Question

Which of the following is the correct order of melting points of alkanes?

• A. $C_5{H}_{12}<C_6{H}_{14}<C_7{H}_{16}<C_8{H}_{18}$
• B. $C_8{H}_{18}<C_7{H}_{16}<C_6{H}_{14}<C_5{H}_{12}$
• C. $C_8{H}_{18}<C_6{H}_{14}<C_7{H}_{16}<C_5{H}_{12}$
• D. $C_5{H}_{12}<C_6{H}_{14}<C_8{H}_{18}<C_7{H}_{16}$

Answer 1

The correct option is A $C_5{H}_{12}<C_6{H}_{14}<C_7{H}_{16}<C_8{H}_{18}$

Melting points of alkanes depends on Van der waals forces of attraction, which in turn depends on surface area and packing efficiency. With increasing number of carbon atoms in an alkane, surface area increases and thus magnitude of force increases. Thus, the melting point of the given alkanes increases as the molecular mass increases: $C_5{H}_{12}<C_6{H}_{14}<C_7{H}_{16}<C_8{H}_{18}$.