Solutions

AP Chemistry

Heat of SolutionHeat of SolutionThe Heat of Solution is the amount of heat energy absorbed (endothermic) or released (exothermic) when a specific amount of solute dissolves in a solvent.

Steps in Solution FormationSteps in Solution FormationHH11 Step 1 - Step 1 - Expanding the soluteSeparating the solute into individual components

Steps in Solution FormationSteps in Solution FormationHH22 Step 2Step 2 -- Expanding the solventOvercoming intermolecular forces of the solvent molecules

Steps in Solution FormationSteps in Solution FormationHH33 Step 3 - Step 3 - Interaction of solute and solvent to form the solution

Predicting Solution FormationPredicting Solution Formation

Solubility ChartSolubility Chart

Vapor Pressure - Henry’s LawThe concentration of a dissolved gas in a solution is directly proportional to the pressure of the gas above the solution

kPCApplies most accurately for dilute solutions of gases that do not dissociate or react with the solvent

Yes Yes CO CO22, N, N22, O, O22

No No HCl, HI HCl, HI

Raoult’s LawThe presence of a nonvolatile solute lowers the vapor pressure of the solvent.

0solventsolventsolution PP

PPsolutionsolution = Observed Vapor pressure of the solution

PP00solventsolvent = Vapor pressure of the pure solvent

solventsolvent = Mole fraction of the solvent

Liquid-liquid solutions in which both components are volatile

Modified Raoult's Law:Modified Raoult's Law:

00BBAABATOTAL PPPPP

PP00 is the vapor pressure of the pure solvent

PPAA and PPBB are the partial pressures

Raoult’s Law – Raoult’s Law – Ideal Ideal SolutionSolution

A solution that obeys Raoult’s Law is called an ideal solution

Negative Deviations from Raoult’s LawNegative Deviations from Raoult’s LawStrong solute-solvent interaction results in a vapor pressure lower than predicted

Exothermic mixing = Negative deviation

Positive Deviations from Raoult’s Positive Deviations from Raoult’s LawLaw

Weak solute-solvent interaction results in a vapor pressure higher than predicted

Endothermic mixing = Positive deviation

Colligative PropertiesColligative Properties

Colligative properties are those that depend on the concentration of particles in a solution, not upon the identity of those particles.

Boiling Point Elevation Freezing Point Depression Osmotic Pressure

Freezing Point DepressionFreezing Point Depression

Each mole of solute particles lowers the freezing point of 1 kilogram of water by 1.86 degrees Celsius.

KKff = 1.86 C kilogram/mol

mm = molality of the solution

ii = van’t Hoffvan’t Hoff factor

solutef mKiT

Boiling Point ElevationBoiling Point Elevation

Each mole of solute particles raises the boiling point of 1 kilogram of water by 0.51 degrees Celsius.

KKbb = 0.51 C kilogram/mol

mm = molality of the solution

ii = van’t Hoffvan’t Hoff factor

soluteb mKiT

Freezing Point Depression and Boiling Point Elevation

Constants, C/m

The van’t Hoff Factor, The van’t Hoff Factor, ii

Electrolytes may have two, three or more times the effect on boiling point, freezing point, and osmotic pressure, depending on its dissociation.

Dissociation Equations and Dissociation Equations and the Determination of the Determination of ii

NaCl(s)

AgNO3(s) MgCl2(s)

Na2SO4(s)

AlCl3(s)

Na+(aq) + Cl-(aq)

Ag+(aq) + NO3-(aq)

Mg2+(aq) + 2 Cl-(aq)

2 Na+(aq) + SO42-

(aq)Al3+(aq) + 3 Cl-(aq)

ii = 2 = 2

ii = 2 = 2

ii = 3 = 3

ii = 3 = 3

ii = 4 = 4

Ideal vs. Real van’t Hoff FactorIdeal vs. Real van’t Hoff Factor

The ideal van’t Hoff Factor is only The ideal van’t Hoff Factor is only achieved in achieved in VERY DILUTEVERY DILUTE solution. solution.

Osmotic PressureOsmotic Pressure

The minimum pressure that stops the osmosis is equal to the osmotic pressure of the solution

Osmotic Pressure Osmotic Pressure CalculationsCalculations

= Osmotic pressure

MM = Molarity of the solution

RR = Gas Constant = 0.08206 Latm/molK

ii = van’t Hoff Factor

iMRT

Suspensions and Suspensions and ColloidsColloids

Suspensions and colloids are NOT solutions. Suspensions: The particles are so large that they settle out of the solvent if not constantly stirred.

Colloids: The particles intermediate in size between those of a suspension and those of a solution.

Types of ColloidsTypes of Colloids

The Tyndall EffectThe Tyndall Effect

Colloids scatter light, making a beam visible. Solutions do not scatter light.

Which glass contains a colloid?

solutioncolloid