Properties of the alkanes

Question 1

What are alkanes the main component of?

Answer 1

Natural gas and crude oil

Question 2

Why are alkanes used in crude oil?

Answer 2

They are some of the most stable organic compounds and their lack of reactivity has allowed crude oil deposits to remain on earth for many years

Question 3

What are alkanes mainly used as?

Answer 3

Fuels

Question 4

Why are alkanes used as fuels?

Answer 4

They react with oxygen to generate heat

Question 5

What is the general formula of alkanes?

Answer 5

CnH2n+2

Question 6

What type of hydrocarbons are alkanes?

Answer 6

Saturated

Question 7

How are the hydrogen and carbon atoms joined in alkanes?

Answer 7

By single covalent bonds

Question 8

How many atoms is each carbon atom in an alkane bonded to?

Answer 8

Four

Question 9

What are the covalent bonds in alkanes called?

Answer 9

Sigma bonds

Question 10

What is a covalent bond?

Answer 10

A shared pair of electrons

Question 11

How is a sigma bond formed?

Answer 11

It is the result of the overlap of two orbitals, one from each bonding atom

Question 12

How many electrons does each overlapping orbital contain?

Answer 12

One

Question 13

How many electrons are in a sigma bond?

Answer 13

Two

Question 14

Where is a sigma bond positioned?

Answer 14

On a line directly between bonding atoms

Question 15

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

Answer 15

Four

Question 16

What are the two possible sigma bonds in alkanes?

Answer 16

C-C and C-H

Question 17

What is each carbon atom in an alkane surrounded by?

Answer 17

Four electron pairs in four sigma bonds

Question 18

What shape do alkanes have?

Answer 18

Tetrahedral arrangement around each carbon atom

Question 19

Why do alkanes have a tetrahedral shape?

Answer 19

Repulsion between the electron pairs in the sigma bonds causes a tetrahedral arrangement

Question 20

What size are the bond angles in alkanes?

Answer 20

109.5 degrees

Question 21

Why are alkane shapes not rigid (can move)?

Answer 21

The sigma bonds act axes around which the atoms can rotate freely, e.g. into a U shape

Question 22

How do oil refineries separate crude oil into fractions?

Answer 22

By fractional distillation in a distillation tower

Question 23

What does each fraction of crude oil contain?

Answer 23

A range of alkanes

Question 24

Why is fractional distillation of alkanes possible?

Answer 24

The boiling points of the alkanes are different

Question 25

What determines an alkane’s boiling point?

Answer 25

Chain length - boiling point increases as chain length increases

Question 26

Why do boiling points increase as the chain length increases?

Answer 26

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

Question 27

How are intermolecular forces related to boiling point?

Answer 27

The greater the intermolecular forces, the higher the boiling point

Question 28

What molecules do London forces act between?

Answer 28

Molecules that are in close surface contact

Question 29

What happens to molecules as chain length increases?

Answer 29

They have a larger surface area, so more surface contact is possible between them

Question 30

Why does chain length affect boiling point?

Answer 30

London forces between molecules will be greater so more energy will be required to overcome the forces

Question 31

What do isomers of alkanes all have in common?

Answer 31

Same molecular mass

Question 32

How does branching affect boiling point?

Answer 32

Branched isomers have lower boiling points compared to straight-chain isomers

Question 33

Why do branched structures have lower boiling points? (2 reasons)

Answer 33

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