Combustion and Chlorination of alkanes

Question 1

What are alkanes used for?

Answer 1

Fuels.

Question 2

Complete combustion of alkanes

Answer 2

alkanes burn completely with a plentiful supply of oxygen

always makes CO2 and H2O

example: CH4(g) + 2O2(g) –> CO2(g) + 2H2O(g)

Question 3

Incomplete combustion of alkanes

Answer 3

burned with limited supply of oxygen

makes: CO or C (soot)

CH4(g) + 3/2O2(g) -> CO(g) + 2H2O(g)
or
CH4(g) + O2(g) -> C(s) + 2H2O(g)

Question 4

What are the products of an internal combustion engine and how can they be removed?

Answer 4

products: NO2x, CO (toxic gas), Carbon and unburned hydrocarbons.

can be removed via catalytic converters.

Catalytic converter is honeycomb made of a ceramic material coated with platinum and rhodium catalysts.
- the honeycomb provides a large SA.
- the platinum oxidises CO and hydrocarbons
- the polluting gases react with each other to form less harmful products when they pass over the catalyst.

2CO(g) + 2NO(g) -> N2(g) + 2CO2(g)
or
C7H16(g) + 22NO(g) -> 11N2(g) + 7CO2(g) + 8H2O(g)

Question 5

explain why sulfur dioxide can be removed from flue gases using calcium oxide or calcium carbonate.

Answer 5

• powdered calcium carbonate (limestone0 or calcium oxide is mixed with water to make an alkaline slurry.
• when flue gases mix with the slurry, the acidic sulfur dioxide gas reacts with the calcium compounds to form a harmless salt (calcium sulfate)

CaO + SO –> CaSO(s)

Question 6

The reaction of methane with chlorine.

Answer 6

CH4 + Cl2 –> CH3Cl + HCl

Question 7

explain the reaction of methane and chlorine as a free-radical substitution mechanism involving initiation, propagation and termination steps.

Answer 7

initiation:
Cl2 –> 2Cl*

the Cl-Cl bond absorbs UV light.

propagation:
CH4 + Cl* –> *CH3 + HCl
*CH3 + Cl2 –> CH3Cl + *Cl

chlorine free radical takes a H atom from methane to form HCl and a methyl free radical.
the methyl free radical takes a chlorine atom to form chloromethane and a chlorine free radical.
chlorine free radical is a catalyst

termination:
*Cl + *Cl –> Cl2
*CH3 + *CH3 –? CH3CH3
*Cl + *CH3 –> CH3Cl

Question 8

Answer 8