12.1 - Properties of alkanes

Question 1

What type of bonds are present in alkanes?

Answer 1

Saturated hydrocabons containing only single covalent bonds

• Each carbon is bonded to 4 other atoms by a single covalent bond called - sigma bonds

Question 2

What are sigma bonds?

Answer 2

Covalent bond - shared pair of electrons

Sigma bond - overlapping of 2 orbitals (each overlapping orbital has one electron so the 2 orbitals have 2 in total)

Question 3

What is the shape and bond angle of an alkane?

Answer 3

Each carbon is surrounded by 4 electron pairs in 4 sigma bonds

• Maximum repulsion between these electron pairs result in 3D TETRAHEDRAL ARRANGEMENT
  109. 5 DEGREES BOND ANGLE

Question 4

Why can atoms in an alkane rotate freely?

Answer 4

The sigma bonds act as axes of rotation so the shapes are not rigid, (double bonds allow no rotation)

Question 5

How are the alkanes of crude oil separated?

Answer 5

Crude oil has 100s of alkanes which are separated by fractional distillation which shows alkanes have many variations in boiling point

Question 6

What is the effect of chain length on boiling point?

Answer 6

Longer the chain length the higher the boiling point
- This is because there are more London forces between the molecules due to larger surface area of contact, so more energy is needed to break the bonds

Question 7

What is the effect of branching on the boiling point of alkanes?

Answer 7

The more branches there are the less the surface area of contact because the branches get in the way of each other, fewer London forces lowering the bp.