Physical properties of the 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 componds, 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 exist as 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.
  6. As shown in the figure below, alkenes have boiling points lower than the alkanes that have same number of carbons in their molecules.
  7. This is because alkenes have less electrons per molecule compare to the corresponding alkanes (that have same number of carbon per molecule).
  8. As a result, the intermolecular force in alkenes is lower than the corresponding alkanes.
IMPORTANT to Know!
  • The strength of the Van der Waals force depends on the number of electrons and the shape of the molecule.
  • More electrons → Stronger Van der Waals force
MUST Know!
Alkenes have boiling points lower than the alkanes that have same number of carbons in their molecules.

Solubility and Conductivity

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

Density

  1. The density of alkenes are low (lower than water).
  2. However, the density increases as the number of carbon atoms per molecule increases.

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