Physical Properties and Chain Length
Physical properties and chain length
Remember that the alkanes are a group of organic compounds that contain carbon and hydrogen atoms bonded together. The carbon atoms link together to form chains of varying lengths. We have already mentioned that the alkanes are relatively unreactive because of their stable C-C and C-H bonds. The boiling points and melting points of these molecules are determined by their molecular structure and their surface area.
Some alkanes, from top left to bottom right: methane, ethane, propane, butane, hexane, octane and icosane.
The more carbon atoms there are in an alkane, the greater the surface area available for intermolecular interactions.
Van der Waals intermolecular forces increase as chain length increases. Image by Duncan Watson.
This increase in intermolecular attractions leads to higher melting and boiling points. This is shown in the table below.
|
Formula |
Name |
Molecular mass (\(\text{g.mol$^{-1}$}\)) |
Melting point (℃) |
Boiling point (℃) |
Phase (at 25℃) |
|
\(\text{CH}_{4}\) |
methane |
\(\text{16.04}\) |
\(-\text{182}\) |
\(-\text{162}\) |
gas |
|
\(\text{C}_{2}\text{H}_{6}\) |
ethane |
\(\text{30.06}\) |
\(-\text{183}\) |
\(-\text{89}\) |
gas |
|
\(\text{C}_{3}\text{H}_{8}\) |
propane |
\(\text{44.08}\) |
\(-\text{188}\) |
\(-\text{42}\) |
gas |
|
\(\text{C}_{4}\text{H}_{10}\) |
butane |
\(\text{58.1}\) |
\(-\text{137}\) |
\(\text{0}\) |
gas |
|
\(\text{C}_{6}\text{H}_{14}\) |
hexane |
\(\text{86.14}\) |
\(-\text{95}\) |
\(\text{68.5}\) |
liquid |
|
\(\text{C}_{8}\text{H}_{18}\) |
octane |
\(\text{114.18}\) |
\(-\text{57}\) |
\(\text{125.5}\) |
liquid |
|
\(\text{C}_{20}\text{H}_{42}\) |
icosane |
\(\text{282.42}\) |
\(\text{37}\) |
\(\text{343}\) |
solid |
Table: The physical properties of some alkanes.
As the molar mass of an alkane increases so does the boiling point. Image by Duncan Watson.
Notice that when the molecular mass of the alkanes is low (i.e. there are few carbon atoms), the organic compounds are gases because the intermolecular forces are weak. As the number of carbon atoms and the molecular mass increases, the compounds are more likely to be liquids or solids because the intermolecular forces are stronger.
The larger a molecule is, the stronger the intermolecular forces are between the molecules. This is one of the reasons why methane (\(\text{CH}_{4}\)) is a gas at room temperature while hexane (\(\text{C}_{6}\text{H}_{14}\)) is a liquid and icosane (\(\text{C}_{20}\text{H}_{42}\)) is a solid.
Tip:
Be careful when comparing molecules with different types of intermolecular forces. For example, small molecule with hydrogen bonding can have stronger intermolecular forces than a large molecule with only van der Waals forces.
The table below shows some other properties of the alkanes that also vary with chain length.
|
Name |
Density (\(\text{g.dm$^{-3}$}\)) |
Flash point (℃) |
Smell |
Phase (at 25℃) |
|
methane |
\(\text{0.66}\) |
\(-\text{188}\) |
odourless |
gas |
|
ethane |
\(\text{1.28}\) |
\(-\text{135}\) |
odourless |
gas |
|
propane |
\(\text{2.01}\) |
\(-\text{104}\) |
odourless |
gas |
|
butane |
\(\text{2.48}\) |
\(-\text{60}\) |
like petrol |
gas |
|
hexane |
\(\text{650}\) |
\(-\text{26}\) |
like petrol |
liquid |
|
octane |
\(\text{700}\) |
\(\text{13}\) |
like petrol |
liquid |
|
icosane |
\(\text{790}\) |
\(>\text{113}\) |
odourless |
solid |
Table: Properties of some of the alkanes.
Density increases with increasing molecular size. Note that compounds that are gaseous are much less dense than compounds that are liquid or solid.
Remember that the flash point of a volatile molecule is the lowest temperature at which that molecule can form a vapour mixture with air and be ignited. The flash point increases with increasing chain length meaning that the longer chains are less flammable although they can still be ignited.
Fact:
It is partly the stronger intermolecular forces that explain why petrol (mainly octane (\(\text{C}_{8}\text{H}_{18}\))) is a liquid, while candle wax (\(\text{C}_{23}\text{H}_{48}\)) is a solid. If these intermolecular forces did not increase with increasing molecular size we would not be able to put liquid fuel into our cars or use solid candles.
The change in physical properties due to chain length does not only apply to the hydrocarbons. The solubility of ketones in water decreases as the chain length increases. The longer the chain length, the stronger the intermolecular interactions between the ketone molecules.
This means that more energy is required to overcome those interactions and the molecule is therefore less soluble in water. Long chains can also fold around the polar carbonyl group and stop water molecules from bonding with it.
The water solubility of a ketone decreases as the chain length increases.
This same idea can be applied to all the compounds with water-soluble functional groups. The longer the chain becomes, the less soluble the compound is in water.
This lesson is part of:
Organic Molecules