Solutions

Occur in all phases The solvent does the dissolving. The solute is dissolved. There are examples of all types of

solvents dissolving all types of solvent.

We will focus on aqueous solutions.

Ways of Measuring

Molarity = moles of solute Liters of solvent

% mass = Mass of solute x 100 Mass of solution

Mole fraction of component A

A = NA

NA + NB

Molality = moles of solute Kilograms of solvent

Molality is abbreviated m Normality - read but don’t focus on

it (unless you plan on being an “Old School Biologist”)

Ways of Measuring

Energy of Making Solutions Heat of solution ( Hsoln ) is the energy

change for making a solution. Most easily understood if broken into

steps. 1.Break apart solvent 2.Break apart solute 3. Mixing solvent and solute

1. Break apart Solvent Have to overcome attractive forces.

H1 >0

2. Break apart Solute. Have to overcome attractive forces.

H2 >0

3. Mixing solvent and solute H3 depends on what you are mixing.

Molecules can attract each other H3 is large and negative.

Molecules can’t attract- H3 is small and negative.Solutions which form are typically exothermic

overall!

This explains the rule “Like dissolves Like”

Energy

Reactants

Solution

H1

H2

H3

Solvent

Solute and Solvent

Size of H3 determines whether a solution will form

H3

Solution

Types of Solvent and solutes

If Hsoln is small and positive, a solution will still form because of entropy.

Typically, greater forms of disorder are favored over ordered matter.

Structure and Solubility Water soluble molecules must have

dipole moments -polar bonds. To be soluble in non polar solvents

the molecules must be non polar. I2 is very slightly soluble in water, but

becomes more soluble in a concentrated KI solution…Explain.

Soap

P O-

CH3

CH2CH2

CH2CH2

CH2

CH2

CH2

O-

O-

Soap

Hydrophobic non-polar end

P O-

CH3

CH2CH2

CH2CH2

CH2

CH2

CH2

O-

O-

Soap

Hydrophilic polar end

P O-

CH3

CH2CH2

CH2CH2

CH2

CH2

CH2

O-

O-

P O-

CH3

CH2CH2

CH2CH2

CH2

CH2

CH2

O-

O-

_

A drop of grease in water Grease is non-polar Water is polar Soap lets you dissolve the non-polar

in the polar.

Hydrophobic ends dissolve in

grease

Hydrophilic ends dissolve in water

Water molecules can surround and dissolve grease.

“Helps get grease out of your way.”

Pressure effects Changing the pressure doesn’t effect

the amount of solid or liquid that dissolves

They are incompressible. It does effect gases.

Dissolving Gases Pressure effects the

amount of gas that can dissolve in a liquid.

The dissolved gas is at equilibrium with the gas above the liquid.

The gas is at equilibrium with the dissolved gas in this solution.

The equilibrium is dynamic.

If you increase the pressure the gas molecules dissolve faster.

The equilibrium is disturbed.

The system reaches a new equilibrium with more gas dissolved.

Henry’s Law.

P= kCPressure =

constant x Concentration of gas

Examples:

1. Soda Can

2. Lake Nyos Tragedy

Lake Nyos Tragedy

•Suffocated 1,700 people within 25 kilometres (16 mi) of the lake, mostly rural villagers, as well as 3,500 livestock.•300 ft fountain of water created an 82 ft wall of water which scoured the lake shore.

Degassing Lake Nyos

Temperature Effects In general: Increased temperature

increases the rate at which a solid dissolves.

However, we can’t always predict whether it will increase the amount of solid that dissolves.

Consult a graph of experimental data for the best results!

20 40 60 80 100

Gases are predictable

As temperature increases, solubility decreases.

Gas molecules can move fast enough to escape the solution.

Thermal pollution of lakes.

Vapor Pressure of Solutions A nonvolatile solvent lowers the

vapor pressure of the solution. The molecules of the solvent

must overcome the force of both the other solvent molecules and the solute molecules.

Raoult’s Law:Psoln = solvent x Psolvent Vapor pressure of the solution =

mole fraction of solvent times the vapor pressure of the pure solvent

Applies only to an ideal solution where the solute doesn’t contribute to the vapor pressure.

