Alkanes

Question 1

How can crude oil be separated and what property allows this to happen?

Answer 1

Fractional distillation

Boiling points of the different hydrocarbon chains

Question 2

What process can break long alkane chains and what is formed?

Answer 2

Cracking

Shorter alkanes and alkenes are made

Question 3

How does the size of an alkane chain effect the boiling point?

Answer 3

The larger the chain the larger the surface area, meaning more points of contact between chains and therefore more intermolecular London forces. This means they have a higher boiling point.

Question 4

How does branching effect the boiling points of alkanes?

Answer 4

Branched alkanes cannot get as close therefore have fewer points of contact so weaker London forces so therefore a lower boiling point.

Question 5

Why are alkanes non- polar?

Answer 5

C-H only have a small electronegative difference therefore there is hardly any bond polarity.
Totally symmetrical alkanes such as methane are completely non polar.

Question 6

What shape is around a C atom and why and what is the angle?

Answer 6

Tetrahedral arrangement of single covalent bonds due to equal amount of repulsion between the 4 bonding pairs of electrons.
109.5

Question 7

Describe a sigma bond:

Answer 7

1. When either 2 s orbitals or 2 p orbitals or 1 s and 1 p orbital overlap head on and the bonded atoms are free to rotate around the sigma bond
2. Sigma bonds are very strong and have a high bond enthalpy
3. They have strong electrostatic attraction due to the high electron density between nuclei

Question 8

Why do alkanes have a low reactivity?

Answer 8

Non polar

Strong sigma bonds with high bond enthalpies

Question 9

What products are formed during complete combustion?

Answer 9

Carbon dioxide and water

Question 10

What products are formed when oxygen is in limited supply?

Answer 10

Carbon monoxide and water

Question 11

What happens if oxygen is in really short supply?

Answer 11

The carbon is not oxidised leaving Carbon and water as products

Question 12

What is the overall reaction equation of the chlorination of methane? And what are the conditions?

Answer 12

CH4+ Cl2= CH3Cl + HCl

Ultra violet light and excess methane

Question 13

Describe the mechanism of the chlorination of methane:

Answer 13

1. Initiation Cl2= 2Cl.
   UV light provides the energy to break the bond homolytically.
2. Propagation
   A) CH4 + Cl. = .CH3 + HCl
   B) .CH3 + Cl2 = CH3Cl + Cl.
   Chain reaction
3. Termination
   .CH3 + Cl. = CH3Cl (major)
   .CH3 + .CH3 = CH3CH3 (minor)
   Cl. + Cl. = Cl2

Question 14

What happens if the halogen is in excess?

Answer 14

Further substitution is favoured