Brown's GCSE/IGCSE KS4 science-CHEMISTRY Revision Notes
Oil, useful products, environmental problems, introduction to
5. ALKENES - unsaturated hydrocarbons - their chemical reactions
The alkenes are a series of
hydrocarbon molecules (made of carbon and hydrogen atoms). They are referred to
as 'unsaturated' hydrocarbons because they have a carbon = carbon C=C double bond and other
atoms can add to them via simple addition reactions. The physical properties and chemical reactions of
alkenes with hydrogen (to form alkanes), bromine to form a dibromoalkanes (used
as a test for alkenes), polymerisation (self-addition to form polymers like
polyethene and with oxygen (combustion, burning)
are fully described with word and symbol equations.
Index of KS4 Science GCSE/IGCSE
Chemistry Oil & Organic Chemistry Pages: 1.
Fossil Fuels : 2. Fractional distillation of crude oil & uses of fractions : 3.
ALKANES - saturated hydrocarbons and combustion : 4.
Pollution, carbon monoxide, nitrogen oxides, what
makes a good fuel?, climate change-global warming :
5. Alkenes - unsaturated hydrocarbons :
6. Cracking - a problem of supply and demand, other products :
7. Polymers, plastics, uses and problems :
8. Introduction to Organic Chemistry - Why so many series of
organic compounds? : 9. Alcohols - Ethanol
- properties, reactions, biofuels :
10. Carboxylic acids and esters : 11. Addition
polymers and condensation polymers :
12. Natural Molecules - carbohydrates - sugars
- starch : 13. Amino acids, proteins,
enzymes & chromatography : 14. Oils, fats,
margarine and soaps :
15. Vitamins, drugs-analgesic medicines & food
additives and aspects of cooking chemistry! : 16. Ozone, CFC's and free
radicals : 17. Extra notes, ideas and links on
Global Warming and Climate Change : Multiple Choice and Gap-Fill Quizzes:
m/c QUIZ on Oil Products (GCSE/IGCSE easier-foundation-level)
m/c QUIZ on Oil Products (GCSE/IGCSE harder-higher-level) :
IGCSE/GCSE m/c QUIZ on other Aspects of Organic Chemistry
3 Easy linked GCSE/IGCSE Oil Products word-fill worksheets
hydrocarbons series (unsaturated)
hydrocarbons containing a carbon...carbon double bond (>C=C<)
as well as single bonds.
have the general formula CnH2n where n = 2, 3, 4
etc. giving the formulae C2H4, C3H6,
The three formula quoted
above match the three names above.
As with naming all organic
molecule series eth.. means 2 carbon atoms in the chain,
prop... means 3 and but.. means 4 etc.
These are called unsaturated molecules because two atoms can join
onto half of the double bond when it opens up.
The first three in the series are shown
in the section
below and are colourless smelly gases.
They are extremely reactive and important compounds in the chemical
industry and are converted into very useful compounds e.g. plastics and
A Cracking demonstration!
You can demonstrate cracking
in the laboratory by heating paraffin grease over an aluminium oxide
catalyst at 400-700oC, and collecting the smaller gaseous
hydrocarbon molecules over water - easily shown to be flammable!
Examples of alkene
(1) is the molecular formula: a
summary of the totals of
each atoms of each element in one molecule
is are 'shorthand' versions of the full structural formula or
displayed formula (3)
is called the full structural formula or displayed formula:
formula shows how all the atoms are
linked with the covalent bonds (the dashes -) ie the C-C bonds and the C-H
Note that carbon must
form four bonds (C-C single bond or a C=C double bond) and hydrogen forms one bond (C-H).
It is the presence of
the carbon = carbon double bond (C=C) which makes alkenes
unsaturated hydrocarbon molecules.
the C=C is referred to as
the 'double bond'
5c. More on ALKENES
- unsaturated hydrocarbons
- a quick summary
- These cannot be obtained directly from
crude oil and must be made by cracking (see
section 6 cracking notes).
- The unsaturated hydrocarbons form an
homologous series called alkenes with a general formula CnH2n
- Unsaturated means the molecule has a C=C double bond to which atoms or
groups can add.
- Alkene examples: Names end in
- C2H4 or or
- The alkenes are more reactive than alkanes
because of the presence of the carbon = carbon double bond. The alkenes
readily undergo addition reactions in which one of the carbon = carbon double bonds
breaks allowing each carbon atom to form a covalent bond with another atom
such as hydrogen or bromine.
- Examples of addition reactions
are: with hydrogen
under pressure and in the presence of a nickel catalyst to form an alkane
- Alkenes react by 'addition' with bromine
and decolourises the orange bromine water because the organic product is
colourless, and this is a simple test to distinguish an
alkene from an alkane.
- Vegetable oils contain unsaturated fats
can be hardened to form margarine by adding hydrogen on to some of the carbon=carbon
double bonds using a nickel catalyst. The process is called hydrogenation,
- Alkenes can add to themselves by
addition polymerisation to
form 'plastic' or polymeric materials.
- Alkenes readily burn, just like
alkanes, to give carbon dioxide and water.
- e.g. ethene + oxygen ==> carbon dioxide +
- C2H4 + 3O2
==> 2CO2 + 2H2O
- However, they are NOT used as fuels for
- They are far too valuable for use to
make plastics, anti-freeze and numerous other useful compounds.
- They burn with a more smokey flame
than alkanes due to less efficient and more polluting combustion.
- Alkenes are isomeric with cycloalkanes
e.g. the molecular formula C6H12
can represent hexene or cyclohexane
- hexene CH3-CH2-CH2-CH2-CH=CH2
- and note that ....
- hexene is an unsaturated
hydrocarbon with a double bond,
- the isomeric cyclohexane does not
have a double bond and is a saturated hydrocarbon,
- so a simple bromine test could
distinguish the two similar colourless liquids,
- because only the hexene would
decolorize the bromine water test reagent.
Multiple Choice Quizzes and Worksheets
KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products
KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products
KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry
3 linked easy Oil Products gap-fill quiz worksheets
ALSO gap-fill ('word-fill') exercises
originally written for ...
... AQA GCSE Science
Useful products from
crude oil AND
... OCR 21st C GCSE Science
Worksheet gap-fill C1.1c Air
pollutants etc ...
... Edexcel 360 GCSE Science
Crude Oil and its Fractional distillation
... each set are interlinked,
so clicking on one of the above leads to a sequence of several quizzes
Level Organic Chemistry revision notes
Revise KS4 Science GCSE/IGCSE/O level
Chemistry Revision-Information Study Notes for revising for AQA GCSE Science, Edexcel
360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science WJEC/CBAC
GCSE science-chemistry CCEA/CEA GCSE science-chemistry
(and courses equal to US grades 8, 9, 10)
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