Physical Properties of Alkanes
Boiling Point of Straight Chained Alkanes
The table below gives some physical properties for the first eight straight chained alkanes in the alkane homologous series.
|Name||Number of Carbon atoms||Molecular Formula
|Structural Formula||Boiling Point °C||Melting Point °C|
From the table of the straight chained alkanes it can be seen that there is a gradual change in the physical properties as the number of carbon atoms in the molecules increases. The graph below shows a smooth increase in boiling point with increasing number of carbon atoms for the first eight straight chained alkanes in the homologous series. This gradual variation makes it possible to predict properties of a compound from the properties of other members of its homologous series.
The graph above shows a smooth gradual increase in boiling point with an increasing number of carbon atoms. This gradual variation makes it possible to predict properties of a compound from properties of other members of its homologous series.
It is important to remember that during boiling and melting of simple molecules no covalent bonds are broken. The boiling point of a compound depends on the attractive forces between the molecules of the liquids. The stronger the attractive forces are, the more energy is required to overcome the intermolecular forces between the molecules in order to covert the liquid into vapour and so the higher is the boiling point.
Melting Point of Straight Chained Alkanes
By plotting a graph of the melting points for the straight chained alkanes in the homologous series, see below, it can be seen that the alkanes do not follow the smooth curve as was the case for boiling points. However, two separate curves can be plotted, one for alkanes with even number of carbon atoms and a lower one for those with an odd number.
This is due to the molecular arrangements the alkanes adopt when in the solid state. In alkanes with an even number of carbon atoms the crystalline structure is such that the molecule chains pack closer together than alkanes with an odd number of carbon atoms. The tight packing of the molecules in the even numbered alkanes results in larger attractive intermolecular forces. Therefore, more energy is required to separate the molecules with even number of carbon atoms than those with odd numbers.
Density & Viscosity of Straight Chained Alkanes
The table below gives the density, viscosity and state of the first 8 straight chained alkanes in the alkane homologous series.
Pascal second x 10-3 (mPa.s) at 25°C
The first four alkanes in the straight chain alkane homologous series are all gases at room temperature. The alkanes from pentane through to C16 are liquids with the remainder larger molecules being solids.
All liquid alkanes are less dense than water and therefore float on it. The viscosity of liquid alkanes increases as the number of carbon atoms increase.
From the physical properties of the alkanes, it can be established that:
The increasing boiling point is due to the increasing intermolecular forces of attraction between molecules of increasing size. The first four alkanes in the series are small chain molecules and therefore have very weak intermolecular attractive forces. For this reason they have very low boiling points and exist as gases at room temperature and pressure. As the alkane molecules get larger the chains can align themselves in such arrangements that the intermolecular forces can operate along the whole length of the molecule.
The increase in viscosity is also due to the increase in intermolecular forces between molecules of increasing size.