Chapter 14: Solutions and Their Properties If you’re not part of the solution, you’re part of the precipitate!

Chapter 14:

Solutions and Their Properties

If you’re not part of the solution,

you’re part of the precipitate!

Solvation

Structure/Interm olecular Forces

Henry's Law

B. pt. E levation Fr. pt. Depression Osom otic Pressure

Colligative Properties

Vapor Pressures Raoult's Law

T em perature Pressure

Heats of Solution

Com positions Solubility Rules

T erm inology

Properties of SolutionsProperties of Solutions

o

AAA PP

Sfbfb mKT ,, TRM S

AS

AA nn

n

Akg

nm S

S

V

nM S

S

Physical Properties of Solutions

Have you ever wondered...•Why antifreeze keeps water from freezing?•Why salt causes ice to melt?•Why cooks add salt to boiling water?•Why root beer foams only when poured?•What force opposes gravity to allow water to climb up a tree?

Solutions

• Solutions are homogeneous mixtures of two or more pure substances.

• In a solution, the solute is dispersed uniformly throughout the solvent.

An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity.

A nonelectrolyte is a substance that, when dissolved, results in a solution that does not conduct electricity.

nonelectrolyte weak electrolyte strong electrolyte

“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)

Energy Changes in SolutionTo determine the enthalpy

change, we divide the process into 3 steps. 1. Separation of solute

particles.

2. Separation of solvent particles to make ‘holes’.

3. Formation of new interactions between solute and solvent.

Three types of interactions in the solution process:• solvent-solvent interaction• solute-solute interaction• solvent-solute interaction

Hsoln = H1 + H2 + H3

Enthalpy Changes in Solution

The enthalpy change of the overall process depends on H for each of these steps.

Start

End

EndStart

Calculating⁄Hsolution

(enthalpy of solution)

KF(s) -> K+(g) + F-(g) ⁄ E = +821 kJ

K+(g) + F-

(g) + H2O -> K+(aq) + F-

(aq)

⁄ E=-819 kJ

So Net -

KF(s) -> K+(aq) + F-

(aq)

⁄ Hsolution = +2kJ

Degree of saturation

• Saturated solutionSolvent holds as much

solute as is possible at that temperature.

Undissolved solid remains in flask.

Dissolved solute is in dynamic equilibrium with solid solute particles.

Degree of saturation

• SupersaturatedSolvent holds more solute than is normally

possible at that temperature.These solutions are unstable; crystallization can

often be stimulated by adding a “seed crystal” or scratching the side of the flask.

Gases in Solution

• In general, the solubility of gases in water increases with increasing mass.

Why?• Larger molecules

have stronger dispersion forces.

Gases in Solution

• The solubility of liquids and solids does not change appreciably with pressure.

• But, the solubility of a gas in a liquid is directly proportional to its pressure.

Increasing pressure above solution forces more gas to dissolve.

Henry’s Law

What happens to the solubility of carbon dioxide in a bottle of soda when the pressure is reduced?

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

Chemistry In Action: The Killer Lake

Lake Nyos, West Africa

8/21/86CO2 Cloud Released

1700 Casualties

Trigger?

• earthquake

• landslide

• strong Winds

Temperature

Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

Solubility is measured as the mass of solute dissolved in 100 g of solvent at a given temperature

Temperature• The opposite is true of

gases. Higher temperature drives gases out of solution.

Carbonated soft drinks are more “bubbly” if stored in the refrigerator.

Warm lakes have less O2 dissolved in them than cool lakes.

Mass Percentage

Mass % of A =mass of A in solutiontotal mass of solution

100

moles of Atotal moles in solution

XA =

Mole Fraction (X)

• In some applications, one needs the mole fraction of solvent, not solute—make sure you find the quantity you need!

mol of soluteL of solution

M =

Molarity (M)

• Because volume is temperature dependent, molarity can change with temperature.

mol of solutekg of solvent

m =

Molality (m)

Because neither moles nor mass change with temperature, molality (unlike molarity) is not temperature dependent.

