Alkanes

Question 1

What is the shape of alkane molecules?

Answer 1

Tetrahedral (4 pairs of bonding electrons)

Question 2

What is the type of bond that joins alkane atoms?

Answer 2

Covalent bond

Question 3

What is a covalent bond?

Answer 3

A bond formed by the sharing of a pair of electrons

Question 4

What holds covalent bonds together?

Answer 4

Van dear waals forces

Question 5

How does the boiling point differ between branched chain alkanes and straight chain isomers?

Answer 5

A branched chain has a lower boiling point because they can’t pack closely together, they have a smaller surface area. Therefore the van der waals forces are reduced.

Question 6

What happens when you burn an alkane?

Answer 6

It is oxidised. This is a combustion reaction. Alkane + oxygen - carbon dioxide + water

Question 7

What happens when you burn alkanes in a limited supply of oxygen?

Answer 7

It produces carbon monoxide and water

Question 8

What is the general formula for an alkane?

Answer 8

CnH2n+2

Question 9

What is bond fission?

Answer 9

Breaking a covalent bond

Question 10

What is heterolytic fission?

Answer 10

Two different substances are formed. A positively charged cation and a negatively charged anion.

Question 11

What is a single covalent bond?

Answer 11

A shared pair of electrons between two atoms.

Question 12

What is homolytic fission?

Answer 12

Two electronically uncharged ‘radicals’ are formed. Radicals are particles that have an unpaired electron. Because of the unpaired electron, these radicals are very reactive.

Question 13

What is it called when a hydrogen atom is replaced by chlorine or bromine?

Answer 13

Free radical substitutions

Question 14

What are the three stages of free radical substitution?

Answer 14

Initiation, propagation, termination

Question 15

What happens in the initiation stage of free radical substitution?

Answer 15

Uv light provides energy to break a bond (e.g. Cl2 becomes 2Cl’ )
The bond splits equally and each atom keeps one electron - this is homolytic fission. The atom becomes a highly reactive free radical due to its unpaired electron.

Question 16

What happens in the propagation phase of radical substitution?

Answer 16

Free radicals are used up and created in a chain reaction.

1. ) Cl’ attacks a methane molecule (e.g. Cl’ + CH4 -‘CH3 + HCl
   2) the new methyl free radical ‘CH3 can attack another Cl2 molecule (‘CH3 + Cl2 - CH3Cl + Cl’)
   3) the new Cl’ can attack another CH4 molecule and so on until all cl2 and Ch4 molecules are wiped out

Question 17

What happens in the termination reaction in free radical substitution?

Answer 17

Free radicals are mopped up
1) if two free radicals join together, they make a stable molecule
2) there are heaps of possible termination reactions. Here are a couple of them to get the idea
Cl’ + ‘CH3 - CH3Cl
‘CH3 + ‘CH3 - C2H6

Question 18

What is the problem with free radical substitution?

Answer 18

You end up with a mixture of products.