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Physical Properties of Alkanes

1. PHYSICAL STATE

Alkanes contain only C-C and C-H sigma bonds which are almost non-polar due to very small difference of electronegativity between carbon and hydrogen atoms. Therefore, alkanes possess weak van der Waals’ forces as the intermolecular forces. Due to the weak interparticle forces, the first four members (C1 to C,J are gases; the next thirteen members, (C5 to C17) are liquids while the higher members are waxy solids.

 

2. BOILING POINTS

Alkanes generally have low boiling points. The boiling points of n-alkanes increase regularly with the increase in the number of carbon atoms. This can be explained on the basis of interparticle forces. With the increase in the number of carbon atoms, the molecular mass and also the molecular size increases. This results in the increase in the magnitude of van der Waals’ forces (London dispersion forces). Consequently, boiling point also increases. It is evident that there is an increase of 20-30 degrees for each carbon atom added to the chain. However, the first few members are exceptions where the increase in boiling point is relatively more.

Among the isomeric alkanes, the branched chain isomers have relatively low boiling points as compared to their corresponding straight chain isomers. Greater the branching of the chain is, lower is the boiling point. This is due to the fact that branching of the chain makes the molecule more compact and brings it closer to a sphere. This decreases the surface area and hence, the magnitude of interparticle London dispersion force~ and leads to the decrease in boiling point. The boiling points of isomeric pentanes are given ahead:

 

Table 40.1. Physical Properties of the first 20 n-Alkanes

3. VOLATILITY

Due to their low boiling points, lower alkanes are highly volatile. Volatility refers to the ability of a liquid to change into vapour state. Among alkanes volatility decreases with increase in chain length. Among isomeric alkanes more the branching, greater is the volatility.

 

4. MELTING POINTS

The melting points of alkanes do not show a very smooth gradation with the increase in molecular mass. For example, let us look at the melting points of propane to n-octane.

Hydro-            C3H8                C4H10               C5HI2               C6H14               C7H16          C8H18

carbon

Melting              84                     135                  143                 178                    182                 216

Point (K)

It is evident that the increase in melting point is relatively more in moving from an alkane having odd number of carbon atoms to the higher alkane while it is relatively less in moving from an alkane with even number of carbon atoms to the higher alkane. This is because the intermolecular forces in a crystal depend not only upon ·he size of the molecule but also upon how well the molecules fit into the crystal lattice. The molecule which fit into crystal lattice more easily due to their symmetrical structures usually have high lattice energy and therefore, high melting points.

For example, let us compare the melting points of isomeric pentanes.

Melting point: 256.4 K

Among these isomers neo-pentane has the highest melting point because its molecules are almost spherical and hence pack more closely.

 

5. SOLUBILITY

In keeping with the popular rule “like dissolves like” hydrocarbons are insoluble in polar solvent like water because they are predominantly non-polar in nature. However, they are soluble in non-polar solvents like ether, carbon tetrachloride, etc.

The liquid hydrocarbons themselves are good solvents for other non-polar organic molecules.

 

6. DENSITY

The densities of alkanes increase with increasing molecular masses but become constant at about 0.8 g cm-3. This means that all alkanes are lighter than water.