12.4 - Solution Equilibrium and Factors Affecting Solubility

Solution EquilibriumStages 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

SaturationSolutions 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 SolidsIn 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 purificationbecause the resulting crystal rejectsimpurities.

Factors affecting the Solubility of Gases in WaterThe 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 WaterThe 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 LawWe 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 LawPractice 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

ConcentrationConcentration 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 soluteA Concentrated solution has a relatively large amount of

soluteMeasurements include molarity, molality, parts by mass,

parts by volume, mole fraction, and mole percent

Measurements

UsesThe different measurements of concentration are useful for different purposes. Molarity for making and diluting solutionsMolality for comparing concentrations at different

temperaturesParts by mass for comparing by weight Parts by volume for comparing liquidsMole fraction and Mole percent for largely varying ratios

Practice ProblemsA 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 MolarityAnswer = 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