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How does boiling point and melting point of hydrocarbons change with

increase in molecular mass ?

With increase in molar mass, the melting point and boiling points of hydrocarbons increases generally.

It is because, with increase in molar mass, the molecule becomes larger and so the van der waal's forces and other binding forces between those molecules become stronger, which holds the particles very firmly together in solid or liquid phase and so it requires more energy to break these bonds and thus the melting and boiling points of the hydrocarbons increases.

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 boiling point and melting point of hydrocarbons increase with increase in molecular mass

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 boiling point and melting point of hydrocarbons increases with increase in molecular mass

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Since hydrocarbons are non-polar compounds, therefore the intermolecular forces of attraction operating between the molecules of these compounds are weak Vander Waal's forces, which depend upon the shape and structure of the molecule. The magnitude of these forces increases with the increase in surface area of contact between the adjacent molecules. More is the surface area, more are the Vander Waal's forces. Now as the molecular mass of the hydrocarbons increases, the surface area of the molecules increases. As a result, the magnitude of Vander Waal's forces increases and hence the melting point increases.

 

 

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