Liquid-Vapour Equilibrium
Text 29 A Liquid-Vapour Equilibrium - раздел Образование, Part One THE PREDICATE A Liquid Of Relatively Low Boiling Temperature, When Placed In A Contain...
A liquid of relatively low boiling temperature, when placed in a container open to the atmosphere, will eventually evaporate entirely. Remembering that molcculcs in the liquid arc "bound" by attractive forccs to their neighbours, wc might ask why some arc able to overcome these forccs and leave die liquid spontaneously. The answer lies in a consideration of the possible magnitudes of molccular kinetic energies, for these, as wc have already mentioned, range from very low to very high values, and arc distributed according to the Maxwcll-Boltzmann law. Therefore, even if the average potential energy which binds the molcculcs to the liquid is substantial, there arc always some molcculcs which have enough kinetic energy to overcome the binding forces and enter the vapour. According to the Maxwcll-Boltzmann law, the fraction of the molcculcs which have kinetic energies greater than some minimum value the value required for the molcculcs to leave the liquid, is proportional to the Boltzmann factor, е*кТ. Therefore, the temperature remaining constant, the fraction of liquid molecules with enough kinetic energy to evaporate remains the same, and evaporation continues. The vessel being open to the atmosphere, vapour molcculcs arc swept away, and evaporation continues until no liquid is left.
Now, let us analyse what happens when a liquid is placed in a closed evacuated container. Immediately, the liquid starts to evaporate at a rate which is primarily determined by the fraction of molcculcs which have enough kinetic energy to overcome attractive forces and leave the surfacc. Initially, the rate of condensation is zero, there being no molcculcs in the vapour. As long as the temperature stays constant, evaporation continues at a constant rate, and the number of molcculcs in the vapour phase increases. Consequently, the rate of condensation starts to increase, for as the pressure of the vapour grows, the number of gas molecules which collide with and reenter the liquid surfacc also increases.
The time dependence of the evaporation and condensation rates is worth considering. While growing, the condensation rate eventually becomes equal to the rate of evaporation. At this time, the number of molcculcs which enter and which leave the vapour per unit time is the same, and, consequently, the pressure of the vapour stops increasing and remains constant. If the system is left undisturbed at a fixed temperature, evaporation and condensation continue at equal rates, and the pressure of the vapour remains unchanged. This, then, is a situation of equilibrium between the two phases. Note particularly that at equilibrium, evaporation and condensation do not stop, but that the constancy of the equilibrium vapour pressure is a consequence of these opposing processes proceeding at equal rates. Thus wc say that phase equilibrium is dynamic in nature.
