Question

The standard molar enthalpy of vaporisation ($\Delta_{vap}H^\ominus$) of A, B and C liquids is 23.3, 41 and 29 kJ mol$^{-1}$ respectively. The correct order of dipole-dipole attractive forces in these liquids is

• A. B>C>A
• B. B>A>C
• C. A>C>B
• D. A>B>C

Hint:

Enthalpy of vaporization ($\Delta_{vap}H^\ominus$) is a measure of the energy needed to overcome intermolecular forces in a liquid to convert it to a gas.
Stronger intermolecular forces lead to a higher enthalpy of vaporization.
Intermolecular forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonding.
If dipole-dipole forces are the primary varying factor or representative of the overall intermolecular attraction, then the order of $\Delta_{vap}H^\ominus$ will reflect the order of dipole-dipole force strength.

Answer 1

The Correct Option is A

The standard molar enthalpy of vaporization ($\Delta_{vap}H^\ominus$) is the enthalpy change required to vaporize one mole of a liquid at its standard state (usually 1 bar pressure, specified temperature - often the normal boiling point). A higher enthalpy of vaporization indicates that more energy is required to overcome the intermolecular attractive forces in the liquid state and convert the liquid into a gas. Therefore, a higher $\Delta_{vap}H^\ominus$ generally implies stronger intermolecular forces. The question asks for the order of dipole-dipole attractive forces. While $\Delta_{vap}H^\ominus$ reflects the *total* strength of all types of intermolecular forces (London dispersion forces, dipole-dipole forces, hydrogen bonding), if we assume that dipole-dipole forces are a significant contributor and other factors are comparable or scale similarly, then a higher $\Delta_{vap}H^\ominus$ would suggest stronger dipole-dipole forces. This is a common simplifying assumption in such comparative questions unless other information (like molecular structure or polarity) is given. Given $\Delta_{vap}H^\ominus$ values: Liquid A: $23.3 \text{ kJ mol}^{-1}$ Liquid B: $41 \text{ kJ mol}^{-1}$ Liquid C: $29 \text{ kJ mol}^{-1}$ Arranging these values in decreasing order: $41 \text{ kJ mol}^{-1}$ (B)>$29 \text{ kJ mol}^{-1}$ (C)>$23.3 \text{ kJ mol}^{-1}$ (A). Since higher $\Delta_{vap}H^\ominus$ implies stronger intermolecular forces, the order of strength of these forces (and by inference, dipole-dipole forces if they are dominant or representative of the total) is: B>C>A. This matches option (a). \[ \boxed{\text{B>C>A}} \]