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Chemistry – Key Topic 7
4.7 Organic chemistry
4.7.1 Carbon compounds as fuels and feedstock
Saturated – as many atoms as possible in each molecule
Unsaturated – molecules containing at least one bond
Hydrocarbon – a compound containing only hydrogen and carbon
Homologous series – a group of related organic compounds that have the same
functional group and react in a similar way. Successive members of a homologous
series differ from each other by an extra carbon atom that has two hydrogen atoms
Organic compounds are used as fuels to run cars, warm homes and to generate
electricity
4.7.1.1 Crude oil, hydrocarbons and alkanes
Alkanes are a homologous series of saturated hydrocarbons. They are the simplest
type of hydrocarbon
- They are used as fuels: complete combustion (good supply of oxygen) resulting in
carbon dioxide and water, incomplete combustion (poor supply of oxygen)
resulting in carbon monoxide/carbon and water
- They are used for cracking: long alkanes short alkanes + alkenes
Crude oil is a finite resource found in rocks. Crude oil is the remains of an ancient
biomass consisting mainly of plankton that was buried in mud over millions of years
- Plankton refers to any organism that Is not actively swimming against the water
flow, instead it floats, drifts or is passively moving in its habitat
- Subjected to immense pressure and heat and anaerobic conditions over millions
of years to make crude oil
Crude oil is a mixture of a very large number of compounds. Most of the compounds
in crude oil are hydrocarbons, which are molecules made up of hydrogen and carbon
atoms only
Most of the hydrocarbons in crude oil are hydrocarbons called alkanes. The general
formula for the homologous series of alkanes is CnH2n+2
- The first 4 members of the alkanes are methane, ethane, propane and butane
, - They contain single covalent bonds only
Small alkanes Big alkanes
Boiling points Lower Higher
Flammability Higher Lower
Cleanliness of flame Cleaner More smoky
Viscosity (how easy it Lower higher
flows)
- 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
- As a result, the long alkane molecules are turned into smaller, useful ones by
cracking
Alkane molecules can be represented in the following forms:
- C2H6 or
-
4.7.1.2 Fractional distillation and petrochemicals
Fraction = a mixture of hydrocarbon molecules with similar chain lengths and
therefore similar boiling points
The many hydrocarbons in crude oil may be separated into fractions, each of which
contains molecules with a similar number of carbon atoms, by fractional distillation (it
separates liquids with different boiling points). This is because crude oil straight from
the ground is not much use as there are too many different substances in it
- Separates hydrocarbons so they can be useful
- Longer chain length = higher boiling point + the bigger the molecule/longer chain
length = stronger intermolecular forces
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 is a temperature gradient (it is hot at the bottom and gets
cooler as you go up)
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