what is the relationship between intermolecular forces, vapor pressure and boiling or evaporation. stronger the intermolecular force the lower the vapor pressure. where lv is the enthalpy of vaporization (often called the latent heat of vaporization, that the saturation vapor pressure always increases with temperature (i.e., des/dT > 0). .. You may complete this practice quiz as many times as you want. To understand that the relationship between pressure, enthalpy of vaporization, and temperature is given by the Clausius-Clapeyron equation.
The simplest case is when all the water is vapor, which occurs when the water vapor pressure is low enough and the temperature and thus saturation vapor pressure is high enough that all the water in the system is evaporated and in the vapor phase.
Suppose we have a cylinder closed on one end and a sealed piston in the other that is in a temperature bath so that we can hold the cylinder and its contents at a fixed temperature i. Initially we fill the cylinder with liquid water and have a small volume of pure water vapor at the top. If we set the bath temperature to, say, K and let the system sit for a while, the vapor will become saturated, which is on the es line.
Vaporization and Vapor Pressure - Chemistry LibreTexts
For isothermal compression, in which energy is removed from the system by the bath in order to keep the temperature constant, a push on the piston will slightly raise the vapor pressure above es and there will be net condensation until equilibrium is obtained again. If we continue to push the piston and the bath keeps the temperature constant, then the water pressure will increase. In the atmosphere, ice or liquid almost always has a surface that is exposed to the atmosphere and thus there is the possibility that water can sublimate or evaporate into this large volume.
Note that the presence or absence of dry air has little effect on the condensation and evaporation of water, so it is not the presence of air that is important, but instead, it is the large volume for water vapor that is important. Conditions can exist in the atmosphere for which the water pressure and temperature are in the liquid or sometimes solid part of the phase diagram.
But these conditions are unstable and there will be condensation or deposition until the condensation and evaporation or sublimation and deposition come into equilibrium, just as in the case of the piston above.
Thus, more water will go into the liquid or ice phase so that the water vapor pressure drops down to the saturation value. When the water pressure increases at a given temperature to put the system into the liquid region of the water phase diagram, the water vapor is said to be supersaturated.
This condition will not last long, but it is essential in cloud formation, as we will see in the lesson on cloud physics. Note also that the equilibrium line for ice and vapor lies below the equilibrium line for supercooled liquid and vapor for every temperature.
This small difference between esi and es can be very important in clouds, as we will also see in the lesson on cloud physics.
In this section, we describe vapor pressure in more detail and explain how to quantitatively determine the vapor pressure of a liquid. As for gases, increasing the temperature increases both the average kinetic energy of the particles in a liquid and the range of kinetic energy of the individual molecules.
The fraction of molecules with a kinetic energy greater than this minimum value increases with increasing temperature. Just as with gases, increasing the temperature shifts the peak to a higher energy and broadens the curve. Some molecules at the surface, however, will have sufficient kinetic energy to escape from the liquid and form a vapor, thus increasing the pressure inside the container.
As the number of molecules in the vapor phase increases, the number of collisions between vapor-phase molecules and the surface will also increase. Eventually, a steady state will be reached in which exactly as many molecules per unit time leave the surface of the liquid vaporize as collide with it condense.
Heat of Vaporization - Definition, Equation and Quiz | Biology Dictionary
At this point, the pressure over the liquid stops increasing and remains constant at a particular value that is characteristic of the liquid at a given temperature.
The rate of evaporation depends only on the surface area of the liquid and is essentially constant. The rate of condensation depends on the number of molecules in the vapor phase and increases steadily until it equals the rate of evaporation.
Equilibrium Vapor Pressure Two opposing processes such as evaporation and condensation that occur at the same rate and thus produce no net change in a system, constitute a dynamic equilibrium. In the case of a liquid enclosed in a chamber, the molecules continuously evaporate and condense, but the amounts of liquid and vapor do not change with time.
3.3 Phase Diagram for Water Vapor: Clausius Clapeyron Equation
The pressure exerted by a vapor in dynamic equilibrium with a liquid is the equilibrium vapor pressure of the liquid. If a liquid is in an open container, however, most of the molecules that escape into the vapor phase will not collide with the surface of the liquid and return to the liquid phase. Instead, they will diffuse through the gas phase away from the container, and an equilibrium will never be established. Volatile liquids have relatively high vapor pressures and tend to evaporate readily; nonvolatile liquids have low vapor pressures and evaporate more slowly.
Thus diethyl ether ethyl etheracetone, and gasoline are volatile, but mercury, ethylene glycol, and motor oil are nonvolatile.