Alkanes: Combustion of Alkanes

Question 1

What are alkanes used for?

Answer 1

• Alkanes are used for fuels
• The combustion of alkanes is a highly exothermic process
• This makes them good fuels because they release a large amount of energy per gram of fuel

Question 2

Describe complete combustion of alkanes

Answer 2

• In excess oxygen, organic compounds can undergo complete combustion forming CO2 and H2O

Question 3

Describe incomplete combustion of alkanes

Answer 3

• If oxygen is limited then incomplete combustion will occur forming either CO or C and H2O
• Carbon monoxide is a toxic gaseous product
• Carbon particulates are also known as soot (solid) and leads to global dimming

Question 4

What kind of alkanes are used in an internal combustion engine?

Answer 4

• Alkanes with chain lengths of 5–10 carbon atoms are used as fuels in internal combustion engines

Question 5

What types of pollutants are released from internal combustion engines?

Answer 5

• NOx
• CO
• Carbon
• Unburned hydrocarbons

Question 6

What is the environmental impact of nitrogen oxides?

Answer 6

• Can form acid rain
• Nitrogen + oxygen –> nitrogen monoxide
• N2 + O2 → 2NO
• Nitrogen monoxide further reacts with oxygen to make nitrogen dioxide
• 2NO + O2 → 2NO2
• Nitrogen dioxide reacts with rain water and more oxygen to form nitric acid, which contributes to acid rain
• 4NO2 + 2H2O + O2 → 4HNO3

Question 7

How can these pollutant gases be removed from internal combustion engines?

Answer 7

• These gaseous pollutants from internal combustion engines can be removed using catalytic converters

Question 8

Describe what reactions occur in a catalytic convertor

Answer 8

• Removal of CO
- 2CO + O2 → 2CO2

• Removal of CO and NO
- 2CO + 2NO → 2CO2 + N2

• Removal of NO
- 2NO → N2 + O2

• Removal of unburnt hydrocarbons
- pentane + oxygen → carbon dioxide + water

• Removal of unburnt hydrocarbons and NO
- octane + nitrogen monoxide → carbon dioxide + nitrogen + water

Question 9

Describe how a catalytic convertor works

Answer 9

• Carbon monoxide, nitrogen oxides and unburnt hydrocarbons are released from engine and passed through catalytic convertor
• The catalyst has a honeycomb pattern covered in platinum and rhodium (pattern increases surface area)
• The pollutants are converted into harmless gases and released from exhaust
• Gases released are carbon dioxide, nitrogen and water

Question 10

What impurities are found in crude oil and how does this lead to pollution?

Answer 10

• Sulfur is found as an impurity in crude oil and other fossil fuels
• Combustion of hydrocarbons containing sulfur leads to sulfur dioxide that causes air pollution and acid rain

Question 11

How does sulfur dioxide become acid rain?

Answer 11

• Sulfur dioxide can dissolve in rain water to form sulfurous acid (H2SO3)
- SO2 + H2O → H2SO3

• Sulfur dioxide may be oxidized to sulfur trioxide
- 2SO2 + O2 → 2SO3

• Sulfur trioxide + water → sulfuric acid
- SO3 + H2O → H2SO4

• Sulfur oxides and nitrogen oxides dissolve in water which contributes to acid rain

Question 12

How does acid rain affect the environment?

Answer 12

• Acid rain destroys trees and vegetation
• Corrodes buildings and statues
• Kills fish in lakes

Question 13

What are flue gases?

Answer 13

• Gases given out by power stations

- A flue is a pipe or opening in a chimney

Question 14

How is sulfur dioxide removed from flue gases?

Answer 14

• The acidic sulfur oxides are removed from the flue gases using a base such as calcium oxide or calcium carbonate
• SO2 + CaO → CaSO3
• This is a neutralisation reaction called flue gas desulfurisation