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 isadded to a supersaturated solution of sodium acetate.Rapid Crystallization

Enthalpy of solutionEnthalpy of solution (or (or enthalpy of dissolutionenthalpy of dissolution))

The heat of solution (The heat of solution (HHsolsol) is defined ) is defined as the sum of the energy absorbed as the sum of the energy absorbed (endothermic), or released (endothermic), or released (exothermic) as the solute dissolves (exothermic) as the solute dissolves in a solvent at constant pressure. in a solvent at constant pressure.

The value of the overall enthalpy The value of the overall enthalpy change is the sum of the individual change is the sum of the individual enthalpy changes of each step. enthalpy changes of each step.

ExamplesExamples Dissolving ammonium nitrate in Dissolving ammonium nitrate in

water is endothermic (solvation does water is endothermic (solvation does not weigh up against energy spent in not weigh up against energy spent in breaking down the crystal lattice) breaking down the crystal lattice)

Adding potassium hydroxide is Adding potassium hydroxide is exothermic, the solute-solvent exothermic, the solute-solvent attractions are stronger than the attractions are stronger than the other steps.other steps.

Solutions with negative heats of Solutions with negative heats of solution have lower vapor pressures.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

CCl4 in C6H6

• polar molecules are soluble in polar solvents

C2H5OH in H2O

• ionic compounds are more soluble in polar solvents

NaCl in H2O or NH3 (l)

The Cleansing Action of Soap

Fat Soluble and Water Soluble Fat Soluble and Water Soluble VitaminsVitamins

Vitamin A is soluble in nonpolar Vitamin A is soluble in nonpolar compounds (like fats).compounds (like fats).

Vitamin C is soluble in water.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/L•atm) that depends onlyon temperature

low P

low c

high P

high c

Concentration UnitsThe 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 solutemass of solute + mass of solvent

= x 100%mass of solutemass of solution

12.3

Mole Fraction (X)

XA = 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 (C2H5OH) solution whose density is 0.927 g/mL?

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 0.927 g/mL = 927 g of solution

mass of solvent = mass of solution – mass of solute

= 927 g – 270 g = 657 g = 0.657 kg

m =moles of solute

mass of solvent (kg)=

5.86 moles C2H5OH

0.657 kg solvent= 8.92 m

Convert % mass to MolarityConvert % mass to Molarity What is the Molarity of a 95% acetic acid What is the Molarity of a 95% acetic acid

solution? (density = 1.049 g/mL)solution? (density = 1.049 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 SolutionsColligative Properties of SolutionsColligative propertiesColligative properties are properties are properties

that depend only on the that depend only on the numbernumber of of solute particles in solution and not on solute particles in solution and not on the the naturenature of the solute particles. of the solute particles.

Vapor Pressure LoweringVapor Pressure Lowering

Boiling Point ElevationBoiling Point Elevation

Freezing Point DepressionFreezing Point Depression

Osmotic PressureOsmotic Pressure

Raoult’s law

Vapor-Pressure Lowering; The vapor pressure of a Solution is lower than pure solute.

P 10 = vapor pressure of pure solvent

X1 = mole fraction of the solvent

P1 = X1 P 10

P1 = X2P01 X2 = mole fraction of the

solute

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

SolubilitySolubility

Boiling-Point Elevation of Nonelectrolyte Solutions

Tb = Tb – T b0

0

T b boiling point of the pure solvent

0

T b boiling point of the solution

Tb = Kb m

m is the molality of the solution

Kb is the boiling-point elevation constant (0C/m)

Osmotic PressureOsmotic Pressure Osmotic pressure is the “funky” colligative Osmotic pressure is the “funky” colligative

property, but it is very important property, but it is very important biologicallybiologically

Osmotic pressure is the pressure required Osmotic pressure is the pressure required to prevent osmosis.to prevent osmosis.

= M RT= M RT

where where is osmotic pressure is osmotic pressure

Colligative properties depend on Colligative properties depend on the concentration of the concentration of particlesparticles

Strong electrolytes, like NaCl, Strong electrolytes, like NaCl, should produce (nearly) two moles should produce (nearly) two moles of solute of solute particlesparticles for mole of NaCl for mole of NaCl that dissolvesthat dissolves

The van’t Hoff factor The van’t Hoff factor ii scales the scales the solute molatity to the correct solute molatity to the correct number of particlesnumber of particles

van’t Hoff factor van’t Hoff factor ii

van’t Hoff factor van’t Hoff factor ii

3.00 2.84 2.70 2.23 SOK

2.00 1.98 1.94 1.85 KCl

2.00 1.97 1.94 1.87 NaCl

Occured 0.001 0.01 0.1 Salt

dDissociate Conc Molal

% 100 If

42

van’t Hoff factor van’t Hoff factor ii

Δ T f = i K f m

Tb = i Kf m

P1 = X1P01

= iMRT