Physical Properties Of Alkanes And Their Variations


Alkanes are referred to as saturated hydrocarbons that is, hydrocarbons having all carbon atoms bonded to other carbon atoms or hydrogen atoms with sigma bonds only. As the alkanes posses weak Van Der Waals forces, the first four members, \(C_1\) to \(C_4\) are gases, \(C_5\) to \(C_{17}\) are liquids and those containing 18 carbon atoms or more are solids at 298 K. They are colourless and odorless. Some salient physical properties of alkanes are:

Structures of Alkanes: All the carbon atoms present in alkane are \(sp^3\) hybridized that is, every carbon atom forms four sigma bonds with carbon or hydrogen atoms. General configuration of alkane is \(C_nH_{2n+2}\).. They exhibit tetrahedral geometry with a bond angle of 109.47° between them.

Solubility of alkanes: Due to very little difference of electronegativity between carbon and hydrogen and covalent nature of C-C bond or C-H bond alkanes are generally non-polar molecules. As we generally observe, polar molecules are soluble in polar solvents whereas non-polar molecules are soluble in non-polar solvents. Hence, alkanes are hydrophobic in nature that is, alkanes are insoluble in water. However, they are soluble in organic solvents as the energy required to overcome the existing Van Der Waals forces and generate new Van Der Waals forces is quite comparable.

Boiling point of alkanes: As the intermolecular Van Der Waals forces increase with the increase of the molecular size or the surface area of the molecule we observe:
• Boiling point of alkanes increases with increasing molecular weight,
• The straight chain alkanes are observed to have higher boiling point in comparison to their structural isomers.

Melting point of alkanes: Melting point of alkanes follow the same trend as their boiling point that is, it increases with increase in molecular weight. This is attributed to the fact that higher alkanes are solids and it’s difficult to overcome intermolecular forces of attraction between them. It is generally observed that even numbered alkanes have higher trend in melting point in comparison to odd numbered alkanes as the even numbered alkanes pack well in solid phase, forming a well organized structure which is difficult to break.

Know more about the Pyrolysis of Hydrocarbons: Alkanes

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The process in which higher hydrocarbons are broken down into lower hydrocarbons by controlled pyrolysis, is called