12 - Alkanes

Question 1

What is the general formula for an alkane?

Answer 1

C(n)H(2n+2)

Question 2

Describe the polarity and intermolecular forces in alkanes.

Answer 2

Non-polar
Form only van der Waals

Question 3

What are alkanes?

Answer 3

Saturated hydrocarbons.

Question 4

What is petroleum?

Answer 4

A mixture containing mostly alkane hydrocarbons that can be separated by fractional distillation.

Question 5

Describe the conditions for thermal cracking.

Answer 5

High (700-1200K) temperature, high pressure (up to 7000kPa/70 atm)

Question 6

Give the conditions for catalytic cracking.

Answer 6

High but lower temperature (720K), slight pressure, zeolite catalyst.

Question 7

What does catalytic cracking produce?

Answer 7

Motor fuels and aromatic compounds.

Question 8

What occurs in thermal cracking?

Answer 8

C-C bonds are broken.

Question 9

What does thermal cracking produce?

Answer 9

Mostly alkenes.

Question 10

Where is the fractionating column hottest?

Answer 10

The bottom

Question 11

Describe the process of fractional distillation. (3)

Answer 11

Crude oil vapourised
Gases enter fractionating column
Gases condense at different points due to their different boiling point
Each fraction is a mixture

Question 12

Why is cracking done?

Answer 12

Shorter chains are more useful
Alkenes are more reactive and are used a chemical feedstock

Question 13

What do alkanes completely combust to?

Answer 13

Carbon dioxide + water

Question 14

What do alkanes incompletely combust to?

Answer 14

Carbon monoxide + C (soot) + water

Question 15

Why are alkanes used as fuels?

Answer 15

They release high amounts of energy.

Question 16

What harmful byproducts are produced by alkane combustion engines?

Answer 16

Carbon monoxide, nitrogen oxides

Question 17

What do catalytic convertors do?

Answer 17

Remove nitrogen oxides, carbon monoxide and unburnt hydrocarbons (particulates) from exhausts into safer compounds ie CO2 or H2O or N2

By reacting them with each other

Question 18

What metal catalyst do catalytic convertors use?

Answer 18

Rhodium catalyst

Question 19

Why are unburnt hydrocarbons/carbon particulates bad?

Answer 19

They can cause respiratory problems

Question 20

What can sulphur dioxide cause?

Answer 20

Acid rain

Question 21

How is sulfur dioxide removed from flue gases?

Answer 21

Flue gas desulfurisation , using CaO and water to produce CaSO4. CaCO3 can also be used, producing CO2 and CaSO4

Question 22

Name the three stages of a free radical substitution.

Answer 22

Initiation, Propagation, Termination

Question 23

Under what condition do alkanes react with halogens to form halogenoalkanes?

Answer 23

UV light

Question 24

What is propagation?

Answer 24

The replacement of a hydrogen with a halogen atom, and the radical halogen reforming.

Question 25

What is a radical?

Answer 25

A highly reactive molecule with an unpaired electron.

Question 26

What occurs in termination?

Answer 26

Two radicals react to form a stable compound with no unpaired electrons.

Question 27

Why might values of enthalpy for combustion of a liquid alkane be different to that using an expression using bond enthalpies?

Answer 27

Enthalpy expression uses gaseous; alkane is not gaseous