Evaporation is a process of spontaneous change of a liquid into vapours. It is continuous at all temperatures.
According to kinetic molecular theory, molecules of liquid possess kinetic energy. All the molecules do not have the same K.E. Some molecules have K.E. higher than the average value. When such types of molecules come to the surface of the liquid, these overcome the intermolecular force and thus escape from the surface of the liquid as vapours. This is called evaporation.
In an open container, at constant temperature evaporation is continuous at the same rate until all the liquid is converted into vapours. Evaporation is continuous at all temperatures.
Factors Affecting the Rate of Evaporation
The rate of evaporation of a liquid depends upon the following factors.
1. Strength of Intermolecular Forces
Different liquids evaporate at different rates.
Stronger the intermolecular force. Lower will be the rate of evaporation and vice versa e.g. at the same temperature, rate of evaporation of gasoline (petrol) is more than water. Because petrol has weaker London forces.
Similarly, the rate of evaporation of water is lower than ether. Because in water strong hydrogen bonding is present.
An increase in temperature increases the rate of evaporation and vice versa. It is because the increase in temperature increases the number of molecules having K.E. higher than the average value. Thus more molecules are escaped from the surface of the liquid. Hence, the rate of evaporation increases.
3. Surface Areas
The larger the surface as higher the rate of evaporation and vice versa. It is because more number of molecule escape from a larger surface area. That is why expanded clothes are dried earlier than unexpanded clothes.
Evaporation Causes Cooling
Temperature is the measure of the average kinetic energy (KE) of the particles of a substance. The greater the kinetic energy higher is the temperature.
When particles having high KE escape from the surface of the liquid, the average K.E of the remaining particles decreases. So, the temperature of the liquid decreases. As the evaporation continues, the liquid absorbs more and more from the surroundings, therefore, the temperature of the surrounding also decreases. Hence, evaporation causes a cooling effect.
The pressure exerted by the vapours of a liquid, in equilibrium with the liquid, at a given temperature is called the vapour pressure of a liquid.
When a liquid is placed in a closed container, then, due to evaporation, molecules leave the surface liquid and change into the vapour state. Some of these vapours, on colliding with other vapours, lose a part of their K.E and are returned to the liquid state. This is the condensation process.
Initially, the rate of evaporation is higher than the rate of condensation. After some time, when the space above the liquid becomes saturated with vapours, the evaporation rate becomes equal to the condensation rate and a dynamic equilibrium is established in the system.
Like gas, vapours of liquid also collide with the walls of the container and exert pressure. This pressure at equilibrium is called the vapour pressure of the liquid.
The vapour pressure is Independent of the amount and surface area of the liquid.
The evaporation is faster from a larger surface area. However, returning molecules also have a larger area to return. Thus, condensation is also faster. So, the number of molecules per unit of the surface area remains the same. Thus, vapour pressure is independent of the amount and surface area of the liquid.
Factors affecting vapour pressure of a liquid
The following factors affect the vapour pressure of a liquid.
(1) Intermolecular Forces (ii) Temperature
(1) intermolecular forces
The stronger the intermolecular forces lower will be the vapour pressure of a liquid and vice versa.
At 20°C H₂O with strong intermolecular hydrogen bonding has low vapour pressure (17 torrs) than ether (torr) in which no hydrogen bonding is present.
Isopentane has a high vapour pressure due to weak London force, while glycerol has a low due to strong hydrogen bonding.
An increase in temperature increases the vapour pressure of a liquid and vice versa. It is because at high temperatures, the K.E. of molecules increases, therefore, evaporation increases. Hence, vapour pressure increases.
The vapour pressure of H₂O increases from 4.579tom to 9.209 torrs from 0 ° C to 10 ° C, but it increases from 527.8 torrs to 760 torrs from 90 ° C to 100 ° C.
Measurement of Vapour Pressure of a Liquid
• Method This method is used to determine the accurate vapour pressure of the liquid.
• Pure liquid is placed in the round bottom flask.
• One end of the tube from the flask is connected to a manometer and the other end is connected to a vacuum pump.
• The liquid in the flask is frozen.
• The air above the liquid is removed with a vacuum pump.
• Frozen liquid is re-melted to release the entrapped air . . .
• Liquid is again frozen and the air above it is again removed. This process is repeated till all the air is removed.
• The flask is then kept in a thermostat at a temperature at which the vapour pressure of the liquid is to be determined.
• The vapours of the liquid push the mercury column in the manometer. Thus, the column of mercury on the side of the liquid becomes lower and on the side of the atmosphere becomes higher.
• After sometimes height of mercury in both limbs of the manometer becomes constant.
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