(a)Burning/combustion

Alkenes burn with a yellow/ luminous sooty/ smoky flame in excess air to form carbon(IV) oxide and water.

In excess air/oxygen

Alkene + Air   ->   carbon(IV) oxide   +  water

In limited air

Alkenes burn with a yellow/ luminous sooty/ smoky flame in limited air to form carbon(II) oxide and water.

Alkene   +   Air   ->   carbon(II) oxide   +  water   

Burning of alkenes with a yellow/ luminous sooty/ smoky flame is a confirmatory test for the presence of the  =C=C= double bond because they have  higher C:H ratio.

A homologous series with  = C = C= double or  -C    C-    triple bond is said to be unsaturated.

A homologous series with       C      C      single bond is said to be saturated.

Most of the reactions of the unsaturated compound

 involve trying to be saturated to form a    C       C    single bond.

Examples of burning alkenes

1.(a) Ethene when ignited burns with a yellow sooty flame in excess air to form carbon(IV) oxide and water.

Ethene   + Air   ->   carbon(IV) oxide   +  water 

C2H4(g)  + 3O2(g)  -> 2CO2(g)   +  2H2O(l/g)

 (b) Ethene when ignited burns with a yellow sooty flame in limited air to form carbon(II) oxide and water.

Ethene   + Air   ->   carbon(II) oxide +water 

C2H4(g)  + 3O2(g)  -> 2CO2(g)   + 2H2O(l/g)

2.(a) Propene when ignited burns with a yellow sooty flame in excess air to form carbon(IV) oxide and water.

2C3H6(g)  + 9O2(g)->   6CO2(g) +6H2O(l/g)

   (b) Propene when ignited burns with a yellow sooty flame in limited air to form carbon(II) oxide and water.

C3H6(g)+ 3O2(g)  ->   3CO(g) +3H2O(l/g)

b)Addition reactions

An addition reaction is one which an unsaturated compound reacts to form a saturated compound.

Addition reactions of alkenes are named from the reagent used to

cause the addition/convert the double  =C=C= to  single bond.

(i)Hydrogenation

Hydrogenation is an addition reaction in which hydrogen in presence of Palladium/Nickel catalyst at high temperatures react with alkenes to form alkanes.

Examples

1.When Hydrogen gas is passed through liquid vegetable and animal oil at about 180oC in presence of Nickel catalyst,solid fat is formed.

Hydrogenation is thus used to harden oils to solid fat especially margarine.

During hydrogenation, one hydrogen atom in the hydrogen molecule attach itself to one carbon and the other hydrogen to the second carbon breaking the double bond to single bond.

3.Both But-1-ene and But-2-ene undergo  hydrogenation to form Butane