy to overcome
Describe & explain how fractional distillation is used to separate hydrocarbons within crude oil:
• mixture of hydrocarbons vaporised, forming gas • gaseous mixture enters fractional distillation tower from bottom • cooler at top, hotter at bottom • hydrocarbons with highest boiling point condense lower, • hydrocarbons with lower boiling points remain gas & rise to cooler area, where they condense further up • separates hydrocarbons by boiling point
How does the boiling point change down the alkane homologous series?
• increased carbon chain length • greater surface area of contact • more London forces between molecules • require more energy to overcome • therefore boiling point increases down the homologous series.