Solutions Ch. 15 (p. 501-529)

Solutions Sugar in water Air Dental fillings Saline Solution: A homogeneous mixture of two or more components in a single physical state Sugar in water Air Dental fillings Saline

Characteristics of a Solution The solute can’t be filtered out. The solute always stays mixed. Particles are always in motion. Volumes may not be additive. A solution will have different properties than the solvent. A solution consists of two components: solvent - component in the greater extent solute - component in the lesser extent (There may be more than one.)

Physical states of solutions Solutions can be made that exist in any of the three states. (See Fig. 15-3, p. 503) Solid solutions dental fillings, 14K gold, sterling silver Liquid solutions saline, liquor, antifreeze, vinegar Gas solutions air, anesthesia gases

Solution Definitions Alloy: solution of two or more metals Soluble: able to dissolve in another substance Insoluble: not able to dissolve in another substance Miscible: pairs of liquids that mix in any amount (Ex. ethanol and water) Immiscible: pairs of liquids that do not mix in any amount (Ex. oil and water) Aqueous: solutions with water as the solvent

Amount of Solute in Solution Concentration: the amount of solute per quantity of solvent or solution There are many systems - we will cover four. Molarity (M) Mass percent Mole fraction (X) Molality (m)

Example: Molarity MNaOH = 10 mol / 2.0 L Calculate the molarity of a 2.0 L solution that contains 10 moles of NaOH. MNaOH = 10 mol / 2.0 L = 5.0 M moles solute mol liters of solution L Molarity (M) = =

Use the same units for both Mass Percent Mass Solute Total Mass Mass % = x 100 Use the same units for both If a ham contained 5 grams of fat in 200 g of ham, what is the mass percent? 5 g / 200g * 100 = 2.5 % On the label, it would say 97.5 % fat free.

Molality moles solute mol kilograms of solvent kg Molality (m) = = Example: Calculate the molality of a 10.0 g of KCl in 80.0 grams of water.

Mole fraction Mole fraction The moles of solute, expressed as a fraction of the total number of moles in the solution. XA = Because the units in the numerator and denominator are the same, mole fraction is a unitless quantity. The sum of all components must equal one. nA nA + nB + nC + . . . .

Example: Mole Fraction What is the mole fraction of sulfur dioxide (SO2) in an industrial exhaust gas containing 128.0 g of sulfur dioxide dissolved in 1500. g of carbon dioxide. XCO2 = 1 – XSO2 = 1 – 0.05539 = .9446

Saturation When a solution contains as much solute as it can at a given temperature. Unsaturated Can still dissolve more. Saturated Have dissolved all you can. Supersaturated Temporarily have dissolved too much. Precipitate Excess solute that falls out of solution.

Properties of Aqueous Solutions There are two general classes of solutes. Electrolytes ionic compounds in polar solvents dissociate in solution to make ions conduct electricity may be strong (100% dissociation) or weak (less than 100%) Nonelectrolytes do not conduct electricity solute is dispersed but does not dissociate

Colligative Properties “Bulk” properties that change when you add a solute to make a solution. Based on how much you add but not what the solute is. Effect of electrolytes is based on number of ions produced. Colligative properties vapor pressure lowering freezing point depression boiling point elevation osmotic pressure

Vapor Pressure Lowering Volatile: has a measurable vapor pressure The introduction of a nonvolatile solute will reduce the vapor pressure of the solvent in the resulting solution. The vapor pressure of a nonvolatile component is essentially zero. It does not contribute to the vapor pressure of the solution. However, the solution’s vapor pressure is dependent on the solute mole fraction.

Demo: Vapor Pressure Lowering Water will end up in the ‘salt’ solution because it’s vapor pressure is lower than the pure water.

Vapor Pressure (A Dynamic Process) liquid

Boiling Point Elevation When you add a nonvolatile solute to a solvent, the boiling point goes up. This is because the vapor pressure has been lowered. DTb = Kb m The boiling point will continue to be elevated as you add more solute until you reach saturation. Examples Cooking pasta in salt water Antifreeze

Example: Boiling point elevation Determine the boiling point for a 0.222 m aqueous solution of sucrose. Kb = 0.512 oC/m for water. DTb = 0.512 oC/m (0.222 m) = 0.114 oC Tb = 100.00 oC + 0.114 oC = 100.11 oC

