12.4 - Solution Equilibrium and Factors Affecting Solubility

Solution Equilibrium Stages in the Equilibrium process of a Solution: Initial - The solute begins to dissolve in the solvent Dissolving - The Solution becomes more concentrated, and the solute may recrystallize (Rate of dissolution> Rate of recrystallization) Dynamic Equilibrium - The Solution is highly concentrated and the Rate of recrystallization = Rate of dissolution

Saturation Solutions in Dynamic Equilibrium are Saturated, any more added solute will not dissolve. Solutions with less than the Equilibrium amount of solute are Unsaturated, and added solute will continue to dissolve. Solutions containing more that the Equilibrium amount of solute are Supersaturated. These solutions are unstable and will rapidly recrystallize if disturbed. Solutions like this can only be attained by manipulating solubility.

Temperature Dependence of the Solubility of Solids In general, the solubility of solids in water increases with temperature. Supersaturated solutions are made by heating water and adding more solute than could be dissolved at room temperature, then letting it cool. This technique is used for purification because the resulting crystal rejects impurities.

Factors affecting the Solubility of Gases in Water The solubility of gases in liquids is affected by both temperature and pressure. As temperature increases solubility decreases. This is why small air bubbles form when heating water. The gases become less soluble and come out of solution.

Factors affecting the Solubility of Gases in Water The solubility of gases in liquids increases with pressure. This is why soda fizzes when the container is opened. The pressurized carbon dioxide escapes, lowering the solubility of the carbon dioxide dissolved in the soda and letting it escape solution.

Henry’s Law The above image depicts the increase of pressure on a system in equilibrium. The increase in pressure causes an increased rate of dissolution of gas molecules, causing an increase of gas molecules in solution until a new equilibrium is formed.

Henry’s Law We can use Henry’s Law to determine the solubility of gases with an increase of pressure Henry’s Law: Sgas=kHPgas Where: Sgas is the solubility of the gas (in M) Pgas is the partial pressure of the gas (in atm) kH is the Henry’s Law constant of the gas which depends on the solute, solvent, and temperature (in M/atm)

Henry’s Law Practice Problem! What pressure of carbon dioxide is required to keep the carbon dioxide concentration in a bottle of soda at 0.12 M at 25℃? Answer = 3.5 atm

12.5 - Expressing Solution Concentration

Concentration Concentration is a measure of how much solute is present in a solution relative to the amount of solvent. A Dilute solution has a relatively small amount of solute A Concentrated solution has a relatively large amount of solute Measurements include molarity, molality, parts by mass, parts by volume, mole fraction, and mole percent

Measurements

Uses The different measurements of concentration are useful for different purposes. Molarity for making and diluting solutions Molality for comparing concentrations at different temperatures Parts by mass for comparing by weight Parts by volume for comparing liquids Mole fraction and Mole percent for largely varying ratios

Practice Problems A solution is prepared by dissolving 17.2 g of ethylene glycol (C2H6O2) in 0.500 kg of water. The final volume of the solution is 515 ml. Calculate your answer in Molarity Answer = 0.538 M 0.005 moles of salt (NaCl) are added to a 20 L barrel of water (H2O). Assuming the density of water is 1g/mL, what is the concentration of the solution in ppm by mass? Answer = 14.61 ppm