Alkanes and Alkenes

Question 1

What are alkanes?

Answer 1

Alkanes are a homologous series of hydrocarbons that contain only carbon-carbon single bond. Alkanes do not have a functional group.

Question 2

Physical properties of alkanes

Answer 2

Gradual trend in:
Melting and boiling point
Viscosity
Flammability

Question 3

Chemical Properties of Alkanes

Answer 3

1. Alkanes are generally unreactive. They consist of only carbon-carbon single bonds and carbon-hydrogen bonds which are strong and difficult to break.
2. The flame produced becomes smokier and sootier due to incomplete combustion of alkanes.

Question 4

Combustion of Alkanes

Answer 4

Alkanes burn readily in excess oxygen to produce carbon dioxide and water vapour.

Question 5

Why are alkanes good fuels?

Answer 5

The combustion of alkanes are highly exothermic.

Question 6

Definition of substitution reaction

Answer 6

Alkanes react with halogens in the presence of UV light.

Question 7

Reagent and reaction condition for substitution reaction

Answer 7

Reagent: Gaseous chloride or liquid bromine

Reaction condition: Ultraviolet light

Question 8

Use of Alkanes

Answer 8

Use of Alkanes is determined according to the number of carbon atoms present:

• fuel for heating or electricity generation
• fuel to provide heat for cooking
• making paraffin wax to be used in candles
• road surfacing

Question 9

Sources of Alkanes

Answer 9

Petroleum or crude oil.
They can be further separated into various fractions containing alkanes of similar carbon-chain length or similar boiling point by fractional distillation

Question 10

What are alkenes?

Answer 10

Alkenes are hydrocarbons that contain one or more carbon-carbon double bonds.

Question 11

Definition of unsaturated

Answer 11

Molecules that contain carbon-carbon double (or triple) bonds

Question 12

Combustion of Alkenes

Answer 12

Similar to alkanes, alkenes also burn readily on excess oxygen to produce carbon dioxide and water vapour

Question 13

Why do alkenes burn with a sootier flame?

Answer 13

Alkenes have a relatively higher percentage of carbon than corresponding alkanes.

Question 14

Definition of addition reaction

Answer 14

Reaction in which an unsaturated organic compound combines with another compound to form a single new saturated organic compound.

Question 15

Hydrogenation: Addition of hydrogen

Answer 15

Reagent: hydrogen gas
Reaction condition: 200•C, nickel catalyst

Undergo addition reaction with hydrogen to form alkanes.

Question 16

Bromination: Addition of Bromine

Note: other halogens work as well

Answer 16

Reagent: [Concentrated] liquid bromine/ [Diluted] bromine gas in CCl4 (inert organic solvent)
Reaction conditions: Room temperature and pressure

Question 17

Hydration: Addition of Hydrogen

Answer 17

Reagent: Steam

Reaction condition: 300•C, 60 atm, phosphoric(V) acid

Question 18

Addition polymerisation

Answer 18

Reagent: alkene

Reaction condition: high temperature, high pressure and presence of catalyst

Question 19

Use of alkenes

Answer 19

1. Making alcohol

2. Making polymer that can be used to manufacture different plastic products

Question 20

Source of Alkenes

Answer 20

Cracking of petroleum/ crude oil

Question 21

Definition of cracking

Answer 21

Breaking down of long-chain hydrocarbons into smaller molecules

Question 22

Cracking

Answer 22

Reagent: Alkane

Reaction condition: high temperature (~600•C), Mixture of Aluminium oxide and silicon dioxide as catalyst