Raoult’s Law Example Determine the vapor pressure of a

solution made by dissolving 125 g of sucrose into 455 mL of water at 25.0ºC.

The vapor pressure of water at this temp is 23.76 torr. (Density of water at this temp is 0.9971 g/cm3)

Solution (get it?) 125g sucrose = .365 mol 155 mL of water = 8.58 mol of water = .959 Pressure = 22.8 torr (down from

23.76 torr)

Ionic vs Covalent Solute? What if we used the same moles of

salt rather than sucrose? Same result? I don’t think so…you get twice the

number of solute particles = twice the effective mole fraction of solute!

of water drops to .921 from .959…so the vapor pressure drops as well!

Aqueous Solution

Pure water

Water has a higher vapor pressure than a solution

Aqueous Solution

Pure water

Water evaporates faster from the water than the solution…

The water condenses faster in the solution so it should all end up there.

Aqueous Solution

Pure water

What is the composition of a pentane-hexane solution that has a vapor pressure of 350 torr at 25ºC ?

The vapor pressures at 25ºC are

• pentane 511 torr

• hexane 150 torr.

• Nonideal solution: both can evaporate What are the component vapor

pressures?

Review Question

Review Question Nonideal Solutions: Ptotal=PA + PB

Psoln = sol A x Psol A + sol B x Psol B

350 torr = A511 torr + B 150 torr(A + B = 1…so B = 1-A now you may substitute…)

350 torr = 511 A+ 150 (1-A)

A = .554 (so B = 1- .554…) B = .446

Check it in the original equation!

Colligative Properties Because dissolved particles affect

vapor pressure - they affect phase changes.

Colligative properties depend only on the number - not the kind of solute particles present

Useful for determining molar mass

Boiling point Elevation

Because a non-volatile solute lowers the vapor pressure it raises the boiling point.

The equation is: T = Kbmsolute

T is the change in the boiling point Kb is a constant determined by the

solvent. msolute is the molality of the solute

Freezing point Depression

Because a non-volatile solute lowers the vapor pressure of the solution it lowers the freezing point.

The equation is: T = Kfmsolute

T is the change in the freezing point Kf is a constant determined by the solvent

msolute is the molality of the solute

Freezing/Boiling Point Addition of a nonvolatile solute

extends the liquid phase! Decreasing the freezing point of the

solution… Increasing the boiling point of the

solution!

1 atm

Vapor Pressure of solution

Vapor Pressure of pure water

1 atm

Freezing and boiling points of water

1 atm

Freezing and boiling points of solution

1 atm

TfTb

Electrolytes in solution Since colligative properties only

depend on the number of molecules. Ionic compounds should have a

greater effect. When they dissolve they dissociate. Individual Na and Cl ions fall apart. 1 mole of NaCl makes 2 moles of ions. 1mole Al(NO3)3 makes 4 moles ions.

Electrolytes have a more significant impact on on melting and freezing points per mole because they generate more particles.

Relationship is expressed using the van’t Hoff factor: i

i = Moles of particles in solution

Moles of solute dissolved The expected value can be determined

from the formula.

The actual value is usually less because…

…at any given instant some of the ions in solution will be paired.

…ion pairing increases with concentration.

…i decreases with increasing concentrations.

We can change our formulas to

T = imK

Liquid-liquid solutions where both are volatile.

Modify Raoult’s Law to Ptotal = PA + PB = APA

0 + APB0

Ptotal = vapor pressure of mixture PA

0= vapor pressure of pure A If this equation works then the

solution is ideal. Solvent and solute are alike.

Ideal solutions

Deviations If Solvent has a strong affinity for

solute (H bonding). Lowers solvents ability to escape. Lower vapor pressure than expected. Negative deviation from Raoult’s law. Hsoln is large and negative

exothermic. Endothermic Hsoln indicates

positive deviation.

Osmotic Pressure ∏ = MRT ∏ = osmotic pressure M = molarity (not molality) R = gas law constant T = Kelvin temp

For electrolyte solutions:

∏ = iMRT