Evaporation, Vapour Pressure and Boiling Point: MDCAT Chemistry notes
Evaporation, Vapour Pressure and Boiling Point MDCAT notes: factors affecting each, manometric measurement, pressure cooker and vacuum distillation.
Evaporation
Evaporation is the escape of high-energy molecules from the surface of a liquid at any temperature below its boiling point. Because the most energetic molecules leave, the average kinetic energy of the remaining liquid falls, so evaporation causes cooling.
The rate of evaporation depends on:
- Surface area: wider surface, faster evaporation. Water in a wide saucepan evaporates sooner than the same water in a glass.
- Temperature: higher temperature, more molecules with enough energy to escape.
- Intermolecular forces: weaker forces, faster evaporation. Acetone evaporates faster than ethanol, water or ethylene glycol.
It does not depend on the amount of liquid.
Vapour pressure
In a closed vessel, evaporation and condensation eventually occur at equal rates. The pressure exerted by the vapour in equilibrium with its liquid at a given temperature is the vapour pressure.
| Factor | Effect on vapour pressure |
|---|---|
| Temperature | Increases with temperature |
| Intermolecular forces (nature, size of molecules) | Stronger forces, lower vapour pressure |
| Surface area | No effect |
| Amount of liquid | No effect |
A larger surface increases both evaporation and condensation equally, so the equilibrium pressure is unchanged. At the same temperature, hydrogen-bonded water has a much lower vapour pressure than diethyl ether, chloroform or carbon tetrachloride.
Volatility order (most volatile first, based on boiling points): acetaldehyde (about 20 °C) > diethyl ether (about 35 °C) > acetone (about 56 °C) > ethanol (78 °C) > water (100 °C) > acetic acid (118 °C) > glycerol (about 290 °C). Glycerol has three OH groups and is the least volatile.
Measuring vapour pressure: manometric method
The liquid is placed in a flask connected to a mercury manometer. The vapour presses on the mercury, so the column facing the vapour is depressed and the other column rises. The vapour pressure is found from the difference between the pressure with the liquid present and the atmospheric pressure.
Boiling point
A liquid boils when its vapour pressure equals the external (atmospheric) pressure. Bubbles can then form throughout the liquid. The normal boiling point is the boiling point at 760 torr (1 atm).
- Lower external pressure, lower boiling point. Water boils below 100 °C at high altitude; ethanol boils below 78 °C if the pressure is reduced.
- Higher external pressure, higher boiling point. To boil water at 110 °C the external pressure must be above 760 torr. In a pressure cooker the pressure is above 1 atm, so water boils above 100 °C and food cooks faster.
- Vacuum distillation: distilling under very reduced pressure lowers the boiling point. It is used for liquids such as glycerol that decompose at their normal boiling point.
The molar heat of vaporization of water is $40.7\ \mathrm{kJ\,mol^{-1}}$ at 100 °C.
The freezing point is the temperature at which the solid and liquid phases have the same vapour pressure.
Common MDCAT traps
- Surface area affects the rate of evaporation but not the vapour pressure.
- Amount of liquid affects neither the rate of evaporation nor the vapour pressure.
- Boiling point is where vapour pressure equals external pressure, not where surface tension or viscosity equals anything.
- Halving the external pressure lowers the boiling point but does not halve it.
- Check units: heat of vaporization of water is 40.7 kJ/mol, not J/mol or kcal/mol.
Quick revision
- Evaporation is a surface phenomenon and causes cooling.
- Vapour pressure depends only on temperature and the nature of the liquid.
- Normal boiling point is measured at 760 torr.
- Pressure cooker raises the boiling point; vacuum lowers it.
- Glycerol is the least volatile of common lab liquids.