& 12.8 Solution Equilibrium and Factors affecting Solubility – Expressing Solution Concentration & Colloids

The Process of Solvation (Dissolving) As sodium chloride is added to water the rate of dissolution is rapid and the ions dissolve in the water. As more salt is added the rate of recrystallization of ions begins to increase until both rates are equal. Here a dynamic equilibrium exists. Now, the solution is said to be saturated. NaCl(aq) ↔ Na + (aq) + Cl - (aq) An unsaturated solution: A solution containing less than the equilibrium amount of solute. A supersatured solution: A solution containing more than the equilibrium amount of solute. Here the solution is unstable, and a disturbance to the system will cause recrystallization to occur.

Factors that Affect Solubility What factor does temperature have on the solubility of most solids? What about gases? As temperature increases, the solubility of most solids will increase as well, and the solubility of gases decrease with increasing temperature.

Let’s Try a Practice Problem! A solution is saturated in both nitrogen gas and potassium bromide at 75 o C. When the solution cooled to room temperature, what is most likely to happen? a.) Some nitrogen gas bubbles out of solution. b.) Some potassium bromide precipitates out of solution. c.) Some nitrogen gas bubbles out of solution and some potassium bromide precipitates out of solution. d.) Nothing happens. b.) Some potassium bromide precipitates out of solution.

Pressures Affect on Gases An increase in pressure, causes an increase in gas solubility, and a new equilibrium to be established. Here, we can use Henry’s Law: S gas = k H P gas (where S gas = solubility of a gas in M, and K H is Henry’s law constant that depends both on a specific solute/solvent relationship (in M/atm), P gas is the partial pressure of a gas (usually in atm))

Let’s Try a Practice Problem! Determine the solubility of oxygen in water at 25 o C exposed to air at 1.0 atm. Assume the partial pressure for oxygen of 0.21 atm. (K H for O 2 in water at 25 o C = 1.3X10 -3 M/atm). S gas = k H P gas S gas = (1.3X10 -3 M/atm)(0.21 atm) = 2.7X10 -4 M

Expressing Solution Concentration A dilute solution – contains a small quantities of solute compared to the quantity of solvent. Concentrated solution – Contains large quantities of solute compared to the quantity of solvent. If you are stranded on an island, and you are thirsty, should you drink the sea water? Why? No. Since sea water is highly concentrated with salt, the salt absorbs the water from the cells that line your digestive tract causing dehydration, and also draws extra water into your intestines causing diarrhea.  No Good!!!

Reporting Solution Concentration Here are ways to report solution concentration: – Molarity (M) = moles of solute / volume of solution (in Liters) (note: this is not per L of solvent!) – Molality (m) = moles of solute / mass of solvent (in kg) – Percent by mass = (mass of solute /mass of solution) X 100 – Percent by volume = (volume of solute / volume of solution) X 100 – For my dilute solutions the following two equations may be used: Parts per million (ppm) = (mass of solute / mass of solution) X 10 6 Parts per billion (ppb) = (mass of solute / mass of solution) X 10 9

Let’s Try a Practice Problem! A solution is prepared by dissolving 50.4 g sucrose (C 12 H 22 O 11 ) in kg of water. The final volume of the solution is 355 mL. Calculate the concentration of the solution in molarity. M = mol solute / L solution M = ((50.5 g C 12 H 22 O 11 ) X (1.00 mol / g)) / L = 4.16X10 -1 M

Colloids Colloid (or colloid dispersion) – a mixture in which a dispersed substance (which is solute-like) is finely divided in a dispersing medium (which is solvent-like). – Examples: fog, smoke, whipped cream, milk and opal. Whether or not a mixture is a colloid is determined by the size of the particles it contains. – If the particles are between 1 nm and 1000 nm, the mixture is a colloid. – Colloid particles scatter light; this is called the Tyndall effect.

& 12.8 pgs #’s 42, 44, 52 (molarity only), 54, and 62 & 64 (a only). Study for Quiz Chapter 12