Slide presentation
Vapour Pressure
General Chemistry • Liquids and Solids
Liquids and solids
Vapour pressure connects particle motion to boiling behavior
Vapour pressure is the pressure produced by particles that escape from a liquid into the gas phase when evaporation and condensation reach dynamic equilibrium.
Learning target
- Explain evaporation and condensation at the particle level.
- Predict how temperature and intermolecular forces affect vapour pressure.
- Compare volatility and connect vapour pressure to boiling point.
- Use the Clausius-Clapeyron relationship as a temperature-pressure model.
Why it matters
Vapour pressure explains everyday observations and laboratory techniques
A liquid’s vapour pressure tells us how easily its particles enter the gas phase. That single idea helps explain boiling, distillation, smell, evaporation rates, and why some liquids must be stored in tightly sealed containers.
Boiling begins when pressure matches outside pressure
A liquid boils when its vapour pressure equals the external pressure. Lower external pressure means a lower boiling temperature.
Different liquids separate because they vaporize differently
A more volatile liquid usually has a higher vapour pressure and tends to enter the gas phase more readily.
Sealed systems matter
In a closed container, escaping particles build pressure. In an open container, many vapour particles leave the system.
Microscopic cause
Particles with enough kinetic energy overcome attractions and escape the liquid surface.
Macroscopic result
The gas above the liquid exerts pressure on the container walls.
Core concept
Vapour pressure is an equilibrium pressure
In a closed container, evaporation does not continue forever. As more vapour particles collect above the liquid, some collide with the surface and condense. At equilibrium, the rate of evaporation equals the rate of condensation.
What “dynamic” means
Particles are still moving. Some continue to evaporate, and some continue to condense. The system looks stable macroscopically because the two rates are equal.
At a fixed temperature, this balance produces a characteristic vapour pressure for that liquid.
Vocabulary and variables
The important words describe particles, pressure, and escaping tendency
A strong vapour-pressure explanation uses both molecular language and measurable quantities.
| Term or variable | Meaning | Common units or comparison | Particle-level interpretation |
|---|---|---|---|
| Vapour pressure, \(P_{\text{vap}}\) | Pressure exerted by vapour above a liquid at equilibrium. | atm, kPa, mmHg, torr | More gas particles above the liquid means more collisions with the container walls. |
| Temperature, \(T\) | Measure of average kinetic energy. | K for gas-law and Clausius-Clapeyron calculations | Higher temperature increases the fraction of particles energetic enough to escape. |
| Volatility | Tendency of a substance to vaporize. | Compared qualitatively as low or high | High volatility usually means weaker attractions and higher vapour pressure. |
| \(\Delta H_{\text{vap}}\) | Enthalpy required to vaporize one mole of liquid. | kJ/mol | Larger values mean stronger attractions must be overcome. |
Key habit
For temperature equations, convert Celsius to Kelvin before calculating: \(T(\text{K}) = T(^{\circ}\text{C}) + 273.15\).
Main relationships
Temperature raises vapour pressure, while stronger attractions lower it
Vapour pressure depends mainly on how much kinetic energy particles have and how strongly they attract one another.
Clausius-Clapeyron connection
For many introductory problems, the temperature dependence of vapour pressure is modeled by:
Here \(P_1\) and \(P_2\) are vapour pressures at Kelvin temperatures \(T_1\) and \(T_2\), and \(R = 8.314\ \text{J mol}^{-1}\text{K}^{-1}\).
More particles can escape
The high-energy tail of the particle energy distribution becomes larger.
Fewer particles can escape
Stronger intermolecular forces hold particles in the liquid more effectively.
\(P_{\text{vap}} = P_{\text{external}}\)
Normal boiling point occurs when \(P_{\text{vap}} = 1\ \text{atm}\).
Interactive simulation
Change temperature and attraction strength to model vapour pressure
Move the sliders. Higher temperature increases escape, while stronger intermolecular forces reduce escape. The index below is a teaching model, not an exact experimental pressure.
At moderate temperature and moderate attractions, evaporation and condensation can balance with a moderate vapour pressure.
