CH 13 Solutions and Colligative Properties: Chemistry Concepts and Calculations

On CH 13- Solubility & Solutions

Solutions

Homogeneous mixtures where the solute and solvent are mixed uniformly.

Solute

Substance there is less of in a solution.

Solvent

Substance there is more of in a solution.

Solution

Result of mixing solute and solvent.

Dissolution

A physical change where molecules remain intact when they dissolve.

Ionic compounds

Dissociate to form solvated ions when dissolved.

Covalent bonds

Do not break during dissolution; only intermolecular forces are affected.

Solubility

The ability of a substance to dissolve in another substance.

Miscibility

The ability of substances to mix in any proportions.

Polar molecules

Molecules that will dissolve in other polar molecules.

Nonpolar molecules

Molecules that will dissolve in other nonpolar molecules.

Energy of Mixing

The energy associated with the mixing of solute and solvent.

Entropy

The natural tendency towards mixing of substances.

Enthalpy

The energy of mixing that can affect solubility.

Dispersion forces

Weak forces always present in nonpolar substances.

Dipole-dipole forces

Forces present when both molecules are polar.

H-bonding

Attractive forces between H-bond donors and acceptors.

Ion-dipole forces

Forces between ionic solutes and polar solvents.

Colligative Properties

Changes in physical properties of solutions that depend on solute concentration.

Vapor pressure lowering

A colligative property where the vapor pressure decreases due to dissolved solute.

Boiling point elevation

A colligative property where the boiling point increases due to dissolved solute.

Freezing point depression

A colligative property where the freezing point decreases due to dissolved solute.

Colligative Property

A solution will have lower vapor pressure and increase the boiling point.

Raoult's Law

Pvap of the solution is given by Psolution = Xsolvent•P°solvent.

P°solvent

Vapor pressure of the pure solvent.

Psolution

Vapor pressure of the solution.

X_solvent

Mole fraction of the solvent.

Decrease in Pvap

∆P = Psolution - P°solvent, proportional to the mole fraction of solute.

∆P

Change in vapor pressure, calculated as ∆P = Xsolute•P°solvent.

Partial Pressures

Partial pressures are proportional to the mole fractions of the species in solution and are additive.

PA

Partial pressure of A in the solution, calculated as PA = CA•P°A.

PB

Partial pressure of B in the solution, calculated as PB = CB•P°B.

Ptotal

Total vapor pressure of the solution, given by Ptotal = PA + PB.

Vapor Pressure of Pure Water

The vapor pressure of pure water at 20 °C is 0.0231 atm.

Mole Fraction of Water

For seawater with a mole fraction of water XH2O = 0.980 at 20 °C, calculate Pvap.

Vapor Pressure at 100 °C

Calculate the vapor pressure of a solution prepared by mixing 1.000 L of ethylene glycol with 1.000 L of water.

Vapor Pressure of Methanol

Methanol has a vapor pressure of 0.81 atm at a given temperature.

Vapor Pressure of Ethanol

Ethanol has a vapor pressure of 0.45 atm at the same temperature.

Partial Pressure of Methanol

Calculate the partial pressure of methanol in a solution prepared from 2 mol methanol and 1 mol ethanol.

Partial Pressure of Ethanol

Calculate the partial pressure of ethanol in a solution prepared from 2 mol methanol and 1 mol ethanol.

Total Vapor Pressure of Solution

Calculate the total vapor pressure of the solution prepared from 2 mol methanol and 1 mol ethanol.

Ideal Solutions

Ideal solutions obey Raoult's Law and hold when solute and solvent have similar-strength interactions.

Vapor Composition and Distillation

A component with a higher vapor pressure will make up a greater composition of the vapors than it does in the solution.

Fractional Distillation

Repeated distillations or evaporation/condensation cycles yield purified liquids.

Boiling-Point Elevation

∆Tb = i•Kb•m, where Tb is raised proportionally to the molality of the solute.

Van't Hoff Factor

i is the Van't Hoff factor, always 1 for molecular solutes.

Molal Concentration of Ethylene Glycol

What molal concentration of ethylene glycol would give an aqueous solution that boils at 105 ºC?

Freezing-Point Depression

∆Tf is lowered (∆Tf = Tf solution - Tf solvent) proportionally to the molality of the solution: ∆Tf = -i•Kf•m

Freezing Point Elevation

The boiling point is raised (∆Tb > 0) and the freezing point is lowered (∆Tf < 0)

Henry's Law

Solubility of gases is usually low, decreases with temperature, and increases with pressure (Sg = khPg)

Van't Hoff Factor, i

The number of dissolved particles per species; for molecular compounds, i = 1 and for electrolytes, i > 1

Osmosis

Across a semipermeable barrier, solvent will move from low to high concentration.

Osmotic Pressure

Movement towards equilibrium acts as a pressure.

Supersaturated Solutions

Solutions with more solute than 'allowed'; if [solute] > [solubility], the extra solute will be eliminated.

Solubility of Gases

Solubility of gases increases with pressure and decreases with temperature.

Molarity of Glycerol Solution

A solution of glycerol in water has a freezing point of -0.56 °C.

Freezing Point of Ethylene Glycol Solution

What is the freezing point of a 2.0 m solution of ethylene glycol in water?

Concentration of Lactic Acid

Two aqueous solutions of equal volume, one containing 20 g of lactic acid (C3H6O3) and one containing 10 g of glucose (C6H12O6) are separated by a semipermeable membrane.

Direction of Water Flow (KNO3 and Glucose)

If a 0.1 M solution of KNO3 and a 0.2 M solution of glucose (C6H12O6) are connected by a semipermeable membrane.

Osmotic Pressure Comparison

Which of the following aqueous solutions will have the highest osmotic pressure?

Types of Solutions

The solute concentration of seawater is approximately 1.15 M.

Isotonic Solutions

Isotonic solutions for red blood cells have the same osmotic pressure as 5.0% m/v glucose.

Hypertonic Solutions

In hypertonic solutions (higher [solute] than in the cell), cells shrink (crenation).

Hypotonic Solutions

In hypotonic solutions (lower [solute] than in cell), cells swell until they burst (hemolysis).

Solubility of Solids

Solubility usually increases at higher T.

Crystallization

You can form crystals by cooling a saturated solution.

Equilibrium in Freezing and Melting

At the freezing (melting) point, the rate of freezing equals the rate of melting.

Ionic Species Dissociation

In concentrated solutions, ionic species do not dissociate fully: some ions pair.

Measured vs Expected i

At higher concentrations, imeasured < iexpected.