Chapter 12- Alkanes

Question 1

What are alkanes?

Answer 1

Saturated hydrocarbons containing only C-C and C-H single bonds.

Question 2

What is crude oil?

Answer 2

A mixture of hydrocarbons, mostly alkanes, both unbranched and branched. Its fractions can be separated by fractional distillation.

Question 3

What is the process of fractional distillation?

Answer 3

-Crude oil heated
-Mixture of liquid and vapour passes into fractionating column with temp. gradient
-Vapours rise up the column through trays until they reach a cool enough tray where they condense
-Shorter chain hydrocarbons condense further up due to their lower boiling points
-Thick residue collects at the base (tar/bitumen) which can be processed further or used for road surfacing

Question 4

What is cracking?

Answer 4

The breaking of longer chain hydrocarbons into shorter chain hydrocarbons through the breaking of C-C bonds

Question 5

Why are longer chain alkanes cracked?

Answer 5

Longer chain alkanes are in lower demand while shorter chain alkanes and alkenes are in higher demand as they are more useful
Alkenes are more reactive than alkanes and can be used as chemical feedstock

Question 6

What are the conditions and products of thermal cracking?

Answer 6

-High temperature: 700-1200K
-High pressure: up to 7000 kPa
-Produces mostly alkenes that can be used as chemical feedstock

Question 7

What are the conditions and products of catalytic cracking?

Answer 7

-Moderately high temperature: 700K
-Moderately high pressure: higher than atmospheric pressure
-Zeolite catalyst (al. oxide and sil. dioxide) in honeycomb structure for high SA
-Produces motor fuels, aromatic compounds (branched alkanes, cycloalkanes) which are separated using fractional distillation

Question 8

How are alkanes often used?

Answer 8

As fuels, eg methane, propane, petrol, etc.

Question 9

How can combustion reactions of alkanes differ?

Answer 9

Depending on the amount of oxygen available, combustion can be complete or incomplete and produce different products
Plentiful: carbon dioxide and water
Limited: carbon monoxide and water
Very little: carbon (soot) and water

Question 10

What are some pollutants that burning crude oil can produce?

Answer 10

CO
NOx
SO2
C
Unburnt hydrocarbons
CO2
H2O

Question 11

What is carbon monoxide?

Answer 11

CO, a poisonous gas produced by incomplete combustion

Question 12

What are nitrogen oxides?

Answer 12

NOx, contribute to acid rain and photochemical smog
Produced in petroleum engines at high temp. causing nitrogen and oxygen in the air to combine

Question 13

What is sulfur dioxide?

Answer 13

SO2, contributes to acid rain
Produced from sulfur-containing impurities in crude oil reacts with water and oxygen to form sulfuric acid

Question 14

What are carbon particles/particulates?

Answer 14

C, can exacerbate asthma and cause cancer
Produced by incomplete combustion with very little oxygen available.

Question 15

What are unburnt hydrocarbons and what do they do?

Answer 15

Hydrocarbons
Bad greenhouse gases, photochemical smog

Question 16

What is carbon dioxide?

Answer 16

CO2, a greenhouse gas
Produced by complete combustion and contributes to climate change

Question 17

What is water vapour?

Answer 17

H2O (g), a greenhouse gas
Always produced in combustion

Question 18

What is flue gas desulfurisation?

Answer 18

The process in which sulfur dioxide is removed from flue gases (the gases given out by power stations).

Question 19

How can sulfur dioxide be removed using calcium oxide (lime) ?

Answer 19

Slurry of CaO and water sprayed into flue gas.
CaO+2H2O+SO2+0.5O2 —> CaSO4.2H2O
Gypsum is produced which is saleable as it is used to make plaster.

Question 20

How can sulfur dioxide be removed using calcium carbonate (limestone) ?

Answer 20

CaCO3 + 0.5O2 + SO2 —> CaSO4 + CO2

Question 21

What do catalytic converters do?

Answer 21

Reduce output of CO, NOx and unburnt hydrocarbons in the exhaust gas mixture.

Question 22

What is a catalytic converter made of?

Answer 22

Ceramic material in a honeycombic structure coated with platinum and rhodium (catalysts).
As the pollutants pass over the catalyst, they react with each other to form less harmful products.

Question 23

How is the output of CO and NO reduced?

Answer 23

2CO and 2NO reacts to form N2 and 2CO2

Question 24

How is the output of unburnt hydrocarbons and nitrogen oxide decreased?

Answer 24

Hydrocarbons and NO react to form N2, CO2 and H2O