12.1 Properties of the Alkanes

Question 1

What are alkanes

Answer 1

The main components of natural gas and crude oil.
Amongst the most stable organic compounds and their lack of reactivity has allowed crude oil deposits to remain in the Earth for many millions of years

Question 2

What are alkanes mainly used for

Answer 2

As fuels, exploiting their reaction with oxygen to generate heat

Question 3

What is the general formula of alkanes

Answer 3

CnH(2n+2)

Question 4

Alkanes are ______ hydrocarbons, containing only ______ and ______ atoms joined together by ______ covalent bonds

Answer 4

Saturated
Carbon
Hydrogen
Single

Question 5

What is a sigma bond

Answer 5

σ- bond

Each carbon atom in an alkane is joined to four other atoms by single covalent bonds

Question 6

What is a σ- bond the result of

Answer 6

Sigma bond
The result of the overlap of two orbitals, one from each bonding atom
Each overlapping orbital contains oe electron so the σ- bond has two electrons that are shared between the bonding atoms

Question 7

How many sigma bonds does each carbon atom in an alkane have

Answer 7

Four

Either C-C or C-H

Question 8

What is the shape around each carbon atom in an alkane

Answer 8

Tetrahedral arrangement

Each bond angle around 109.5

Question 9

How do oil refineries separate the crude oil into fractions

Answer 9

By fractional distillation in a distillation tower

Question 10

How is fractional distillation possible

Answer 10

Because the boiling points of the alkanes are different, increasing as their chain length increases

Question 11

Why does the boiling point of the alkanes increase

Answer 11

The weak, intermolecular London forces hold molecules together in solids and liquids but once broken, the molecules move apart from each other and the alkane becomes a gas. The greater the intermolecular forces, the higher the boiling point.

Question 12

What happens to the boiling point as chain length (of an alkane) increases, and why?

Answer 12

London forces act between molecules that are in close surface contact, as chain length increases, the molecules have a larger surface area, so more surface contact is possible between molecules
London forces between will be greater so more energy required to overcome the forces

Question 13

Increased chain length,

Answer 13

Greater surface area of contact

Stronger London forces

Question 14

Isomers of alkanes have the same…

Answer 14

…molecular mass

Question 15

What do you find when you compare the boiling points of branched isomers with straight-chain isomers

Answer 15

You find that the branched isomers have lower boiling points

Question 16

Why do branched isomers have lower boiling points than straight-chain isomers

Answer 16

There are fewer surface points of contact between molecules of the branched alkanes, giving fewer London forces.
Branches also get in the way and prevent branched molecules getting as close together as straight-chain molecules, decreasing the intermolecular forces further