Okay so this is a bit of a jump from the last topic I did in my Chemistry section, but a lot of the stuff in chemistry is quite hard to take notes on. The alkanes are covered in the introduction to organic chemistry and are a homologous series of the hydrocarbons. A homologous series is a group of hydrocarbons which each has the same functional group, and they can be divided further depending on the number of carbon atoms they contain.
Fractional Distilation
This is the separating of hydrocarbons in crude oil by heating it and gradually cooling it down in factions.
The crude oil is first vaporised by heating and passed into the fractionating column, which is hotter at the bottom than the top. The vapour moves up through the column through a series of bubble caps. When the temperature reaches a point lower than a hydrocarbon’s boiling point, then it cools and condensed into a liquid. The liquid fractions are than siphoned off into storage containers. Shorter chained hydrocarbons have lower boiling points and as such condense near the top of the column, where as long chained hydrocarbons with high boiling points condense closer to the bottom. The residue which collects as the bottom of the column is known as bitumen and is used for making tar for surfacing roads.
Boiling points of alkanes
As the chain length of hydrocarbons increases, the boiling point increases because they are more van der Waals’ forces between the molecules as there are more points of contact. It takes more energy, and therefore a higher temperature, to overcome these forces and separate the molecules.
When molecules are branched, they will have a lower boiling point than their unbranched isomer. This is because there are less points of contact between branched molecules and they cannot be packed as tightly together, therefore there are less van der Waals’ forces and a lesser intermolecular attraction to be overcome. Therefore less energy is needed to separate the molecules.
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