2.2.5 Physical Properties of Alkenes

1. Alkenes are covalent compounds that consist of simple molecules.
2. The molecules of alkenes are held together by weak Van der Waals forces (intermolecular forces).
3. As covalent compounds, the physical properties of alkenes are similar to alkanes.

Boiling Points

MUST Know!
The first 3 alkenes are gases.
The Melting and boiling point of alkenes increases as the number of carbon per molecule increases. (You must also know the explanation).

1. Alkenes have low melting/boiling points, owing to the weak intermolecular force.
2. At room temperature,  ethene, propene and butene exist as gases while pentene to decene exists as a liquid.
3. As shown in the diagram below, the boiling point of alkenes increases as the number of carbon atoms per molecule increases.
4. This is because as the number of carbon atoms per molecule of alkene increases, the molecular size increases, and hence the inter molecular forces increases.
5. As a result, more heat energy is needed to overcome this forces during melting and boiling, and hence the melting and boiling points increases.

1. As shown in the figure above, alkenes have boiling points lower than the alkanes that have the same number of carbons in their molecules.
2. This is because alkenes have fewer electrons per molecule compare to the corresponding alkanes (that have the same number of carbon per molecule).
3. As a result, the intermolecular force in alkenes is lower than the corresponding alkanes.

MUST Know!
Alkenes have boiling points lower than the alkanes that have the same number of carbons in their molecules.

Solubility and Conductivity

1. Like most of the other covalent compound, alkenes are insoluble in water but dissolve in organic solvents.
2. Alkenes do not conduct electricity, because there are no free moving ions in them.

Density