Vapour Pressure & Raoult's Law
Master the thermodynamics of solutions. Decode the mathematical graphs of Raoult's Law, surface blocking by non-volatile solutes, and the ultimate vapor phase composition traps.
Module Focus: The Dynamic Equilibrium
Vapour Pressure is the pressure exerted by the vapor in thermodynamic equilibrium with its condensed phases (liquid or solid) at a given temperature in a closed system. Understanding how this pressure changes when we mix liquids (volatile solutes) or dissolve solids (non-volatile solutes) is the foundation of all colligative properties.
1. Factors Affecting Vapour Pressure
Vapour pressure is inversely proportional to the strength of Intermolecular Forces (IMF).
Liquids with weak IMF (like diethyl ether) escape easily into the vapor phase, resulting in a high VP (highly volatile). Liquids with strong IMF (like water via H-bonding) have a low VP.
Vapour pressure is directly proportional to temperature.
As temperature increases, more molecules gain enough kinetic energy to overcome IMF and escape into the vapor phase. (Governed by the Clausius-Clapeyron equation).
Vapour pressure is INDEPENDENT of the surface area of the liquid and the volume of the container!
While a larger surface area increases the rate of evaporation, it also increases the rate of condensation equally. The final equilibrium pressure remains exactly the same.
2. Raoult's Law (Volatile Solute in Volatile Solvent)
French chemist François-Marie Raoult stated that for a solution of volatile liquids, the partial vapour pressure of each component in the solution is directly proportional to its mole fraction in the liquid mixture.
For a binary ideal solution of components A and B:
$P_B = P_B^0 \cdot X_B$
According to Dalton's Law, Total Pressure ($P_T$):
Where $P^0$ is the vapour pressure of the pure liquid, and $X$ is its mole fraction in the solution.
3. The Mega Trap: Vapor Phase Composition ($y_A$)
Mole fractions in the liquid solution ($X_A, X_B$) are NOT the same as the mole fractions in the vapor phase above the liquid ($y_A, y_B$). The vapor is always richer in the more volatile component.
According to Dalton's Law of Partial Pressures, the mole fraction in the vapor phase is the ratio of its partial pressure to the total pressure.
4. Raoult's Law (Non-Volatile Solute)
When a non-volatile solid (like glucose or urea) is dissolved in a volatile solvent (like water), the vapour pressure of the solution becomes less than the pure solvent.
Because the solute is non-volatile, it does not contribute to the vapor pressure ($P_{\text{solute}} = 0$).
The solute particles occupy space on the liquid's surface, blocking solvent molecules from escaping. Thus, fewer solvent molecules enter the vapor phase.
The pressure of the solution ($P_s$) is just the partial pressure of the solvent ($P_A$). So, $P_s = P_A^0 X_A$.
Since $X_A + X_B = 1$, we get:
RLVP equals the mole fraction of the solute ($X_B$).
NEET Grand Test: Raoult's Law
15 High-Yield Questions targeting vapor phase composition, graphical plots, and RLVP numericals.
Join the Ultimate Chemistry Crash Course
Master Physical Chemistry, Solutions, and Thermodynamics. Get access to our full suite of Rapid Revision modules, formula sheets, and mock tests specifically designed for the NTA NEET pattern.
Explore All NEET Modules →
No comments:
Post a Comment