Mass/M

ass

Mol

es/M

oles

Moles/Mass

Moles/L

Changing Molarity to Molality

If we know the density of the solution, we can calculate the molality from the molarity, and vice versa.

Colligative Properties

• Colligative properties depend only on the number of solute particles present, not on the identity of the solute particles.

• Among colligative properties areVapor pressure lowering Boiling point elevationMelting point depressionOsmotic pressure

Vapor Pressures of Pure Water and a Water Solution

The vapor pressure of water over pure water is greater than the vapor pressure of water over an aqueous solution containing a nonvolatile solute.

Solute particles take up surface area and lower the vapor pressure

Vapor Pressure

As solute molecules are added to a solution, the solvent becomes less volatile (has decreased vapor pressure).

Solute-solvent interactions contribute to this effect.

Vapor Pressure

Therefore, the vapor pressure of a solution is lower than that of the pure solvent.

Lowering Vapor Pressure• Raoult’s Law:

• Where: PA = vapor pressure with solute,

• PA = vapor pressure without solute (pure solvent), and

A = mole fraction of A (the pure solvent).

Colligative PropertiesColligative Properties

AAA PP

Boiling Point Elevation and Freezing Point Depression

Solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.

Boiling Point ElevationThe change in boiling point is proportional to the molality of the solution:

Tb = Kb m

where Kb is the molal boiling point elevation constant, a property of the solvent.Tb is added to the normal

boiling point of the solvent.

Freezing Point Depression• The change in freezing

point can be found similarly:

Tf = Kf m

• Here Kf is the molal freezing point depression constant of the solvent.

Tf is subtracted from the normal freezing point of the solvent.

Boiling-Point Elevation

• Molal boiling-point-elevation constant, Kb, expresses how much Tb changes with molality, mS :

• Decrease in freezing point (Tf) is directly proportional to molality (Kf is the molal freezing-point-depression constant):

Colligative PropertiesColligative Properties

Sbb mKT

Sff mKT

In both equations, T does not depend on what the solute is, but only on how many particles are dissolved.

Colligative Properties of Electrolytes

Because these properties depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) show greater changes than those of nonelectrolytes.

e.g. NaCl dissociates to form 2 ion particles; its limiting van’t Hoff factor is 2.

Colligative Properties of Electrolytes

However, a 1 M solution of NaCl does not show twice the change in freezing point that a 1 M solution of methanol does.

It doesn’t act like there are really 2 particles.

van’t Hoff Factor

One mole of NaCl in water does not really give rise to two moles of ions.

van’t Hoff Factor

Some Na+ and Cl− reassociate as hydrated ion pairs, so the true concentration of particles is somewhat less than two times the concentration of NaCl.

The van’t Hoff Factor

• Reassociation is more likely at higher concentration.

• Therefore, the number of particles present is concentration dependent.

The van’t Hoff Factor

We modify the previous equations by multiplying by the van’t Hoff factor, i

Tf = Kf m i

i = 1 for non-elecrtolytes

Osmosis

• Semipermeable membranes allow some particles to pass through while blocking others.

• In biological systems, most semipermeable membranes (such as cell walls) allow water to pass through, but block solutes.

OsmosisIn osmosis, there is net movement of solvent from the area of higher solvent concentration (lower solute concentration) to the are of lower solvent concentration (higher solute concentration).

Water tries to equalize the concentration on both sides until pressure is too high.

Osmosis• Osmotic pressure, , is the pressure required to stop

osmosis:

Colligative PropertiesColligative Properties

TRV

n

TRnV

TRM S TRM S

Molar Mass from Colligative Properties

We can use the effects of a colligative property such as osmotic pressure to determine the molar mass of a compound.

Solvation

Structure/Interm olecular Forces

Henry's Law

B. pt. E levation Fr. pt. Depression Osom otic Pressure

Colligative Properties

Vapor Pressures Raoult's Law

T em perature Pressure

Heats of Solution

Com positions Solubility Rules

T erm inology

Properties of SolutionsProperties of Solutions

o

AAA PP

Sfbfb mKT ,, TRM S

AS

AA nn

n

Akg

nm S

S

V

nM S

S