3.3.2 Alkanes
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_____, _____ and _____ have no isomers
Methane, ethane and propane
Name 4 physical properties of alkanes
• Non-polar • As chain length increases = boiling points increase • Insoluble in water • Relatively unreactive
Explain why alkanes are almost non-polar & what does this mean
∵ electronegativities of carbon and hydrogen = similar ∴ intermolecular forces = weak van der Waals forces (Larger the molecule = stronger the van der Waals forces)
Explain why as the chain length increases, the boiling points increase
Intermolecular forces increase
Alkanes with branched chains have _____ melting points than straight chain alkanes (with same no. of C)
LOWER melting points than straight chain alkanes (with same no. of C)
Why do alkanes with branched chains have lower melting points than straight chain alkanes (with same no. of C)?
∵ less surface area & weaker van der Waals forces
Solid alkanes have a ____ feel
waxy
Shorter chains of alkanes are ___ at room temp.
gases
Why are alkanes insoluble in water?
Hydrogen bonds holding water molecules together = much stronger than van der Waals forces between alkane molecules
What do alkanes react with?
halogens (& burn under suitable conditions)
What is petroleum (crude oil)?
Mixture of hydrocarbons - mostly alkanes
How did crude oil form?
Formed millions of years ago by breakdown of plant and animal remains at high pressure and temperature
Why does crude oil contains other elements?
Contains some elements that were in original plants and animals
Describe the stages of fractional distillation
• Crude oil = vaporised at 350°C • Vaporised crude oil goes into fractionating column & rises up through trays • Largest hydrocarbons don’t vaporise = run to the bottom & form a gooey residue • Crude oil vapour goes up = gets cooler • Each fraction condenses at different temperature • Drawn off at different levels in column • Hydrocarbon with lowest boiling points don’t condense • Drawn off as gases at top of column
What is cracking?
Breaking long-chain alkanes into smaller hydrocarbons, involves breaking C-C bonds
Give 2 conditions of thermal cracking
• 900°C - high temperature • Up to 70 atm - high pressure
What does thermal cracking produce?
Produces a lot of alkenes
How do can we avoid the decomposition (of C and H) in thermal cracking?
Alkanes kept in these conditions for very short time (e.g. a second)
Name 3 conditions of catalytic cracking
• Uses zeolite catalyst (hydrated aluminosilicate) • 1-2 atm - slight pressure • 450°C - high temperature
What is the structure of a zeolites & why?
Have honeycomb structure = enormous surface area & are acidic
What is the benefit of using a catalyst?
• Cuts costs ∵ reaction can be done at a low pressure & low temperature • & catalyst speeds up reaction - saving time (& time is money)
What does catalytic cracking normally produce?
Mostly produces aromatic hydrocarbons and motor fuels (+ branched alkanes and cycloalkanes)
What do aromatic compounds contain?
Benzene rings
What is the structure of benzene rings?
Have 6 carbon atoms with 3 double bonds