3.3.2.3. Combustion of Alkanes

Question 1

Why are alkanes used as fuel?

Answer 1

When they are burned down, they release energy via combustion reaction (exothermic)

Question 2

What are the 2 types of combustion?

Answer 2

Complete and incomplete

Question 3

When does complete combustion happen?

Answer 3

When there is an excess of oxygen

Question 4

What are the products of complete combustion?

Answer 4

CO2 and H2O

Question 5

When does incomplete combustion happen?

Answer 5

When there is a limited amount of oxygen

Question 6

What are the products of incomplete combustion?

Answer 6

CO or C and H2O

Question 7

Why is carbon monoxide known as a silent killer?

Answer 7

It is odourless, tasteless and colourless.
- For safety, have regular maintenance checks or get a CO alarm

Question 8

How does the internal combustion engine work?

Answer 8

Small amount of fuel and lots of oxygen is drawn into the combustion chamber.
The fuel is ignited with an extremely high temperature (a spark)
The fuel the burns explosively, which forces the movement of engine parts.
Products exit via exhaust.

Question 9

What causes side reactions in the combustion engine?

Answer 9

Pressure and extreme temperatures

Question 10

What are some side products produced in the combustion engine?

Answer 10

Nitrogen: oxides of nitrogen (air is drawn into the chamber, which is 78% nitrogen). Can get nitrogen monoxide (NO) or nitrogen dioxide (NO2).
Sulphur: From impurities in crude oil which end up in fuels. Usually SO2.

Question 11

Describe the greenhouse effect

Answer 11

1. Short wavelength radiation from the Sun enters the Earth’s atmosphere.
2. Objects/ materials absorb this and re-emit it as longer wavelength radiation
3. Greenhouse gasses trap and absorb this longer wavelength radiation, causing these molecules to vibrate more and increase the temperature of the Earth
   * only molecules with polar bonds can be greenhouse gasses

Question 12

Describe global warming

Answer 12

Mainly due to ‘enhanced greenhouse effect’ and increasing CO2 levels, so more long wavelength radiation is absorbed and trapped leading to higher temperatures in the atmosphere

Question 13

What contributes to acid rain?

Answer 13

Nitrogen Oxides (NOX)

Question 14

How is acid rain formed?

Answer 14

SO2 reacts with acid rain in the air to produce sulphurous acid (H2SO3), which can further be oxidised in the presence of oxygen in the air to become H2SO4.

Question 15

What are the impacts of acid rain?

Answer 15

Destroys trees and vegetation
Corrodes limestone and buildings and statues
Kills fish in lakes

Question 16

What causes photochemical smog?

Answer 16

Pungent and toxic gasses and solid particles suspended in the air close to the surface of the Earth

Question 17

What forms photochemical smog?

Answer 17

NOX, SO2, Soot and unburnt hydrocarbons reacting with sunlight

Question 18

What are the health impacts of photochemical smog?

Answer 18

Irritated eyes
Severe lung disease

Question 19

How can we remove pollutants?

Answer 19

Catalytic converters
Removal of suphur

Question 20

What do catalytic converters remove?

Answer 20

Carbon monoxide
Nitrogen oxides
Unburnt hydrocarbons
- that are released from the internal combustion engine

Question 21

What is the goal of a catalytic converter?

Answer 21

By converting toxic gasses into less toxic gasses, can remove CO, NOx and unburnt hydrocarbons like octane

Question 22

What is the structure of catalytic converters and how do they work?

Answer 22

Honeycomb structure made of a ceramic material and covered in a thin layer of platinum/rhodium/palladium catalysts

Question 23

Why is a honeycomb structure used in a catalytic converter?

Answer 23

Increased surface area so faster rate of reaction

Question 24

Why is a thin layer of catalyst used in a catalytic converter?

Answer 24

Less metal needed, so cheaper

Question 25

What are the reactions that happen in a catalytic converter?

Answer 25

2NO+2CO -> N2+2CO2
C10H22 +31NO -> 10CO2 +11H2O +31/2 N2

Question 26

When are sulphur impurities in fuels removed?

Answer 26

Prior to being fuelled in cars
- but is not economically viable for fuel used in power stations, so instead it is removed through flue gas desulphurisation

Question 27

How does flue gas desulphurisation work?

Answer 27

Gasses are passed through a slurry of calcium oxide and calcium carbonate (CaO and CaCO3).
These are both bases, so neutralise the SO2 to form CaSO3, which is then oxidised to form CaSO4 used in construction

Question 28

What are the equations for flue gas desulphurisation?

Answer 28

CaO + SO2 -> CaSO3 (calcium sulphite)
CaCO2 +SO2 -> CaSO2 + CO2
CaSO3 + [O] -> CaSO4

Question 29

What type of environmental problem do nitrogen oxides cause?

Answer 29

Can form NO2, which can form acid rain

Question 30

What type of environmental problem can carbon monoxide cause?

Answer 30

Toxic - death

Question 31

What type of environmental problem can unburnt hydrocarbons cause?

Answer 31

Photochemical smog

Question 32

What type of environmental problem can soot cause?

Answer 32

Global dimming

Question 33

Why does carbon dioxide cause global warming?

Answer 33

Double bonds can absorb infrared radiation, which causes the molecule to vibrate, so IR transferred to other molecules