14.1 Alkanes

Question 1

Producing alkanes

Answer 1

1) Addition of H to an alkene in a hydrogenation reaction
2) Cracking of a longer chain alkane into a smaller alkane and alkene

Question 2

Hydrogenation (addition of H to an alkene)

Answer 2

• exothermic
• Pt/Ni catalyst, which is finely divided to increase its surface area, therefore increases rate of reaction
• Heat

Question 3

Cracking of a longer chain alkane into a smaller alkane and alkene

Answer 3

• the break down of large, less useful hydrocarbons into smaller, more useful ones
• Al2O3
• Heat
• Products have a low molecular mass)
• Alkene is more reactive as it has a C=C
• steel chamber doesn’t contain oxygen to prevent oxygen

Question 4

How cracking can be used to obtain more useful alkanes and alkenes of lower Mr from heavier crude oil fractions

Answer 4

• crude oil is a mixture of hydrocarbons and is extracted from the earth through a drilling process
• at the oil refinery, crude oil is separated into useful fuels by fractional distillation
• fractional distillation is used as the fractions have different b.p. points

Question 5

Complete combustion

Answer 5

• alkanes are burnt in excess oxygen to produce carbon dioxide + water

Example: octane

2C8H18 + 25O2 = 16CO2 + 18H20

Question 6

Incomplete combustion

Answer 6

• alkanes are burnt in a limited supply of oxygen to form carbon monoxide + water

Example: octane

2C8H18 + 17O2 = 16CO + 18H20

Question 7

Free-radical substitution

Answer 7

• hydrogen atom gets substituted by a halogen
• since alkanes are unreactive, UV light is needed
• 3 steps: initiation, propagation and termination

Question 8

Unreactivity of alkanes

Answer 8

1) Strength of C-C and C-H bonds
2) Lack of polarity

Question 9

Strength of C-C and C-H bonds

Answer 9

• lots of heat is required to break these strong C-C and C-H single, covalent bonds

Question 10

Lack of polarity

Answer 10

• the electronegativities of C and H in alkanes are almost the same
• both atoms share the electrons in the covalent bond almost equally
• as a result, alkanes are non-polar and therefore do not react with polar reagents

Question 11

Pollutants, their effect and removal

Answer 11

table in notes

Question 12

CO and is effect

Answer 12

• toxic gas as it will bind to haemoglobin in blood, which can then no longer bind to oxygen
• as no oxygen can be transported around the body, victims will feel dizzy, lose unconsciousness and if not removed, they can die