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A-level Chemistry: 3.3.2 Alkanes Part 1

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This deck covers key concepts and facts about alkanes, including their physical properties, reactions, and the processes of cracking and combustion.

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_____, _____ and _____ have no isomers

Methane, ethane and propane
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Key Terms

Term
Definition
_____, _____ and _____ have no isomers
Methane, ethane and propane

Name 4 physical properties of alkanes

  1. Non-polar

  2. As chain length increases = boiling points increase

  3. Insoluble in water

  4. Relatively un...

Explain why alkanes are almost non-polar & what does this mean

∵ electronegativities of carbon and hydrogen = similar ∴ intermolecular forces = weak 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
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A-level Chemistry: 3.3.2 Alkanes Part 1
TermDefinition
_____, _____ and _____ have no isomers
Methane, ethane and propane

Name 4 physical properties of alkanes

  1. Non-polar

  2. As chain length increases = boiling points increase

  3. Insoluble in water

  4. 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

  1. Crude oil = vaporised at 350°C

  2. Vaporised crude oil goes into fractionating column & rises up through trays

  3. Largest hydrocarbons don’t vaporise = run to the bottom & form a gooey residue

  4. Crude oil vapour goes up = gets cooler

  5. Each fraction condenses at different temperature

    • Drawn off at different levels in column

  6. 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
Why are benzene rings stable?
Because the electrons are delocalised around the carbon ring
Why doesn't cracking produce 2 alkanes?
Since there isn't enough hydrogen atoms to produce 2 alkanes = one of new chains has C=C
How are the products obtained from cracking separated?
By fractional distillation
Why are alkanes are great fuels?
Burning small amount = huge amount of energy
Give the word equation for complete combustion
Alkanes + Oxygen → Carbon Dioxide + Water

Give the word equations for incomplete combustion

Alkanes + Oxygen → Carbon Monoxide + Water (+ sometimes Carbon Dioxide)

Alkanes + Oxygen → Carbon + Water

Why is carbon monoxide gas poisonous?
Binds to same sites on haemoglobin molecules in red blood cells as oxygen molecules ∴ oxygen cannot be carried around the body
Name 2 issues with soot (particulates)
Soot causes breathing problems & builds up in engines = can't work properly

Describe the greenhouse effect

  • Greenhouse gases in atmosphere = good at absorbing infrared energy (heat)

  • Emit some energy they absorb back towards Earth, keeping it warm = greenhouse effect

How does rising temperature affect water vapour?

  • Higher temps = more water vapour in air = more greenhouse effect

  • Greater cloud formation and clouds reflect solar radiation

How can carbon monoxide be removed from exhaust gases?
By catalytic converters on cars