Hydrocarbons contain only the elements hydrogen and carbon.
Found as fossil deposits of crude oil and natural gas.
They are made from naturally decaying plants and animals over millions of years
Crude oil is a mixture of around 150 hydrocarbons which are mainly straight chain alkanes.
Each source varies in its mixture slightly.
Crude oil does not ignite easily. Only when separated into its constituent components is it valuable
Fractional distillation of crude oil:
Crude oil is a mixture of different hydrocarbons with a large range of boiling points.
Fractional distillation is used to separate crude oil into fractions.
Each fraction is made up of many different hydrocarbons but with more similar boiling points.
Further distillation of these fractions can occur when a desired hydrocarbon is needed.
Boiling points of the alkanes:
The graph shows - the larger the carbon chain, the higher the boiling point:
Weak Van der Waal's forces of attraction exist between the alkanes.
The more carbons and hydrogen's there are, the more electrons there are in the molecule.
The more electrons there are in the molecule, the stronger the Van der Waal's forces of attraction.
This means that as the carbon chain increases, so does the strength of the Van der Waals forces of attraction.
This means that more energy is needed to overcome these attractive forces.
Each isomer has the same number of electrons but the strength of these forces are clearly not the same. This is indicated by the different boiling points.
The shape of the molecule must have an effect on the boiling points. The more branches the lower the boiling point which means weaker Van Der Waals.
The long shape of pentane allows close packing and maximum surface area for Van Der Waals. This gives optimum interactions
More branches means the molecules can’t pack as closely.
It also means that the surface area for Van Der Waals is reduced, therefore the energy needed to overcome Van der Waals forces is reduced (lower boiling point)
Questions 1-2 P119
The combustion of alkanes:
Producing branched chains:
Straight chain alkanes have a tendency to pre ignite in a combustion engine whereas branched chains do not.
Branched chain alkanes burn cleaner than straight chain alkanes so they are usually converted using:
Producing cyclic hydrocarbons:
Again as straight chain alkanes have a tendency to pre ignite they are also converted to cyclic and aromatic hydrocarbons.
Any hydrogen produced is used in other processes - ammonia production, margarines.
These also burn cleaner than straight chain alkanes so they are usually converted using:
Branched and cyclic alkanes burn much more efficiently than straight chain alkanes hence the conversions (above).
Questions 1-4 P121
Questions 6-9 P143
The crude oil economy:
90% of all crude oil is used as a source of fuels to generate electricity or used for transport.
Plastics, pharmaceuticals, cosmetics etc are also sourced from crude oil.
Our reliance on crude oil is worrying as oil deposits are running out!
The use of crude oil for fuels:
The majority of crude oil is the alkanes - chains and branches. These are good fuels.
As the availability of oil decreases the price increases.
Crude oil as a fuel contributes to atmospheric pollutions:
|Carbon monoxide||- toxic gas|
|Carbon dioxide||- global warming|
|Nitrogen oxides||- acid rain / forest destruction|
|Sulphur oxides||- acid rain|
All of these reasons has precipitated a shift away from the use of fossil fuels
The greenhouse effect - global warming:
How do gases absorb radiation?
Solutions to the Greenhouse Effect:
Carbon Capture and Storage, CCS:
Storage as carbonates:
Fuels of the future:
Biofuel is made from the fermentation of plant material such as rape or sugar cane to produce ethanol.
The idea is that as the plant grows it absorbs CO2. When the ethanol is used as a fuel that CO2 is released. This makes it 'carbon neutral'.
The ethanol can be used as a fuel or added to petrol to make petrol burn more efficiently.
Biodiesel is made from rapeseed. It can be used pure in an diesel engine but is more commonly added to normal diesel.
Questions P123 Qu 1 - 3
Halogenation of the alkanes:
Further substitution CH2Cl2, CHCl3, CCl4
The double bond consists of a sigma bond (s) and a pi bond (p).
The p bond consists of 2 lobes one on each side of the sigma bond.
The 2 lobes overlap to produce a p bond.
To ensure maximum overlap, ethane must be a planar molecule.
The asymmetric shape of the 2 bonds locks the molecule around the double bond
This means that there is no free rotation about these bonds.
The p bond changes the shape around the carbon atom to a trigonal planar with a bond angle of 120o:
Questions P127 1-4
The reactivity of the C=C double bond:
The extra electrons in the C=C makes it more reactive as it is more electron rich.
The relative strengths of the 2 bonds are:
|Bond||Bond enthalpy Kj mol-1|