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i am studying liquid vapour equilibrium and there is the set up (below) for the experiment.The conditions:temperature is constant throughout. in this case is the rate of evaporation of the liquid constant throughout the process till equilibrium is reached...or is the rate of evaporation faster in the beginning and becomes slow later as equilibrium is reached?and the rate of evaporation will be constant at eqilibrium?

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  • $\begingroup$ Not quite clear what you are asking. In equilibrium, the net rate of evaporation is zero (unless there is also an absorber or some other sink somewhere, which would lead to steady-state transport). $\endgroup$ – Pieter Apr 5 '18 at 7:03
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There are always molecules leaving the liquid water surface and going into the vapour phase - evaporation. The rate at which molecules in the liquid phase leave the surface depends on a number of factors including the surface area of the liquid/vapour interface and the temperature.
At the same time water molecules in the vapour phase are going into the liquid phase - condensation. The rate at which molecules in the vapour phase enter the liquid depends on a number of factors including the surface area of the liquid/vapour interface and the number density of the molecules in the vapour phase which in turn is related to the vapour pressure.

In the first diagram there are no molecules in the vapour phase so the net rate of transfer of molecules between the vapour and liquid phase is equal to the rate at which molecules are leaving the liquid phase.

As the number density of the molecules in the vapour phase increases the net rate of molecules leaving the liquid decreases as there are now some molecules coming back into the liquid although the rate at which the molecules are leaving the liquid is still the same and this is your second diagram.

The number density of vapour molecules increases until there is no net migration of liquid molecules into the vapour phase as the rate at which molecules leave the liquid is equal to the rate at which molecules enter the liquid.
There is a dynamic equilibrium and the number density of the vapour molecules stays constant and the pressure exerted by the vapour is called the saturated vapour pressure.

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The rate of evaporation is faster in the beginning and the rate gradually decreases till it reaches equilibrium.

For evaporation in a closed vessel , at first the pressure inside the vessel increases .As the molecules in the gas phase increases , the molecules of the substance in the gas phase may strike the liquid and maybe captured by the liquid .This is condensation .Initially the rate of condensation is low , but as the number of particles in the gas phase increases , the rate of condensation increases and the rate of evaporation decreases till at equilibrium , the number of molecules entering the gas phase at any time will be equal to the number of molecules returning to the liquid phase . Thus at equilibrium ,

         Rate of evaporation =Rate of condensation.

The pressure exerted by the gas molecules in equilibrium with the liquid at a particular temperature is called the vapor pressure of the liquid.

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