Vapour Pressure
Dynamic Equilibrium, Boiling Points, and the Clausius-Clapeyron Equation.
When a liquid is kept in a closed container, molecules with high kinetic energy escape from the surface into the vapor phase (Evaporation). Simultaneously, some vapor molecules strike the liquid surface and are recaptured (Condensation). Eventually, a Dynamic Equilibrium is established.
1. What is Vapour Pressure?
Vapour Pressure is defined as the pressure exerted by the vapor of a liquid when it is in dynamic equilibrium with its liquid phase in a closed container at a given temperature.
2. Factors Affecting Vapour Pressure
Vapour pressure is a characteristic property of a liquid at a specific temperature. It depends on exactly two things:
1. Nature of the Liquid (Intermolecular Forces)
- Liquids with weak intermolecular forces (like ether or acetone) allow molecules to escape easily. They have a high vapour pressure and are called volatile liquids.
- Liquids with strong intermolecular forces (like water, which has hydrogen bonding) hold tightly to their molecules. They have a low vapour pressure.
2. Temperature
- As temperature increases, the average kinetic energy of the liquid molecules increases.
- More molecules gain enough energy to overcome the intermolecular forces and escape into the vapor phase.
- Therefore, Vapour Pressure increases non-linearly (exponentially) with Temperature.
3. Vapour Pressure Graph & Boiling Point
The Boiling Point of a liquid is defined as the temperature at which its vapour pressure becomes exactly equal to the external atmospheric pressure.
Interpretation: Diethyl ether has weaker intermolecular forces, so its vapour pressure reaches $1 \text{ atm}$ at a much lower temperature ($34.6^\circ C$) compared to water ($100^\circ C$).
- Normal Boiling Point: Boiling point when external pressure is $1 \text{ atm}$ ($760 \text{ mmHg}$). For water, it is $100^\circ C$.
- Standard Boiling Point: Boiling point when external pressure is $1 \text{ bar}$ ($0.987 \text{ atm}$). For water, it is $99.6^\circ C$.
- Pressure Cooker Effect: By sealing a pot, the pressure inside builds up above $1 \text{ atm}$. The water must reach a higher temperature before its vapour pressure equals this new, higher internal pressure, cooking food faster.
4. Clausius-Clapeyron Equation
The quantitative relationship between Vapour Pressure ($P$) and Absolute Temperature ($T$) is given by the Clausius-Clapeyron Equation. This is crucial for solving numericals in JEE Advanced and NEET.
Or using Base-10 Logarithms ($2.303 \log x = \ln x$):
- $P_1, P_2$: Vapour pressures at temperatures $T_1$ and $T_2$ (in Kelvin).
- $\Delta H_{vap}$: Enthalpy (Heat) of Vaporization of the liquid.
- $R$: Universal Gas Constant ($8.314 \text{ J K}^{-1}\text{mol}^{-1}$).
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