Solutions

Definitions A solution is a homogeneous mixture consisting of two parts. The solvent is usually in greater quantity and does not change phase in the process of forming the solution. The solute is usually in lesser quantity and may change phase in the process of forming the solution

Enthalpy of Solution Energy is needed to: Energy is released when: Separate the solute into individual components (ions or molecules or atoms) Overcome the interparticle forces in the solvent Energy is released when: Interaction between the solute and solvent occur The sum of these enthalpies (Hess’s Law) gives the enthalpy of solution (∆Hsoln)

Rate of Solution The speed of dissolution is related to: Nature of solute and solvent Temperature Solute surface area Agitation

Solubility Like dissolves like Gas solubility depends on temp and pressure Solid solubility depends only on temp Solubility curves (p520)

Saturation An unsaturated solution contains less than the maximum A saturated solution contains the maximum solute per solvent for the given conditions (often undissolved solute will be seen in the solution) An equilibrium exists between dissolving and recrystallizing A supersaturated solution contains over the maximum solute per solvent This is unstable and rapid crystallization will occur if the solution is disturbed

Concentration Describes the amount of solute dissolved in the solution. “concentrated” is a generic term meaning a large amount of solute per amount of solvent “dilute” is a generic term meaning a small amount of solute per amount of solvent

Molarity The number of moles of solute per liter of solution Will change with temperature due to volume changes in the mixture (thermal expansion) Used for small-scale work (such as research and education) M = mol/L

Molality The number of moles of solute per kilogram of solvent Unaffected by temperature changes Industrial uses m = mol/kg

Percent The number of units of solute per units of the solution May be by mass or volume or a combination % by mass = (mass solute/mass solution)(100) % by volume = (vol. of solute/vol. of solution)(100)

Parts per Million (or Billion) For very small solute amounts, ppm or ppb is often used. The “part” is usually grams, but could be a volume measure. ppm = 106(parts solute/parts solution) ppb = 109(parts solute/parts solution)

Mole Fraction The number of mols of the part per total mols May be mols of solute per total mols…or mols of solvent per total mols The symbol chi is used for mol fraction: Χ= mol part/total mols

Normality The number of equivalents per liter of solution An equivalent is the mass of the solute that delivers either 1 mol of electrons for a redox reaction, or the mass that donates or accepts 1 mol of protons in an acid/base reaction. Appropriate for UIL to learn (will not be tested in this class)

Converting between units Converting between molality, percent by mass, and mol fraction is convenient because all of the terms are related to mass and mols… Converting between molarity and any of the others requires knowledge of the density of the solution because a conversion between mass units and volume units of the solution is necessary

Practice Grab a calculator!

Vapor Pressure of a Solution Vapor pressure is the pressure created above the surface of an evaporating liquid, or above the surface of a sublimating solid. Addition of a nonvolatile solute will cause a decrease in the vapor pressure of a solvent. Raoult’s Law: Psoln = ΧsolventPsolvent Addition of ionic solids requires consideration of mols of ions in the solution Use of a volatile solute requires: Psoln = ΧAPA + ΧBPB

Colligative Properties of Solutions Colligative properties are characteristics of solutions that depend only on the number of solute particles, and not the identity of the solute, in the solution. There are 3 of these characteristics: Boiling point elevation Freezing point depression Osmotic pressure As with vapor pressure, these properties can be useful in determining an unknown solute’s molar mass.

van’t Hoff factor Ionic solutes will break apart into ions as they dissolve in polar solvents, so the number of mols of particles dissolved will actually be greater than the number of mols of particles added. EX: 1 mol NaCl = 1 mol Na+ + 1 mol Cl- (2 mols of dissolved particles) EX: 1 mol CaCl2 = 1 mol Ca2+ + 2 mol Cl- (3 mols of dissolved particles) The van’t Hoff factor (i) is the number of moles of separate ions per mol of solute compound

Osmotic Pressure Osmotic pressure is a measure of the pressure gradient that exists between the solution and pure solvent across a semipermeable membrane where solvent can pass, but solute cannot. Addition of solute particles to a solvent increases the osmotic pressure gradient: ∏ = iMRT ∏ is osmotic pressure, M is the molarity of the solute, i is the van’t Hoff factor, R is the gas constant (match units to pressure), and T is the Kelvin temperature

Boiling point elevation and Freezing point depression Addition of solute particles to a solvent causes: an increase in the boiling point of the solvent a decrease in the freezing point of the solvent The change in the b.p. or f.p. of the solvent is equal to the product of the solvent constant and the molality of the solute: ∆t = kim k is a solvent constant, i is the van’t Hoff factor (ions), and m is the molality of the solute