Phase Diagram (Boiling Point Elevation) Diagram: www.sparknotes.com

Freezing Point Depression When you add a solute to a solvent, the freezing point goes down. DTf = Kf m The more you add, the lower it gets. This will only work until you reach saturation. Examples “Salting” roads in winter Making ice cream

Example: Freezing Point Depression Determine the freezing point for a 0.222 m aqueous solution of sucrose. Kf = 1.86 oC/m for water. DTf = -1.86 oC/m (0.222 m) = -0.413 oC Tf = 0.00 oC - 0.413 oC = -0.41 oC

Example Constants (p. 522/526) Normal Kb Normal Kf Solvent Tb, oC oC/m Tf, oC oC/m Water 100.0 +0.512 0.0 -1.86 Benzene 80.1 +2.53 5.5 -5.12 Camphor 207.4 +5.61 178.8 -39.7 Ethanol 78.3 +1.22 -117.3 -1.99

Phase Diagram (Freezing Point Depression) Diagram: www.sparknotes.com

Electrolytes and Colligative Properties Ionic substances have a greater effect per mole than covalent. 1 mol/kg of water for glucose = 1 molal 1 mol/kg of water for NaCl = 2 molal ions 1 mol/kg of water for CaCl2 = 3 molal ions Effects are based on the number of particles!

Van’t Hoff Factor Van’t Hoff factor (i): number of ions created. Glucose (nonelectrolyte)—i = 1 NaCl (Na+ + Cl-)—i = 2 CaCl2(Ca2+ + 2 Cl-)—i = 3 Equations become: Tb = iKbm Tf = iKfm

Example: Electrolytes Determine the boiling point for a 1.35 m aqueous solution of MgCl2. i = 3 (Mg2+ + 2 Cl-) Kb = 0.512 oC/m for water. DTb = (3)0.512 oC/m (1.35 m) = 2.07oC Tb = 100.00 oC + 2.1 oC = 102.1 oC

Determining Molar Mass EXAMPLE: A 10.0 gram sample of an unknown nonvolatile, nonelectrolyte compound is dissolved in 100 g of water. The boiling point of the solution is elevated to 0.433 ◦C. What is the molar mass of the unknown sample? (Kb for water is 0.52 ◦C/m) Given: munk=10.0 g msolv=100 g = .100 kg Tb = 0.433 ◦C i=1 Kb=0.52 ◦C/m

Determining Molar Mass (page 2) Given: munk=10.0 g msolv=100 g = .100 kg Tb = 0.433 ◦C Kb = 0.52 ◦C/m Equation: Tb = Kbm m= Tb/Kb = 0.433/0.52 = 0.83 m (Step 1) By Definition: m = mol solute/kg solvent Rearranging: mol solute = m x kg solvent (Step 2) = 0.83 m * .100 kg = 0.083 mol Conclusion: 10 g/0.083 mol = 120 g/mole (Step 3)

How Solutions Form When an ionic solid is placed in a polar solvent, the outer ions are exposed to the polar molecules. Water will pull the ions from the solid and surround them - solvate them. Solvation of ions is an exothermic process which helps overcome the lattice energy that holds the crystal together. Hydration is this same process—with water as the solvent. (Solvation is entropically favorable, as well.)

Solution of solids

Factors Affecting Solubility Nature of Solvent and Solute 2) Temperature 3) Pressure

Nature of Solvent & Solute “Like dissolves like.” Materials with similar polarity are soluble in each other. Dissimilar ones are not Ionic substances are not soluble in nonpolar solvents like hexane. A large amount of energy is need to separate the ions. A nonpolar solvent can’t solvate ions so there is no solvation energy to offset the lattice energy.

Pressure and solubility of gases Henry’s Law At constant temperature, the solubility of a gas is directly proportional to the pressure of the gas above the solution. cg = kpgas This law is accurate to within 1-3% for slightly soluble gases and pressures up to one atmosphere. 0 1 2 Pressure (atm) (g/100g water) Solubility 0.010 0.005 0.000 O2 N2 He

Solubilities of solids Ionic substances are not soluble in nonpolar solvents like hexane. A large amount of energy is need to separate the ions. A nonpolar solvent can’t solvate ions so there is no solvation energy to offset the lattice energy. Predicting the solubility of ionic solids in water is difficult because a number of competing factors are involved.

Solution of solids While covalent compounds do not dissociate, they are solvated in solution.

Saturated Solutions At saturation, the solute is in dynamic equilibrium. The concentration is constant. Solute species are constantly in motion, moving in and out of solution.

Why do some curves decrease with increasing temperature and some increase?