CBSE Class 11 Chemistry Hydrocarbon Notes Set B

Class XI

Chemistry

Chapter 13: Hydrocarbons

TOP Concepts:

3. Kolbe’s electrolytic method: Alkali metal salts of carboxylic acids undergo decarboxylation on electrolysis of their aqueous solutions and form hydrocarbons having twice the number of carbon atoms present in the alkyl group of the acid.

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4. Physical properties of alkanes:

a. Physical state: C1 to C4 are gases, C5 to C17 are liquids and those containing 18 carbon atoms or more are solids at 298 K. Reason: Hydrocarbons are non- polar because of the covalent nature of C-C and C-H bonds. Thus, they have weak van der Waals forces.

b. Boiling points: Alkanes have low boiling points because they have weak van der Waals forces. Higher alkanes with higher molecular masses have high boiling points than lower alkanes with low molecular masses. This is because magnitude of van der Waals forces increases with increase in molecular mass or increase in surface area. Boiling points of branched chain alkanes are lower than the corresponding straight chain alkanes. This is because with branching the molecules become more compact and hence the surface area decreases.

c. Melting points: The alkanes with even number of carbon atoms have higher melting points as compared to the immediately next lower alkane with odd number of carbon atoms. This is because alkanes with even number of carbon atoms have symmetrical structure and hence it results in closer packing in the crystal structure.

d. Solubility: Alkanes are soluble in non- polar solvents and are insoluble in polar solvents since alkanes are non- polar molecules. 

5. Chemical properties of alkanes:

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