Vapour Pressure Definition:
“The pressure exerted by vapours of a liquid at a given temperature when equilibrium is established between evaporation and condensation is called vapour pressure”.
Consider a liquid in a closed container. In the beginning rate of evaporation will be very fast there is no condensation. But with the passage of time evaporation rate becomes slow and slow while the condensation rate becomes fast and fast and ultimately the two rates become equal i.e. equilibrium will be established and the number of vapours becomes constant. The pressure exerted by vapours at equilibrium is called vapour pressure.
Factors Affecting Vapour Pressure:
Intermolecular forces:
Vapour pressure depends upon the nature of the liquid. Different liquids have different intermolecular forces. The liquid having stronger intermolecular forces shows low vapour pressures and vice versa e.g. water has lower vapour pressure than ether and spirit at the same temperature. Water has a strong hydrogen bonding spirit also has hydrogen bonding but is weaker than water while ether has London forces. So intermolecular forces in water are stronger and therefore its vapour pressure is lower than the other two.
Vapour Pressure ∝ 1/ IMF
| Vapour Pressure→ Compound ↴ | Vapour Pressures at | ||||||
| 0oC | 20oC | 40oC | 60oC | 80oC | 100oC | 120oC | |
| Diethyl ether (B.P=34.7oC) | 185 | 442 | 920 | 1730 | 3000 | 4865 | 7495 |
| Chloroform (B.P=61.6 oC) | – | 170 | 362 | 734 | – | – | – |
| Carbon Tatrachloride (B.P=76.8 oC) | 32.9 | 87 | 224 | 448 | 843 | 1463 | |
| Ethyl alcohol (B.P=78. 5 oC) | 12 | 43 | 132 | 347 | 814 | 1780 | 3535 |
| Water (B.P=100 oC) | 5 | 18 | 55 | 149 | 355 | 760 | 1489 |
| Glycerin (B.P=290 oC) | – | 0.0016 | – | – | – | – | – |
| Mercury (B.P=356. 58 oC) | 0.00018 | 0.0012 | 0.0061 | 0.0252 | 0.0888 | 0.2729 | 0.7457 |
The order of strength of intermolecular is:
Mercury > Glycerin > Water > Ethyl alcohol > Carbon tetrachloride > Chloroform > Diethyl ether
Temperature:
The higher the temperature more will be the more kinetic energy of molecules and more will be the more vapours produced. As a result, high will be vapour pressure. The reverse is true for lower temperatures.
Vapour Pressure ∝ Temperature
Impurities:
If impurities are present in a liquid, less vapour will be formed and thus low will vapour pressure.
Measurement of Vapour Pressure:
There are two methods of measuring vapour pressure.
Barometric Method:
This is a convenient method of determining vapour pressure. The height of the mercury column is noted. Then 1-2 drops of test liquid are introduced into the barometric tube through its lower end with help of a dropper. The liquid rise above the surface of mercury. A part of the liquid vaporizes. Due to pressure exerted by vapours height of the mercury column falls and when becomes constant it is noted. The difference in height (∆h) gives vapour pressure.
Although this method is very convenient but is not much accurate.
Manometric Method:
This method is comparatively accurate. The test liquid is taken in a round bottom flask. A T-shaped tube is attached to it whose one end is connected to a vacuum pump and another end is connected to a manometer. The liquid is boiled to remove dissolved air in it. The liquid is then frozen and the air on it is removed by a vacuum pump.
The liquid is again boiled to remove more air and is again frozen, the air on it will be removed by a vacuum pump. This procedure is repeated again and again to remove maximum air dissolved in liquid. Now the flask is placed in a thermostat and desired temperature is set. The vapours of liquid are allowed to enter the manometer by opening the stopper. Mercury column in manometer facing vapours is depressed while that faces atmospheric pressure, rises.
A difference in heights of mercury columns in two limbs is noted from which then vapour pressure is determined.
Pvap = Patm + ∆h
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Frequently Asked Questions – FAQ
What is meant by vapour pressure?
Vapour pressure is the force a vapour produces when it is in equilibrium with a substance’s liquid, solid, or both forms.
What is the vapor pressure of water at 25 C?
Water has a vapour pressure of 0.0313 atm, or 23.8 mm of mercury, at ambient temperature (25 °C; 760 mm Hg = 1 atm).
What is the unit of vapor pressure?
The torr is the most often used measurement of vapour pressure. 1 torr equals 1 mm Hg (one millimetre of mercury). The majority of substances have very low vapour pressures. For instance, at ambient temperature (22 °C or 72 °F), water has a vapour pressure of roughly 20 torr.
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