This unit aims to elaborate on how alkenes are formed and the polymerisation process

These are unsaturated hydrocarbon with general formulae Cn H2n where n is type number of carbon atoms. They must have at least one carbon bauble bond, C = C.

Unsaturated hydrocarbon  

This is one which has at least one multiple bond (carbon – carbon double bond) as in alkenes (carbon-carbon – carbon triple bond as in alkines, C=C)


Laboratory preparation of Ethane

Methods I

By dehydration of Ethanol, C2H50H Using concentrated sulphuric acids.


  • The acid must be concentrated
  • Heating (temperature of 180)

Ethane is collected over water in which it is only slightly soluble.

Method II

By dehydration of Ethanol by passing it over hot Aluminium

(iii) Oxide

Temperature 4000 C

CH3CH20H (g) C2H4 (g) + H20 (l)

Refer to pg 308 (Atkinson 4th Edition)

Set up of the apparatus – Fig 23.3 Pg 308 Atkinson 4th Edition.


Chemical tests for ethane

  1. Ethane decolourises chlorine water and bromine water


All other alkenes decolourize chlorine water and bromine water

  • The chlorination and brominating reactions are example of addition reactions.

Addition reaction

Is one where one molecule adds to another to form a single product?

 Reaction with potassium Permanganate

Ethane’s (and other alkenes) decolourize acidified potassium manganese (vii) (acidified potassium permanganate).

There is a colour change from purple to colourless. This is another test for alkenes.

 Reactions with Hydrogen; – Hydrogenation

It is also an addition reaction. Ethane and other alkenes react with hydrogen to give the corresponding alkenes (saturated product).

Condition for reactions

  • Nickel as catalyst
  • Temperature of 2000C


Industrial application of the Hydrogenation Reaction

This reaction is used in the conversion of liquid oils to solid fat

Oil + Hydrogen       Ni (cat)      Fat


This is the reaction which takes place in manufacture of margarine.

Polymerisation of Ethane:

Polymerisation is linking of many units of simple molecules (monomers) to form a large molecule (polymer/macromolecule/complex molecules)

Monomer is a simple building unit.

Polymer – complex molecule or large molecule formed by linking many units of monomers.


n is a larger n0.

Addition polymer – Is a polymer formed from unsaturated monomers e.g. polythene the monomers link up or join without losing simple molecules or atoms.

The R.F.M of the polymer is a simple multiple of that of the monomer.

 Uses of polythene

-Used for packing goods.

-Contain 4 chemical (not affect by chemery)

-Manufacture of wash bottles.

-Manufacture of certain bags.

-Roofing materials.

chloroChoral ethane polymerizes to form polychloroethene (polyvinychloride, pvc)

N is a large number

Uses of Pvc

-Waste pipes

-Rain coats

-Electrical insollenta insulators.


Uses of polypropene


Man made polymers e.g. polythene, pvc, polypropene, Nylon, Bakelite.

Natural polymers – polymers whose formation is not controlled by man e.g. cellulose, starch, and insulin, fibre like cotton wool, natural rubbers, and proteins.

  • Natural polymer

Are polymer whose form formation is not controlled by man e.g. starch, cellulose, glycogen, pectoral rubber, plant fibres like cotton ,sisal, jelled proteins, polypeptide, animal fibres, wool and silk.

  • Condensation polymers

Monomers bond with elimination of simple molecules like water, methane e.t.c Example, Starch, glycogen, cellulose, protein, nylon.


Are substances which when soft can be moulded (can be made into different shades) All synthetic polymers are plastic in nature hence they are plastic.

Classified of plastics

They are divided into two groups.

  1. Thermo plastics (thermo softening plastics)

Are plastics which when heated become soft melt therefore can be recycled. On cooling they harden and retain the new shape e.g. Polythene, polypropene (pvc) Perspex, polyvinile chloride. They have low melting point.

  1. Thermosetis (thermosetting plastics)

Are plastics which don’t soften or melt on heat at high temperature they cannot be re shaped after manufacture therefore non recyclable. E.g Bakelite, electric switches, sockets are made of Bakelite.

Molecules in thermo sets have cross linkages. Their structures are three dimensional networks.


Are polymers which can drown out into thread? They contain linear molecules. Forces between molecules are weak but those between atoms are strong – example of fibres.

  • Natural fibres –have 2 two types
  1. Animal fibres e.g. wool, silk.
  2. Vegetable or plant fibres e.g. cotton e.t.c
  • Synthetic fibres, nylon, polyesters, terlylene.

 Advantages of synthetic polymers

i-They are easy to manufacture

ii-They are unaffected acids, alkalis or water

iii-They are light.

Iv-Thermoplastic can be recycled.

v-They can be made into complicated shapes.

VI-They can be easily coloured.

Vii-They are durable.

Viii-They are relatively cheaper compared to natural polymers.


-Some of the have low quality.

-People working in factories producing these polymers are exposed to poisonous fumes.

-They are non biodegradable, can accumulate in the environment causing pollution.

Advantages of natural polymers

-They are biodegradable

-They are natural got so don cause pollution during manufacture

-Some can be eaten, protein

-Some are a source of raw material, cotton.


– Some are not as strong as synthetic polymers

– Expensive

– Non-durable.

Advantages of plastics over metals

-Plastics are resistant to acids, alkaline and atmospheric corrosion hence plastic containers can be used to store acids and alkalis on the other hand metals react and corrode.

– Plastics are lighter than metals.


-Plastic articles melt or burn easily .When they burn they produce pollutant gasses like carbon monoxide.

-Plastics are pollution-They don’t rot and don’t dissolve in water because they are easily carried by wind and hence littering the environment. Metal articles on the other hand they settle in one place and they easily controlled.

-One advantage with metals is that they resist high temperature and they are strong and durable. This is why they are used in engineering work, building bridges, houses e.t.c.


Unsaturated hydrocarbons which contain at least one carbon-carbon triple bond C≡C

General formula CnH2n –2 where n is the number of carbon atoms.

Ethane C2 H2                                                                              Structural formula

Molecular formula


Ethane is used in the oxy-acetylene flame used for welding and cutting metals.


Ethane burns in limited air with e yellow sooty flame; this is because of high percentage of carbon in the compound.

Reaction of Ethane



Chlorination of Ethane

Like Alkenes, Alkynes decolourize chlorine water, bromine water.






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