Alkanes and Alkenes

Question 1

Why is cracking necessary

Answer 1

Cracking is needed to increase the supply of shorter chain hydrocarbons as there is not enough to match the demand of them.

Question 2

Example of cracking

Answer 2

The Cracking of hexane (C6H14) to produce butane (C4H10) and ethene (C2H4)

Question 3

what is an homologous series

Answer 3

A series of organic compounds that has similar features:
Same general formula
Same functional group
Similar chemical reactions
Physical properties show graduations

Question 4

what is a functional group

Answer 4

A group of atoms bonded in a specific arrangement that controls the property of the homologous series

Question 5

What is meant by isomerism

Answer 5

Compounds that have the same molecular formula but different structural formulae

Question 6

What is the general formula for alkenes

Answer 6

CnH2n

Question 7

Why are alkenes classified as unsaturated hydrocarbons

Answer 7

As they contain a double covalent bond (=)

Question 8

Reaction between alkenes w/ bromine

Answer 8

Addition reaction
example:
Ethene(C2H4) + Bromine(Br2) = dibromoethane(C2H4Br2

Question 9

Use of bromine water to distinguish alkanes and alkenes

Answer 9

Alkane - no difference in bromine water (as there is no double bonds.)
Alkene - bromine water decolourised (due to double bonds.)

Question 10

What is substitution

Answer 10

Substitution - A reaction where one atom is swapped with another atom

Question 11

Describe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking (using silica or alumina as the catalyst and a temperature in the range of 600-700°c)

Answer 11

Cracking = large hydrocarbon molecules are broken into small, more useful hydrocarbons

Fractions containing large hydrocarbon molecules are heated at 600 – 700°c to vaporise them.

Vapours then pass over a hot catalyst of silica/alumina

breaks covalent bonds in the molecules = thermal decomposition reactions

As a result, cracking = smaller alkanes and alkenes. The molecules are broken up in a random way which produce a mixture of alkanes and alkenes.

Question 12

How is an addition molecule formed

Answer 12

By the joining up of many small molecules called monomers

Question 13

Uses of polyethene

Answer 13

Polyethene - plastic bags (low density) and Plastic bottles (high density)

Question 14

How do you deduce the structure of a monomer from the repeat unit of an addition polymer.

Answer 14

Identify the repeating unit.

Change the single bond in the repeat unit to a double bond in the monomer.

Remove the bond from each end of the repeat unit.

Question 15

what are the problems with the disposal of additional polymers

Answer 15

Landfills:

Waste polymers are disposed of in landfill sites but this takes up valuable land as addition polymers are non-biodegradable so micro-organisms such as decomposers cannot break them down.
This causes sites to quickly fill up

Incineration:

Polymers release a lot of heat energy when they burn and produces carbon dioxide which is a greenhouse gas that contributes to climate change.
If incinerated by incomplete combustion, carbon monoxide will be produced which is a toxic gas that reduces the capacity of the blood to carry oxygen.

Recycling:

Polymers can be recycled but different polymers must be separated from each other.
This process is difficult and expensive.

Question 16

uses of polypropene

Answer 16

ropes and carpets

Question 17

uses of polychloroethene

Answer 17

drainpipes, insulation on wires and plastic sheets

Question 18

uses of polytetrafluoroethene

Answer 18

non stick cookware and pipework

Question 19

What is addition

Answer 19

Addition - A reaction in which one molecule combines with another to form a larger molecule with no other products

Question 20

What is combustion

Answer 20

Combustion - This is the scientific term for burning. In a combustion reaction, a substance reacts with oxygen to form carbon dioxide (or carbon monoxide if incomplete combustion) and water.