3. Alkenes
C2H4
C3H6
What’s the What’s the
formula for general
these? formula for CnH2n
alkenes?
4. Test for an unsaturated hydrocarbon (an alkene)
Shake
5. Cracking
Hydrocarbons with shorter carbon chains are more
useful, so sometimes we cut longer chains into shorter
ones using the process of cracking.
A long alkane is heated, vaporised and passed over a
ceramic catalyst to produce a shorter alkane and an
alkene.
C8H18 → C4H10 + C4H8
C10H22 → C7H16 + C3H6
Note
Any combination of alkene and alkane can be made, including
straight and branched chains, so long as the numbers of atoms
balance.
6. Hydrocarbons are compounds made of only hydrogen and
carbon atoms.
Hydrocarbons – for example methane (CH4) – burn very well producing
only carbon dioxide (CO2) and water (H2O):
CH4 + 2O2 → CO2 +2 H2O
Alkanes
These are the simplest hydrocarbons. They are ‘saturated’ which means
they only contain single bonds. They are pretty unreactive but burn well
making them good fuels.
Alkenes
These are hydrocarbons containing a C=C double bond. The double bond
makes them quite reactive and they are used as a starting material to
make many other organic compounds.
7. The equation below shows the cracking of a hydrocarbon compound
into two different compounds, A and B.
State two differences between the structures of compounds A and B.
Compound A has fewer C atoms
or Compound B has fewer H atoms Compound A is reactive
Compound A has C = C/double bond or can be used to make
or Compound A is unsaturated many substances
Why is compound A useful in industry? or can be used in
polymerisation/making
plastics/named plastic
or can be used as a fuel
8. In 1939 Roy Plunkett opened the valve on a new cylinder of tetrafluoroethene
gas. No gas came out!
He cut the cylinder open and found that the gas had changed into a white
solid. This solid was an addition polymer
Give the name of the addition polymer that formed inside the cylinder.
poly(tetrafluoroethene) or polytetrafluoroethene
The structure of this polymer can be represented by the diagram below.
Draw the structure of the monomer, tetrafluoroethene, from
which it is formed
9. The diagram shows an apparatus that can be used to carry out cracking
reactions in a laboratory.
Why is aluminium oxide or broken porcelain used?
catalyst or speeds up the reaction
10. Paraffin contains decane. The cracking of decane can be represented by the
equation below. A decane molecule is split into two smaller molecules.
Complete the equation by adding the formula of the other product.
C10H22 (l) →.................... (l) + C2H4 (g)
decaneS
