Alkanes

Question 1

Hydrocarbon

Answer 1

A compound containing only hydrogen and carbon

Question 2

Use of crude oil

Answer 2

A source of hydrocarbons, separated as fractions with
different boiling points by fractional distillation,
which can be used as fuels or for processing into
petrochemicals

Question 3

Saturated hydrocarbons

Answer 3

Hydrocarbons with no C=C double bonds, e.g. alkanes and cycloalkanes

Question 4

Shape around carbon atoms in alkanes

Answer 4

Tetrahedral, as the carbon is bonded to 4 other atoms. Bond angle of 109.5deg.

Question 5

Boiling points of alkanes

Answer 5

• Increases as chain length increases
• Van der Waals forces must be broken for alkanes to boil
• As chain length increases, more Van der Waals forces are present, as points of contact increases
• Branched alkanes have lower boiling points, as there are fewer points of contact, and more space between molecules. This leads to fewer Van der Waals’ forces.
• More intermolecular forces results in more energy needed to break them resulting in higher boiling points

Question 6

Complete combustion of alkanes

Answer 6

• Produces CO(2) and H(2)O

- Short chained alkanes burn more cleanly than long chained alkanes

Question 7

Incomplete combustion of alkanes

Answer 7

• Occurs when there is a limited supply of oxygen

- Produces CO and H2O

Question 8

Dangers of incomplete combustion

Answer 8

• Produces CO (colourless, odourless gas)

- Prevents haemoglobin binding with oxygen, starving tissues of oxygen, potentially fatal

Question 9

Catalytic cracking

Answer 9

• Short chained alkanes are much more useful than long chained alkanes - for fuels and polymer production
• Short chained alkanes are in demand, but there aren’t enough
• Cracking breaks down long chained alkanes into shorter chained alkanes and alkenes.
• Random process, don’t know where chain is going to break
• Zeolite catalyst used at 450degC.

Question 10

Producing branched alkanes

Answer 10

Process called isomerisation.

Question 11

Producing cyclic hydrocarbons

Answer 11

Process called reforming. Aliphatic hydrocarbons can be converted into cyclic/aromatic hydrocarbons, producing hydrogen, as well as the hydrocarbon, which can be used to make ammonia/in margarine production

Question 12

What’s the point of branched, cyclic and short chained hydrocarbons?

Answer 12

They have better octane ratings (how well a fuel burns). They promote efficient combustion in car engines.

Question 13

Problems with fossil fuels

Answer 13

• They emit CO(2) when burnt, building up in our atmosphere, resulting in global warming and climate change
• Over-reliance on fossil fuels, which are non-renewable and are running out: need to develop renewable energy sources, e.g. alcohols and biodiesel that are based on plants. However, if rich countries desire plant based fuel sources, poorer countries may lose out on space for growing food crops

Question 14

Substitution reactions of alkanes

Answer 14

• with halogens to make halogenoalkanes
• in presence of UV light/at 300degC
• radical substitution

Question 15

Radical

Answer 15

A species with an unpaired electron

Question 16

Mechanism of radical substitution of alkanes (using methane and chlorine as an example)

Answer 16

Initiation:
Cl-Cl -> Cl’ + Cl’ . This is homolytic fission in UV light
Propagation, making a chain reaction (lots of Cl’):
CH(4) + Cl’ -> ‘CH(3) + HCl (radical stealing a H)
‘CH(3) + Cl(2) -> CH(3)Cl + Cl’
Termination:
‘CH(3) + Cl’ -> CH(3)Cl
‘CH(3) + ‘CH(3) -> C(2)H(6)
Cl’ + Cl’ -> Cl(2)

Question 17

Limits of radical substitution

Answer 17

• Reaction is uncontrollable

- Many products are formed by radicals reacting together - many pathways the reaction could take