Vapour Pressure — Revision Notes

⚡ 30-Second Revision

Definition: — Pressure exerted by vapour in equilibrium with liquid in a closed system.
Dynamic Equilibrium: — Rate of evaporation = Rate of condensation.
Factors Affecting VP:

- Temperature (T): VP $\uparrow$ with T $\uparrow$ (exponentially). - Intermolecular Forces (IMFs): VP $\downarrow$ with stronger IMFs.

Factors NOT Affecting VP: — Surface area, amount of liquid, volume of container.
Boiling Point (BP): — T at which $P_{vapour} = P_{external}$.
Volatility: — Higher VP $\implies$ higher volatility $\implies$ weaker IMFs.
Clausius-Clapeyron: — $\ln\left(\frac{P_2}{P_1}\right) = -\frac{\Delta H_{vap}}{R}\left(\frac{1}{T_2} - \frac{1}{T_1}\right)$ (qualitative understanding for NEET).

2-Minute Revision

Vapour pressure is the pressure exerted by the vapour of a liquid when it reaches dynamic equilibrium with its liquid phase in a closed container at a specific temperature. This equilibrium means the rate of molecules escaping from the liquid (evaporation) equals the rate of molecules returning to the liquid (condensation).

The two main factors influencing vapour pressure are temperature and intermolecular forces. As temperature increases, more molecules gain enough kinetic energy to escape, leading to an exponential increase in vapour pressure.

Conversely, liquids with strong intermolecular forces (like hydrogen bonding) hold their molecules more tightly, resulting in lower vapour pressure. Vapour pressure is an intensive property, meaning it does not depend on the amount of liquid or its surface area.

A crucial concept is that a liquid boils when its vapour pressure becomes equal to the external atmospheric pressure. Therefore, at higher altitudes where atmospheric pressure is lower, liquids boil at lower temperatures.

5-Minute Revision

Vapour pressure is a critical property of liquids, defined as the pressure exerted by the vapour that is in dynamic equilibrium with its liquid phase in a closed system at a given temperature. This dynamic equilibrium is a state where the rate of evaporation (liquid to gas) precisely matches the rate of condensation (gas to liquid), leading to a constant concentration of vapour molecules and thus a stable pressure.

Key Factors:

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Temperature: — The most significant factor. An increase in temperature provides molecules with higher average kinetic energy, enabling a larger fraction to overcome intermolecular forces and escape into the vapour phase. This leads to an exponential increase in vapour pressure. For example, water's vapour pressure at $20^circ\text{C}$ is $17.5,\text{mmHg}$, but at $60^circ\text{C}$ it's $149.4,\text{mmHg}$.
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Intermolecular Forces (IMFs): — The strength of attractive forces between liquid molecules directly impacts vapour pressure. Liquids with weak IMFs (e.g., diethyl ether with dipole-dipole and London dispersion forces) are volatile, meaning molecules escape easily, resulting in high vapour pressure. Liquids with strong IMFs (e.g., water or ethanol with hydrogen bonding) are less volatile, requiring more energy for molecules to escape, leading to lower vapour pressure.

What doesn't affect it: Vapour pressure is an intensive property, so it's independent of the amount of liquid, the surface area exposed, or the volume of the container (as long as equilibrium can be established).

Boiling Point Connection: The boiling point of a liquid is the temperature at which its vapour pressure becomes equal to the external atmospheric pressure. This explains why water boils at $100^circ\text{C}$ at sea level ($760,\text{mmHg}$ atmospheric pressure) but at a lower temperature at higher altitudes where atmospheric pressure is reduced. Understanding this relationship is vital for NEET.

Example: Compare the vapour pressures of pentane ($C_5H_{12}$) and butan-1-ol ($CH_3CH_2CH_2CH_2OH$) at the same temperature. Pentane is a nonpolar molecule with only weak London dispersion forces. Butan-1-ol has a hydroxyl group, allowing for strong hydrogen bonding. Therefore, pentane will have a significantly higher vapour pressure than butan-1-ol at the same temperature due to its much weaker intermolecular forces.

Prelims Revision Notes

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Definition: — Vapour pressure ($P_{vapour}$) is the pressure exerted by the vapour in dynamic equilibrium with its liquid phase in a closed system at a specific temperature.
2
Dynamic Equilibrium: — Occurs when the rate of evaporation (liquid $\to$ vapour) equals the rate of condensation (vapour $\to$ liquid). No net change in the amount of liquid or vapour.
3
Factors Affecting Vapour Pressure:

* Temperature: Vapour pressure increases exponentially with increasing temperature. Higher T $\implies$ higher average kinetic energy $\implies$ more molecules escape $\implies$ higher $P_{vapour}$. * Intermolecular Forces (IMFs): Stronger IMFs $\implies$ lower $P_{vapour}$. Molecules are held more tightly, requiring more energy to escape. Examples: Hydrogen bonding > Dipole-dipole > London dispersion forces.

1
Factors NOT Affecting Vapour Pressure:

* Amount of liquid. * Surface area of the liquid. * Volume of the container (as long as equilibrium is established).

1
Boiling Point: — The temperature at which the vapour pressure of a liquid becomes equal to the external atmospheric pressure ($P_{vapour} = P_{external}$). At higher altitudes, $P_{external}$ is lower, so boiling point is lower.
2
Volatility: — A measure of how easily a liquid vaporizes. High volatility $\implies$ high $P_{vapour}$ $\implies$ weak IMFs.
3
Effect of Non-Volatile Solute: — Adding a non-volatile solute to a solvent lowers the solvent's vapour pressure (Raoult's Law). This is because solute particles reduce the surface area available for solvent molecules to evaporate.
4
Clausius-Clapeyron Equation (Qualitative): — $\ln P = -\frac{\Delta H_{vap}}{RT} + C$. Shows the exponential relationship between vapour pressure and temperature. $\Delta H_{vap}$ is the molar enthalpy of vaporization.

Vyyuha Quick Recall

Very Pressured Tigers In Mountains Boil Easily.

Vapour Pressure
Temperature (increases VP)
Intermolecular Molecules (stronger IMFs, lower VP)
Boiling Equals (VP equals external pressure)