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Vapour pressure: Definition, measurement, effects and BP relationship

1.0 Definition

The pressure with which vapours come out of a liquid is called vapour pressure.

2.0 Explanation

Another way of getting insights about vapour pressure is that place a jar of water under a bell jar fitted with a pressure gauge. Space under the bell jar is evacuated. The water immediately starts to evaporate and pressure of the water molecules in gas phase is indicated by the gauge. As time passes, the pressure in the bell jar rises. After some time the pressure stops rising. At this stage space above the water jar is saturated with water vapours. The number of water molecules in vapours phases is constant. Kinetically it means that the number of molecules escaping (evaporation) into the empty space becomes equal to the number of molecules coming back into the liquid (condensation). The system is said to have attained equilibrium state. Since there is no net increase in number of molecules in the gas phase after this time, the pressure no longer increases at that temperature. This constant pressure exerted by the vapours in equilibrium with its liquid is called equilibrium vapour pressure or simply the vapour pressure. The vapour pressure is the pressure exerted by vapours in equilibrium with the pure liquid at a given temperature.

3.0 Calculation/measurement of vapour pressure

Accurate measurement of vapour pressure of a liquid can be made with a manometer connected to a flask which in turn is connected to vacuum pump. The liquid whose vapour pressure is to be determined is placed in the flak. The liquid in the flask is frozen and air in the space above the liquid surface is removed by the vacuum pump. The frozen liquid is melted. The flask is pleased in a thermostat set at a given temperature. Vapours of the liquid reach the manometer. The vapours push down the Hg in one limb of the manometer and rises up into the other limb. Actually the pressure of the liquid in the flask is equal to the sun of the atmospheric pressure and the vapour pressure of liquid and it is given by the following equation

                                    P = Pa + h

Where P = vapour pressure of the liquid at 1 atm pressure

Pa = atmospheric pressure

h = difference in the heights of the mercury levels in the two limbs of the manometer.

FACTORS AFFECTING VAPOUR PRESSURE

  • Nature of the liquid

Vapour pressure of a liquid depends on the nature of liquid. Liquids having low boiling points exert more vapour pressure than higher boiling liquids at a given temperature e.g. benzene (B.P. 80 oC and ethyl ether (B.P. = 34 6 oC) have high vapour pressure than water (B.P. = 100 oC)

  • Intermolecular attraction          

In liquids where intermolecular attractions are strong, only a few molecules of high kinetic energies could escape and thus evaporation occurs with a slower rate. On the other hand if the intermolecular attractions are weak, a large number of molecules will have sufficient kinetic energy to leave the surface of liquid and the rate of evaporation will be high. It means that liquids having strong intermolecular attractions release few liquid molecules in the form of vapours to come to equilibrium which create low vapour pressure. While liquids having weak intermolecular attractions, release more number of liquid molecules in the form of vapours to come to equilibrium which exert high vapour pressures.

Water has the lower vapour pressure than diethyl ether. It means that the attractive faces are greater in water compared to ether. This is so because in water there are strong hydrogen bonds and in ether which is non polar have relatively weak London forces.

  • Temperature

The vapour pressure of a liquid increases with increase of temperature. As the temperature of a liquid is increased more of its molecules have enough kinetic energy to escape from the attractive forces of their neighbors thus rate of evapouration of liquid increases. As the temperature increase, the fraction of the molecules which can escape form the liquid surface increases so that at equilibrium, the pressures of gas phase is higher.

Vapour pressure of water at different temperature is given here in the table.

Temperature

Vapour Pressure

Temperature Vapour pressure
0oC 4.6mm 80oC 355 mm
25oC 23.7mm 100oC 760 mm
50oC 93.9mm 120oC 1489mm
  • Volume

Vapour pressure of a liquid is independent of the volume of the container. It does not depend on the amount of the liquid and surface area.

4.0 Boiling Point and its relationship with external pressure and vapour pressure

When a liquid is heated, its vapour pressure increases with rise in temperature. This is possible because more and more molecules escape into the space. The vapour pressure continues to increase unit it becomes equal to atmospheric pressures or to any other pressure to which the liquid is subjected. Al this temperature the liquid begins to boil. At this point the vapour pressure of a liquid becomes equal to the atmospheric pressure or to any other pressure to which the liquid to subjected.

When a liquid is heated, the kinetic energy of its molecules increase and the temperature also increases. At the boiling point, the kinetic energy of the molecules becomes maximum and any further heating at this stage will not increase the temperature rather this heat will only be utilized to break the intermolecular forces and convert the liquid into its vapour. The amount of heat required to vaporize one mole of a liquid at its boiling point is called its molar heat of vaporization. Molar heat of vaporization of water is 40.6 KJ mole.

A liquid cannot exist at equilibrium if its vapour pressure exceeds external pressure and this causes the liquid to evapourate rapidly as manifested by the formation of bubbles.

            V.P of liquid =Atm pressure         

Boiling point depends upon vapours of the liquid at atmospheric pressure or any other pressure to which the liquid is subjected.

The following figure shows the variation of vapour pressure of water, ethyl alcohol and diethyl ether with temperature. It shows that the liquid reach up to their boiling points when their vapour pressures are equal to 760 torr at sea level. The start of these curves at O oC is interesting. Water takes start at 4.8 torr while diethyl ether at around 200 torr due to difference in the strengths of their intermolecular forces. The curve for water goes along with temperature axis to a greater extent at the beginning as compared to others. It means that water can hurriedly overcome its intermolecular forces at low temperature. It is clear from the curves that the vapour pressure increases very rapidly when the liquids are closer to their boiling points.

boiling point VS vapour pressure

5.0 Applications of vapour Pressure

Variation of B.P with pressure finds important application in pressure cookers              

A pressure cooker takes advantage of the fact that the boiling point changes with external pressure since pressure inside the p-cooker is higher than 1 atmosphere so water boils at a temperature above 100 oC, so food cooks faster.

 Vacuum distillation 

The boiling point of a liquid depends upon the pressure exerted on the surface of the liquid. It gets lowered by decreasing the pressure over the surface of a liquid. The distillation of a liquid at reduced pressure is called vacuum distillation.

application of vapour pressure in vacuum distillation

The liquid with high boiling points can be easily distilled by their process at relatively lower temperature. The vacuum distillation is employed to distill those liquids which can decompose at their boiling point.

Study More

Evaporation for kids 

Evaporation General 

About Syed Noman ud din

Syed Noman ud din is an Electrical Engineer and working in Industry from last 3 years. He writes technical articles for electrical and electronic engineers. He has also published several research publications in renowned international journals.

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