M4 ,, Alkenes

Question 1

Bonding in Alkenes?

Answer 1

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formula CnH2n-2
Unsaturated = contain a double C=C bond Hydrocarbon = contains C and H only
Bonding in Alkenes
The carbons at either end of the double bond and the 4 atoms they are bonded to are all in a plane (flat). These 6 atoms are joined by single bonds
The bonds around each C in the C=C bond are trigonal planar in shape. There are 3 areas of electrons round each C (1 double bond and 2 single bonds) which repel equally to give a bond angle of 120°
The double bond is formed by sideways overlap of p orbitals producing a π bond (sausage-shaped clouds of electrons above and below the plane of the single bond framework)

Question 2

Addition of hydrogen produces an alkane

Answer 2

Addition of hydrogen produces an alkane
CH2=CH2 + H2 → CH3CH3 ethane
Needs a Ni catalyst
One mole of hydrogen needed per double bond
This reaction is used to produce margarine from unsaturated vegetable oil

Question 3

Addition of halogens?

Answer 3

produces a dihalogenoalkane

CH2=CH2 + Br2 → CH2BrCH2Br (1,2-dibromoethane) The colour change in this reaction is from orange to colourless
This is used as a test for unsaturation (to show the presence of a double bond)

Question 4

Addition of hydrogen halides ?

Answer 4

produces a halogenoalkane

Question 5

Addition of steam?

Answer 5

produces an alcohol

CH2=CH2 + H2O → CH3CH2OH
Needs a strong acid catalyst e.g. phosphoric acid, H3PO4

Temperature must be above 100°C so that H2O is in the form of steam

Question 6

Mechanical Recycling ?

Answer 6

Waste polymers can be melted down and re-used following separation into types
(PTFE etc). However it is expensive to collect and sort waste polymers

Question 7

Combustion for energy production ?

Answer 7

Waste polymers can be burnt as a fuel but this may produce toxic waste products (see below)

Question 8

Feedstock recycling?

Answer 8

Waste polymers can be cracked and the products used to make fuels and other polymers (after separation)