These are detailed Revision Notes for Topic Seven (Organic Chemistry) of AQA GCSE Chemistry. They are written by me, using a combination of class notes, text books and revision guides. I have also uploaded the other chapters in my store.
Organic Chemistry
Crude Oil:
Crude oil is a fossil fuel that is found underground in rocks. It was
formed over millions of years from the remains of sea creatures
Crude oil is a form of ancient biomass as it was made from the remains
of creatures that lived many years ago. It is a finite resource because
we cannot make more of it
It is a mixture of many different compounds most of these are
hydrocarbons.
Hydrocarbon – A compound that contains carbon and hydrogen only.
Saturated – A molecule that only contains single bonds
Unsaturated – A molecule that contains one or more double covalent
bonds
Alkane – A homologous series of saturated hydrocarbons with the
general formula C n H 2 n+ 2
Alkene – A homologous series of unsaturated hydrocarbons with the
general formula C n H 2 n
Functional Group - Atom or group of atoms responsible for most of the
chemical reactions of a compound.
Homologous series - A family of compounds with the same general
formula, the same functional group and similar chemical properties.
Fractional Distillation:
For crude oil to be useful, the hydrocarbons in it must be separated.
They are separated by fractional distillation.
1. The oil is heated until most of it has turned into gas. The gases
enter a fractionating column and the liquid is drained off.
2. In the column there’s a temperature gradient – it’s hot at the
bottom and gets cooler as you go up.
3. The longer hydrocarbons have higher boiling points. They
condense back into liquids and drain out of the column early on,
when they’re near the bottom. The shorter hydrocarbons have
lower boiling points. They condense and drain out much later,
near to the top of the column where it’s cooler.
4. You end up with the crude oil mixture separated out into different
fractions. Each fraction contains a mixture of hydrocarbons that
all contain a similar number of carbon atoms, so have similar
boiling points.
Naming Chemicals:
1 = Meth 2 = Eth
, 3 = Prop 7 = Hept
4 = But 8 = Oct
5 = Pent 9 = Non
6 = Hex 10 = Dec
Alkanes end in -ane
Alkenes end in -ene
Isomers – same molecular formula but a different arrangement of
atoms
Alkane combustion:
The main use of alkanes is as fuels, they are good fuels because they
release lots of energy as they burn
When alkanes burn they react with oxygen. Complete combustion
takes place if there is a good supply of oxygen from the air. The carbon
atoms in the alkane are oxidised to form carbon dioxide. The hydrogen
atoms are also oxidised to form water.
Alkane + Oxygen Carbon dioxide + water
If there is a poor supply of oxygen incomplete combustion happens.
The carbon monoxide or carbon in the form of soot are formed. Carbon
monoxide is toxic, and the soot causes a smoky flame.
Alkane + Oxygen Carbon monoxide + water
Alkane + oxygen Carbon + water
Cracking:
Short-chain hydrocarbons are flammable so make good fuels and are in
high demand. However, long-chain hydrocarbons form thick gloopy
liquids like tar which aren’t that useful, so a lot of the longer alkane
molecules produced from fractional distillation are turned into smaller,
more useful ones by a process called cracking.
There are two types of cracking:
o Catalytic cracking – vaporise and pass over hot catalyst
o Steam cracking – mix with steam at a high temperature
As well as alkanes, cracking also produces alkenes
Reactions of Alkenes:
1. Put about 1cm³ of bromine water into a test tube.
2. Add 1 drop of cyclohexene, put a bung in and shake.
3. Repeat with a second test tube and cyclohexane.
4. Note all observations
Observations = cyclohexene decolourizes, cyclohexane stays the same
An alkene decolorizes bromine water from orange to colourless.
An alkane will not react with bromine water, leaving it orange.
Addition reactions are
H H H H H H reactions in which
| | / | | |
H – C – C = C + Br₂ H – C – C – C – H
| | \ | | |
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