O1: 12 Alkanes

Question 1

What are alkanes?

Answer 1

Saturated hydrocarbons.

Question 2

What is petroleum?

Answer 2

A mixture consisting mainly of alkane hydrocarbons that can be separated by fraction distillation.

Question 3

What does cracking involve?

Answer 3

Breaking C-C bonds in alkanes.

Question 4

What are the conditions for thermal cracking?

Answer 4

High pressure and high temperature.
Produces a high percentage of alkenes.

Question 5

What are the conditions for catalytic cracking?

Answer 5

Slight pressure, high temperature, and the presence of a zeolite catalyst.
Used mainly to produce motor fuels and aromatic hydrocarbons.

Question 6

What are alkanes used for?

Answer 6

Fuels.

Question 7

What pollutants can combustion produce?

Answer 7

Nitrogen oxides, CO, C, and unburned fuels.

Question 8

How can the gaseous pollutants produced from engines be removed?

Answer 8

Use catalytic converters which converts them into water vapour and less harmful gases.

Question 9

What does the combustion of hydrocarbons containing sulfur produce?

Answer 9

Sulfur leads to sulfur dioxide that causes air pollution.

Question 10

How can SO2 be removed from flue gases?

Answer 10

A slurry of calcium oxide and water is sprayed into the flue gas which reacts with the calcium oxide and water to form calcium sulfite.
Calcium sulfite oxidises to form calcium sulfate (gypsum).

Question 11

What are the economic reasons for cracking alkanes?

Answer 11

Longer, less useful chains are broken up into more useful, shorter chains.

Question 12

What is the reaction of methane with chlorine?

Answer 12

CH4 (g) + Cl2 (g) -> CH3Cl (g) + HCl (g)
A free-radical substitution, involving initiation, propagation, and termination.

Question 13

What happens in the initiation stage of the reaction of methane with chlorine?

Answer 13

1. The Cl-Cl bond is broken using UV light to form two chlorine atoms.
2. The Cl-Cl bond breaks homolytically
   
   • Cl-Cl -> 2Cl.

Question 14

What happens in the propagation stage of the reaction of methane with chlorine?

Answer 14

1. The chlorine free radical takes a H atom from methane to form HCl, leaving a methyl free radical.
   
   • Cl. + CH4 -> HCl + .CH3
2. The methyl free radical reacts with a chlorine molecule, producing another chlorine free radical and a chloromethane molecule.
   
   • .CH3 + Cl2 -> CH3Cl + Cl.
3. The new Cl. free radical reacts with more methane, repeating the two steps.

Question 15

What happens in the termination stage of the reaction of methane with chlorine?

Answer 15

The free radicals can be removed in three ways:
1. Two chlorine free radicals react together to form chlorine.
- Cl. + Cl. -> Cl2
2. Two methyl free radicals react together to form ethane.
- .CH3 + .CH3 -> CH3 CH3
3. A chlorine free radical and a methyl free radical react together to form chloromethane.
- Cl. + .CH3 -> CH3Cl