Alkanes

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Alkanes are saturated hydrocarbons that have the general formula CnH2n.

Hydrocarbon: a molecule consisting of carbon and hydrogen atoms only.

Saturated hydrocarbon: a hydrocarbon with only single bonds between the carbon atoms

Petroleum/crude oil is a mixture consisting of mainly straight and branched chain alkane hydrocarbons that can be separated out by fractional distillation.

It also includes lesser amounts of cycloalkanes and arenes - both of which are hydrocarbons containing a ring of carbon atoms, as well as impurities such as sulfur compounds.

e.g:

(http://docsdrive.com/images/ansinet/jas/2011/img3-2k11-2084-2091.gif)

Fractional distillation:

- A process used to separate a mixture of liquids that have different boiling points.

It works by heating a mixture from the bottom of the fractional distillation collumn. This creates a temperature gradient from hot to cool up the collumn. This means that when the mixture is heated, hydrodarbons with a low boiling point evapourate and turn to vapour and move up the collumn, whilst hydrocarbons with a higher boiling point remain as a liquid at the bottom and are tapped out at that point.

The hydrocarbons in the vapour that has risen up the collumn are tapped out based on their boiling points. Because each of the hydrocarbons in the mixture have a specific and different boiling point due to their size and structure - when they reach an area in the collumn that matches their boiling point they condense back into a liquid at a specific fraction and are tapped.

(http://www.bbc.co.uk/staticarchive/d1fc03f39806642998b1bd6ea1dda2c8e2e2b674.gif)

So overall, alkanes with a higher boiling point are tapped out at the bottom of the collumn whilst those with a lower boiling point are tapped out higher up in the collumn.

WHY ARE THERE DIFFERENT BOILING POINTS:

  • Chain length:

Trend: The boiling point of alkanes increases as the number of carbon atoms in the hydrocarbon chain increase.

Explaination: The more atoms means a greater amount of van der waals forces and therefore more energy is required to separate the molecules, giving these hydrocarbons a greater boiling point. This is in contrast to hydrocarbons with a shorter hydrocarbon chain as they have less van der waals forces.

  • Straight or branched chain?

Trend: Straight chain hydrocarbons have higher boiling points than branched hydrocarbons - the greater the branching, the lower the boiling point

Explaination: Straight chain molecules pack together more easily and have a larger contact surface for van der waals forces to take place. Because there are greater interactions and van der waals forces in straight chained molecules than branched ones and therefore more energy is required to overcome these forces giving straight molecules higher boiling points.

This is in contrast to branched hydrocarbons which do not pack together as easily and therefore have less points of interaction and fewer van der waals forces acting between molecules. This gives branched chains a lower boiling point as less energy is required to overcome the few van der waals forces present in the molecules.(http://www.4college.co.uk/as/poly/induced2.gif)

Products and uses of fractional distillation:

(http://3.bp.blogspot.com/-5U1xZUJXTFs/U76sqzrzN7I/AAAAAAAACB8/zFvARC6pQuQ/s1600/Screen+shot+2014-07-10+at+19.08.18.png)

Cracking:

The naptha fraction from crude oil is in high demand for use as a petrol and in the chemical industry. The longer chain…

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