Chemistry Assignment Help With Physical Properties of the Alkanes

Physical Properties Of Alkanes

(I) The first four alkanes (methane to butane) are colourless gases, the next thirteen (pentane to heptadecane) are colourless liquids and those containing 18 carbon atoms or more are solids at ordinary temperatures.

(II) Their boiling points show gradual rise as the carbon content increases. In general, the boiling point difference between two successive members of the homologous series (except for the first few members) is about 20-30°C. Among the isomeric alkanes, the straight chain (i.e, normal isomer has a higher boiling point than the branched chain isomer. The greater the branching of the chain, the lower the boiling point.

(III) Their melting points also points also show a rise with the increasing number of carbon atoms, but the rise is not as regular as in the case of boiling points. For example,

n-Alkane C4H10 C5H12 C6H12 C7H16 C8H18

m.p. °C –135 – 130 – 95 – 90 – 57

(IV) Alkanes are made up of carbon and hydrogen atoms only. Since these two elements have almost similar electronegativities, alkanes are non-polar. Therefore, non-polar alkanes are soluble in non-polar solvents like carbon tetrachloride, benzene, etc. but insoluble in polar solvents like water, alcohol, etc.

(V) The densities of alkanes show a definite rise with increasing molecular weight, but they reach a limiting constant value of about,.0.8 g/rnl with n-hexadecane (C16H34). Thus, alkanes are always lighter than water.

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