Question

The normal boiling point of a liquid X is $400{\text{K}}$. Which of the following statement is true about the process ${{\text{X}}_{\left( {\text{l}} \right)}} \to {{\text{X}}_{\left( {\text{g}} \right)}}$?
A. At $400{\text{K}}$ and $1{\text{atm}}$ pressure $\Delta {\text{G = 0}}$
B. At $400{\text{K}}$ and ${\text{2atm}}$ pressure $\Delta {\text{G = }}$ +ve
C. At $400{\text{K}}$ and ${\text{0}}{\text{.1atm}}$ pressure $\Delta {\text{G = }}$ -ve
D. At $410{\text{K}}$ $1{\text{atm}}$ and pressure $\Delta {\text{G = }}$ +ve

Answer 1

Boiling point is the temperature at which liquid changes to gaseous state. At boiling point, vapor pressure is equal to the pressure of gas above it. It is an indicator of the strength of the attractive forces between the liquid molecules.

Complete step by step solution:
Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid state. The normal boiling point of a liquid is the temperature at which its vapor pressure is equal to $1{\text{atm}}$. It also depends upon the pressure, molecular weight, structure of molecular and intermolecular interactions and the impurities.
When the pressure is less than $1{\text{atm}}$, the boiling point of the liquid is less than its normal boiling point. Thus the reaction will be spontaneous. When pressure is greater than $1{\text{atm}}$, the boiling point of the liquid is greater than its normal boiling point. Thus the reaction will be non-spontaneous.
Higher the altitude, lower the temperature at which water boils since the pressure at high altitude is very high.
Temperature and pressure is constant at boiling point. Thus change in temperature is zero, thereby change in enthalpy will be zero. $\Delta {\text{G}}$ is dependent on the change in temperature. Thus $\Delta {\text{G = 0}}$.

Hence, the correct option is A.

Note: Boiling occurs when a temperature is reached at which the thermal energy of the particles is great enough to overcome the attractive forces that hold them in liquid. In the liquid state of a covalent bond, the weak intermolecular interactions are more. Thus boiling occurs at much lower temperature.