Graph reasoning
Vapour pressure increases nonlinearly with temperature
The same temperature change has a larger pressure effect at higher temperatures. Liquids with weaker attractions sit higher on the vapour-pressure curve.
Highlight a liquid type
Medium-attraction liquids have moderate vapour pressure. Their boiling point is higher than a weak-attraction liquid but lower than a strong-attraction liquid.
When \(P_{\text{vap}}\) reaches the external pressure, bubbles can form throughout the liquid and boiling occurs.
Worked example
Estimate vapour pressure at a new temperature
A liquid has \(P_1 = 44.6\ \text{mmHg}\) at \(20.0^{\circ}\text{C}\). Its \(\Delta H_{\text{vap}}\) is \(38.6\ \text{kJ/mol}\). Estimate \(P_2\) at \(35.0^{\circ}\text{C}\).
-
Convert temperatures to Kelvin.
\(T_1 = 293.15\ \text{K}\) and \(T_2 = 308.15\ \text{K}\).
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Use joules for \(\Delta H_{\text{vap}}\).
\(38.6\ \text{kJ/mol} = 38600\ \text{J/mol}\), so the units match \(R = 8.314\ \text{J mol}^{-1}\text{K}^{-1}\).
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Substitute into Clausius-Clapeyron.
\(\ln(P_2/44.6) = -(38600/8.314)(1/308.15 - 1/293.15)\).
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Solve for the pressure ratio.
\(\ln(P_2/44.6) \approx 0.771\), so \(P_2/44.6 \approx e^{0.771} \approx 2.16\).
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Calculate the final pressure.
\(P_2 \approx 44.6 \times 2.16 = 96.3\ \text{mmHg}\).
Reasonableness check
The temperature increased, so the vapour pressure should increase. The estimated value is higher than \(44.6\ \text{mmHg}\), so the direction makes sense.
Common misconception
High vapour pressure does not mean strong intermolecular forces
A common mistake is to think that high pressure above a liquid means the liquid particles strongly attract one another. For vapour pressure, the relationship is usually the opposite.
Mistake
“This liquid has high vapour pressure, so its particles must be strongly attracted.”
Correction
High vapour pressure usually means particles escape easily, so the intermolecular attractions are relatively weak.
Practice check
Predict vapour pressure from particle-level evidence
Two liquids are compared at the same temperature in identical closed containers.
Liquid A
Particles have relatively weak intermolecular attractions. Many particles are observed in the gas phase at equilibrium.
Liquid B
Particles have strong intermolecular attractions. Only a few particles are observed in the gas phase at equilibrium.
Question
Which liquid has the higher vapour pressure, and which liquid is expected to have the higher normal boiling point?
Show answer
Liquid A has the higher vapour pressure because more particles escape into the gas phase at the same temperature. Liquid B is expected to have the higher normal boiling point because stronger attractions require a higher temperature before \(P_{\text{vap}}\) reaches \(1\ \text{atm}\).
Apply the topic
Use vapour pressure reasoning in calculations and conceptual questions
A good solution connects three levels: particle attractions, energy distribution, and the measured pressure above the liquid.
Use the calculator to connect pressure, temperature, and the Clausius-Clapeyron relationship.
Practice questions Vapour Pressure QuestionsCheck whether you can explain trends, equilibrium, volatility, and boiling behavior.
How to apply this topic in future problems
First identify whether the problem is asking about temperature, intermolecular forces, volatility, boiling point, or a numerical pressure change. Then choose particle reasoning, graph reasoning, or the Clausius-Clapeyron equation.
Final summary
The essential takeaways
It forms in a closed container when evaporation and condensation occur at equal rates.
Higher temperature gives more particles enough energy to escape from the liquid surface.
Particles are held more tightly, so fewer enter the gas phase at the same temperature.
A liquid boils when \(P_{\text{vap}}\) equals the external pressure.
More volatile liquids usually have higher vapour pressure and lower boiling points.
The Clausius-Clapeyron relationship requires absolute temperature values.