Physical Properties of alkanes

 1.      Physical nature:

(a)     State:

(i)         All are colourless & possess no characteristic odour.

(ii)       Lower alkanes (C1 to C4) are gases, middle one (C3 to C17) are liquids and higher are solids.

(iii)      The boiling point of alkanes increases with increase in molecular weight due to increase in van der waals forces with increase in molecular weight i.e.,

               B.pt. order : pentane < hexane < heptanes

(iv)      Also the branching in alkanes gives a decrease in surface area (as the shape approaches to spherical) which results in decrease in van der Waals forces. That is why b.pt. of isomeric alkanes who the order : pentane > isopentane > neopentane

(v)       The melting points of alkanes do not show a regular trend. Alkanes with even number of carbon atoms have higher m.pt. than their adjacent of odd number of carbon atoms.

    M.pt. order :     propane             <    ethane             <        methane

                         (-187.7oC)            (-172.0oC)                   (-182.5oC)

 

(vi)      The abnormal trend in m.pt. is probably due to the fact that alkanes with odd carbon atoms have their end carbon atom on the same side of the molecule and in even carbon atom alkane, the end carbon atom on opposite side. Thus alkanes with even carbon atoms are packed closely in crystal lattice to permit greater intermolecular attractions.

intermolecular-attractions        

 

(b)      Density : The density of alkanes increases with increase in molecular weight and becomes constant at 0.76 g/ml. Thus all alkanes are lighter than water.

 

(c)       Solubility :

(i)          Alkanes being non polar and thus insoluble in water but soluble in non polar solvents e.g., C6H6, CCI4, ether etc.

(ii)         The solubility of alkanes decreases with increase in molecular weight.

(iii)        Liquid alkanes are themselves good, non polar solvents.

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