Lecture 21sf

Lecture 21sf - Liquid Vapor Equilibrium Water is placed in...

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Liquid Vapor Equilibrium Water is placed in a container with a cover making it a closed system. A pressure gauge is attached, and all air is removed with a pump. Will the pressure gauge read anything? Water has a tendency to evaporate—change from liquid to vapor. The molecules of the liquid are moving, with a range of kinetic energies. The more energetic molecules can have enough energy to break their intermolecular forces and change to vapor. In an open system, the vapor molecules diffuse and dissipate into the open space, and, as long as the surroundings maintain a constant temperature, the same fraction of energetic molecules continue to evaporate (break intermolecular forces) and eventually all the water evaporates. But in a closed system, as the molecules of liquid evaporate, the vapor molecules build up pressure in the closed space. As the pressure of the vapor increases, it becomes increasingly likely that the colliding vapor molecules will condense back into the liquid. The rate of evaporation is constant, based on the temperature. The rate of condensation increases as the pressure builds up, until the two rates are equal.
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This is called phase equilibrium. The pressure at this point is called the vapor pressure . Vapor pressure The pressure exerted by a vapor in equilibrium with its liquid. What does the vapor pressure depend upon? Vapor pressure depends on temperature. At a higher temperature, molecules move faster with a higher kinetic energy. In the kinetic energy distribution, at a higher temperature, more molecules have enough energy to break their intermolecular forces, leading to a higher evaporation rate and a higher temperature. Higher temperatures mean higher vapor pressures. Vapor pressure depends on the nature of the molecule (what substance are we dealing with). In liquids having molecules with stronger intermolecular forces, more energy is required to evaporate. At a given temperature, such liquids will have lower vapor pressures. Stronger intermolecular forces means a lower vapor pressure. Does vapor pressure depend on the volume of the container? Let’s look at a specific example. The vapor pressure of H 2 O at 20C is 17.5 Torr. The system shown below is at equilibrium.
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Now suppose we lift the cover up, still keeping a closed system, but with higher volume. What happens to the pressure now?
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At first the pressure would decrease, due to the increased volume. But the system would no longer be at equilibrium. The rate of evaporation would then be greater than the rate of condensation rate. More liquid would evaporate until equilibrium is again established. At equilibrium, the pressure would again be 17.5 Torr (assuming a constant 20C temperature). Now suppose we push the cover down, still keeping a closed system, but with
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This note was uploaded on 02/20/2012 for the course 160 161 taught by Professor Kim during the Fall '08 term at Rutgers.

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Lecture 21sf - Liquid Vapor Equilibrium Water is placed in...

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