nvolatile solutes do not have an appreciable vapor pressure of their own, and they decrease the vapor pressure of a solvent (over a solution) when added to a solvent. This can be understood by the dynamics depicted in figure 13.5.2. In part (a) you have a pure volatile substance (solvent) and the vapor pressure (Po) is the equilibrium pressure of the solvent when the rate of evaporation equals the rate of condensation (review 11.6, Vapor Pressure as Equilibrium Pressure). Note there are 5 red lines representing the evaporating molecules and 5 black lines representing the condensing molecules (so the rate of condensation equals evaporation and the number of vapor molecules is constant). A non-volatile solute is introduced (b), and when a solute molecule is near the surface it can't escape. This effectively reduces the surface area for evaporation, and so fewer molecules transfer to the vapor phase, but those condensing have no such reduction in surface area (a vaporized solvent molecule can lose energy and condense if it his a surface solute or solvent molecule). So in (b) there are 6 black arrows entering the liquid, but only 4 red arrows leaving. The system is no longer at equilibrium and more solute condense than evaporate, reducing the vapor pressure until the rate of evaporation equals condensation and a new equilibrium has been reached (c). The result is a reduction in t

The relationship between the solubility of a gas and its pressure is a linear one, and can be described by Henry's law.

As you increase the pressure of a gas, the collision frequency increases and thus the solubility goes up, as you decrease the pressure, the solubility goes down..

When a gas phase molecule hits the surface of a liquid it may be deflected back into the gas or dissolved into the solution, in the latter case becoming a solute particle. If a dissolved molecule reaches the surface of the liquid, a fraction will have enough kinetic energy to escape, and so particles are being exchanged across the liquid/gas boundary all the time. When the rate at which the gas phase particles enter and leave are equal you have a dynamic equilibrium, where the concentration in each phase becomes a constant value. The solubility is a measure of the concentration of the dissolved gas particles in the liquid and is a function of the gas pressure. As you increase the pressure of a gas, the collision frequency increases and thus the solubility goes up, as you decrease the pressure, the solubility goes down..

Step 1: Separate particles of the solute from each other Step 2: Separate particles of the solvent from each other Step 3: Combine separated solute and solvent particles to make solution

Grams Solute per 100 g H2O (S)

For example, we used the concept of mole fraction to describe the partial pressure of a gas, where the partial pressure was the mole fraction times the total pressure, but that told you nothing about how close the molecules were to each other

saturated solution it does not dissolve, typically falling to the bottom as a precipitate

Phase diagram of

Figure 12.7

Practice

Intermolecular Force" is a misnomer, even though it is commonly used, as these are the forces between ions with molecules possessing a dipole moment, and ions do not have to be molecular

Figure 11.2.1