in a homologous series what difference comes in the physical and chemical properties as the molecular mass increases

Asked by pr_prashant | 27th Nov, 2010, 09:38: AM

Expert Answer:

Dear Student

Homologous Series: Homologous series is a series of compounds which have same general molecular formula and chemical properties. But there occur variation in the physical properties. Each consecutive member differs from the next by one carbon atom and two hydrogen atoms (-CH2 group). The compounds of a homologous series have similar chemical properties.The physical properties however show a regular gradation.

For example: 
ALKANES: General formula is CnH2n+2
All of these are saturated molecules. as the molecular weight gradually increses a CH2 is increses.
Chemical properties: The chemical properties remain almost the same through out the series. As these molecules are saturated covalent hydrocarbons, they show same type of chemical reactions.

Physical properties: As we move in the series. with the increase in the CH2 fragment, the molecular weight increases.

Most alkanes of low molecular weight are gases while the next higher molecular weight alkanes are liquids and alkanes with more than C18 are colourles solids. 

 Boiling point and melting point of liquid alkanes and solid alkanes respecively increase with increase in molecular weight. 
Alkanes are insoluble in water but soluble in organic solvents. Liquid alkanes are lighter than water but as molecular weight increase the density of the alkanes also increase.

We hope that clarifies your query.

Answered by  | 27th Nov, 2010, 09:54: AM

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