DAT General Chemistry: Vapor Pressure and Volatility

DAT High-Yield Concept

Dynamic Equilibrium: Established in a closed system when the rate of liquid evaporation equals the rate of vapor condensation ( $\ce{H2O(l) <=> H2O(g)}$ ).
Intermolecular Forces (IMFs) Inverse Relationship: Stronger IMFs result in lower equilibrium vapor pressure and lower volatility. Weaker IMFs produce higher vapor pressure and higher volatility.
Temperature Dependence: Vapor pressure increases exponentially with temperature because a larger fraction of liquid molecules gains kinetic energy exceeding the intermolecular attraction threshold.
Container Independence: Equilibrium vapor pressure depends only on temperature and substance identity—it is completely independent of liquid volume or surface area.

Evaporation, Condensation, and Dynamic Equilibrium

Evaporation (vaporization) is an endothermic process where surface molecules of a liquid overcome attractive intermolecular forces (IMFs) to transition into the gas phase. Because molecules in a liquid possess a Maxwell-Boltzmann distribution of kinetic energies, a fraction of molecules at any temperature above absolute zero has sufficient kinetic energy to escape.

When liquid is placed in a closed container, escaping gas molecules cannot diffuse away. As vapor density grows, gaseous molecules collide with the liquid surface and are recaptured through condensation. Initially, the rate of evaporation exceeds the rate of condensation. Eventually, the rate of condensation rises until it equals the rate of evaporation, establishing a dynamic equilibrium:

\ce{Liquid <=> Vapor}

The microscopic process of evaporation and condensation at the liquid surface. (Source: MikeRun · CC BY-SA 4.0 · Wikimedia Commons)

Key Concept: Dynamic Equilibrium

Dynamic equilibrium does not mean equal amounts of liquid and vapor. It means the rates of evaporation and condensation are identical, yielding a constant vapor partial pressure above the liquid at a given temperature.

Vapor Pressure and Volatility

Vapor pressure is the partial pressure exerted by a vapor in dynamic equilibrium with its liquid phase in a closed system at a specified temperature. Volatility refers to the measurable tendency of a substance to evaporate into the gas phase.

The magnitude of vapor pressure and volatility is governed directly by the strength of intermolecular forces:

Weak IMFs: Compounds held by weak London dispersion forces (e.g., nonpolar alkanes) require minimal energy for molecules to escape. They exhibit high evaporation rates, high equilibrium vapor pressures, and high volatility.
Strong IMFs: Compounds stabilized by hydrogen bonding (e.g., water, $\ce{H2O}$ ) require significant thermal energy to overcome cohesive interactions. They exhibit lower vapor pressures and low volatility.

DAT Mnemonic: W-I-M-P = Weak IMFs = More Pressure (High Vapor Pressure & High Volatility).

Worked Example: Predicting Volatility and Vapor Pressure Trends

Question: Rank the following compounds in order of increasing vapor pressure at $\pu{25 degC}$ : $\ce{CH3CH2OH}$ (ethanol), $\ce{CH3OCH3}$ (dimethyl ether), and $\ce{CH3CH2CH2OH}$ (1-propanol).

Reasoning & Strategy:

Identify predominant IMFs: $\ce{CH3OCH3}$ is polar but lacks an O-H bond, so its strongest IMFs are dipole-dipole interactions. Both $\ce{CH3CH2OH}$ and $\ce{CH3CH2CH2OH}$ undergo hydrogen bonding.
Compare hydrogen-bonding compounds: $\ce{CH3CH2CH2OH}$ has a larger nonpolar alkyl chain than $\ce{CH3CH2OH}$ , increasing London dispersion forces in addition to hydrogen bonding. Thus, total IMFs: $\ce{CH3OCH3}$ < $\ce{CH3CH2OH}$ < $\ce{CH3CH2CH2OH}$ .
Apply inverse rule: Weakest IMFs give highest vapor pressure.

Answer: $\ce{CH3CH2CH2OH}$ < $\ce{CH3CH2OH}$ < $\ce{CH3OCH3}$ .

DAT Common Exam Trap: Confusing rate of evaporation with equilibrium vapor pressure. Increasing liquid surface area speeds up how fast equilibrium is reached, but does not alter the final equilibrium vapor pressure value!

Temperature Dependence of Vapor Pressure

As temperature increases, the average kinetic energy of liquid particles increases. According to the Boltzmann distribution, an exponentially larger fraction of molecules possesses kinetic energy exceeding the escape threshold. Consequently, vapor pressure increases non-linearly with rising temperature for all liquids.

When the vapor pressure of a liquid equals the external atmospheric pressure ( $\pu{1.0 atm}$ or $\pu{760 mmHg}$ ), the liquid reaches its normal boiling point (discussed in depth in the phase equilibrium lesson).

Vapor pressure diagram of Water - Experimental data taken from Dortmund Data Bank (Source: WilfriedC · CC BY 3.0 · Wikimedia Commons)

Comparison Table: Volatile vs. Non-Volatile Substances

Property	Volatile Substance	Non-Volatile Substance
Intermolecular Forces	Weak (e.g., dispersion forces)	Strong (e.g., hydrogen bonding, ionic)
Vapor Pressure at $\pu{25 degC}$	High	Low / Negligible
Rate of Evaporation	Fast	Slow
Examples	Diethyl ether, acetone, pentane	Water, glycerol, molten salts

Critical Thinking Questions

If a rigid closed flask containing liquid water and water vapor in dynamic equilibrium at $\pu{25 degC}$ is compressed to half its gas volume at constant temperature, what happens to the equilibrium vapor pressure once equilibrium is re-established?
Why does diethyl ether evaporate rapidly off human skin creating a cooling sensation, whereas water evaporates much more slowly at the same ambient temperature?
Two sealed vessels at $\pu{25 degC}$ contain pure liquid ethanol: Vessel A holds $\pu{100 mL}$ in a $\pu{1 L}$ flask, while Vessel B holds $\pu{500 mL}$ in a $\pu{5 L}$ flask. Compare the vapor pressures in both vessels at equilibrium.

References

Sources returned by Google Search grounding during lesson verification.

11.5: Vapor Pressure - Chemistry LibreTexts — chem.libretexts.org
Intermolecular forces and vapor pressure - Khan Academy — khanacademy.org

Vapor Pressure and Volatility

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