12.1 Properties of Alkanes

Question 1

What are alkanes?

Answer 1

Homologous series

of saturated hydrocarbons

with general formula C_nH_2n+2

Question 2

Describe the bonding within alkanes:

Answer 2

each C has 4 𝝈-bonds

Question 3

What is a sigma bond?

Answer 3

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.

Question 4

Describe the shape of alkanes:

Answer 4

• 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

Question 5

What are the factors which affect the boiling point of alkanes?

Answer 5

• CHAIN LENGTH
• SIDE CHAINS

Question 6

Describe & explain the factors which affect the boiling point of alkanes:

Answer 6

• 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

Question 7

Describe & explain how fractional distillation is used to separate hydrocarbons within crude oil:

Answer 7

• 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

Question 8

How does the boiling point change down the alkane homologous series?

Answer 8

• 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.