Physical Properties of Solutions Chapter 12

A solution is a homogenous mixture of 2 or more substances The solute is(are) the substance(s) present in the smaller amount(s) The solvent is the substance present in the larger amount Definitions

A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature. An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature. A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature. Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate. Rapid Crystallization

Enthalpy of solution (or enthalpy of dissolution)  The heat of solution (H sol ) is defined as the sum of the energy absorbed (endothermic), or released (exothermic) as the solute dissolves in a solvent at constant pressure.  The value of the overall enthalpy change is the sum of the individual enthalpy changes of each step.

Examples  Dissolving ammonium nitrate in water is endothermic (solvation does not weigh up against energy spent in breaking down the crystal lattice)  Adding potassium hydroxide is exothermic, the solute-solvent attractions are stronger than the other steps.  Solutions with negative heats of solution have lower vapor pressures.

“like dissolves like” Two substances with similar intermolecular forces are likely to be soluble in each other. non-polar molecules are soluble in non-polar solvents CCl 4 in C 6 H 6 polar molecules are soluble in polar solvents C 2 H 5 OH in H 2 O ionic compounds are more soluble in polar solvents NaCl in H 2 O or NH 3 (l)

The Cleansing Action of Soap

Fat Soluble and Water Soluble Vitamins  Vitamin A is soluble in nonpolar compounds (like fats).  Vitamin C is soluble in water.

Temperature and Solubility Solid solubility and temperature Generally, solubility increases with increasing temperature Some substances, solubility decreases with increasing temperature (gives off heat as they dissolve)

Temperature and Solubility Gas solubility and temperature solubility usually decreases with increasing temperature

Pressure and Solubility of Gases The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law). c = kP c is the concentration (M) of the dissolved gas P is the pressure of the gas over the solution k is a constant (mol/Latm) that depends only on temperature low P low c high P high c

Concentration Units The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. Percent by Mass % by mass = x 100% mass of solute mass of solute + mass of solvent = x 100% mass of solute mass of solution 12.3 Mole Fraction (X) X A = moles of A sum of moles of all components

Concentration Units Continued M = moles of solute liters of solution Molarity (M) Molality (m) m = moles of solute mass of solvent (kg)

What is the molality of a 5.86 M ethanol (C 2 H 5 OH) solution whose density is g/mL? m =m = moles of solute mass of solvent (kg) M = moles of solute liters of solution Assume 1 L of solution: 5.86 moles x MM ethanol = 270 g ethanol (solute) 1000 mL x g/mL = 927 g of solution mass of solvent = mass of solution – mass of solute = 927 g – 270 g = 657 g = kg m =m = moles of solute mass of solvent (kg) = 5.86 moles C 2 H 5 OH kg solvent = 8.92 m

Convert % mass to Molarity  What is the Molarity of a 95% acetic acid solution? (density = g/mL) 1000 mLx 1.049g/ml = the mass of solution = 1049 g 95% of the solution is acetic acid 1049 g solution x 0.95 = 997 g solute 997 g X 1 mol/60.05 g Acetic acid = 16.6 mol solute Since we assumed 1 L, that’s 16.6 mol / 1 L or 16.6 M

Colligative Properties of Solutions Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.  Vapor Pressure Lowering  Boiling Point Elevation  Freezing Point Depression  Osmotic Pressure

Raoult’s law Vapor-Pressure Lowering; The vapor pressure of a Solution is lower than pure solute. P 1 0 = vapor pressure of pure solvent X 1 = mole fraction of the solvent P 1 = X 1 P 1 0 P 1 = X 2 P 0 1 X 2 = mole fraction of the solute

Lowering the Pressure Above the Solution (by opening bottle) Decreases Gas Solubility

Boiling-Point Elevation of Nonelectrolyte Solutions  T b = T b – T b 0 0 T b boiling point of the pure solvent 0 T b boiling point of the solution  T b = K b m m is the molality of the solution K b is the boiling-point elevation constant ( 0 C/m)

Osmotic Pressure  Osmotic pressure is the “funky” colligative property, but it is very important biologically  Osmotic pressure is the pressure required to prevent osmosis. = M RT where  is osmotic pressure

 Colligative properties depend on the concentration of particles  Strong electrolytes, like NaCl, should produce (nearly) two moles of solute particles for mole of NaCl that dissolves  The van’t Hoff factor i scales the solute molatity to the correct number of particles van’t Hoff factor i

  Δ T f = i K f m    T b = i K f m   P 1 = X 1 P 0 1    = iMRT