Solutions

Homogeneous mixtures of two or more substancesAppear to be pure substances

Transparency Separation by filtration is not possible

SolutionsDefinition and Characteristics

Uniform distribution of particlesNo settling or separation

Uniform mixing is energetically favored by nature

Composition may be varied

Most homogeneous mixtures we encounter can be described as a solution,

i.e., a combination of a solute and a solvent.

Solute Solvent Substance oxygen nitrogen air water vapor air humid air

carbon dioxide water soda water ammonia water household ammonia

acetic acid water vinegar ethylene glycol water antifreeze

salt water sea watersugar water syrup

mercury silver dental amalgam

zinc copper brass carbon iron steel copper silver sterling silver tin lead solder

Solvent and Solute in an Aqueous Solution

Solvent and Solute in an Aqueous Solution

Solution Formation

When one substance (the solute) dissolves in another (solvent) it is

said be soluble.

When one substance does not dissolve in another it is said be

insoluble.

Solubility depends on two factors:

tendency toward mixing (greater entropy)intermolecular forces

The Dissolving and Crystallization Processes

Dissolving

Crystallization

The Dissolving Process Involves

1.Breakdown of the attractive forces between and

2.Breakdown of the attractive forces between and

3. Establishment of attractive forces between and (sometimes referred to as solvation or hydration)

The Dissolving and Crystallization Processes

Dissolving

Crystallization

The Crystallization Process Involves

1.Breakdown of the attractive forces between and

2. Establishment of attractive forces between and

3. Establishment of attractive forces between and

What happens when you dissolve an ionic compound in water??

What happens when you dissolve a polar molecule in water??

What Happens When an Ionic Compound Dissolves in Water?

dipole-dipole attractions

What Happens When a Polar Covalent Compound Dissolves in Water?

What happens when you try to dissolve a nonpolar molecule in water??

What happens when you try to dissolve a nonpolar molecule in water??

Non polar solvents, such as ethanol, carbon tetrachloride, ether, and hexane, are also commonly used to dissolve nonpolar solutes, such as grease and oils.

General Solubility Rule: “Like Dissolves Like”

Polar solutes form solutions with polar solvents.

Nonpolar solutes form solutions with nonpolar solvents.

Selected Polar and Nonpolar Solvents

! ! POLAR SOLVENTS NONPOLAR SOLVENTS water, H2O hexane, C6H14

methanol, CH3OH heptane, C7H16

ethanol, C2H5OH toluene, C7H8

acetone, C3H6O carbon tetrachloride, CCl4

methyl ethyl ketone, CH3CH2C(O)CH3 chloroform, CHCl3

formic acid, HCOOH methylene chloride, CH2Cl2

acetic acid, CH3COOH ethyl ether, CH3CH2OCH2CH3

A solution forms when these three

types of attractions are of similar

strength.

Solubility

The is usually a limit to the solubility of one substance in another.

Two liquids which are mutually soluble are said be miscible.

(Gases are always soluble in each other.)

The maximum solubility of a substance under a given set of conditions* is its solubility.

Solubility varies with temperature and pressure (for gases).

Enthalpy of SolutionThe Solution Cycle

solute (aggregated) + heat → solute (separated)

solvent (aggregated) + heat → solvent (separated)

∆Hsolute>0, endothermic

∆Hsolvent>0, endothermic

solute (separated) + solvent (separated)

→ solution + heat

∆Hmix<0, exothermic

∆Hsoln = ∆Hsolute + ∆Hsolvent + ∆Hmix

∆Hsoln = ∆Hsolute + ∆Hhydration

∆Hhydration

∆Hsoln = ∆Hsolute + ∆Hsolvent + ∆Hmix

Separated components

Solution

Heat of Hydration

∆Hsolution ∆Hhydration

-∆Hlattice

Dissolving ionic compounds in water.

NaCl

NaOH

NH4NO3

Dissolving NaCl in Water

Dissolving ionic compounds in water.

NaCl

NaOH

NH4NO3

Dissolving NH4NO3 in Water

Dissolving ionic compounds in water.

NaCl

NaOH

NH4NO3

Dissolving NaOH in Water

∆Hsolution for Various Compounds

Compound ∆Hsolution (kJ/mol)

KOH -57.6

LiBr -48.8

NaF +0.92

NaCl +3.9

NH4NO3 +25.7

AgNO3 +36.9

Entropy and the Solution ProcessFormation of a solution does not necessarily

lower the potential energy of the system. For example, neon and argon mix even though there is

very little difference between the attractive forces involved between molecules.

Gases mix because “free energy” is released through the increase in the entropy of the system.

Entropy is the measure of the “energy dispersal” of a system.

Energy has a spontaneous drive to spread out over as large a volume as possible.

Entropy in the Solution Process

ΔG = ΔH - TΔS

“Free energy” - a measure of spontaneity

Temperature Dependance of Solubility of Solids in Water

Solubility is generally reported as grams of solute that will dissolve in 100 g of water.

For most solids, solubility increase with an increase in temperature.

Solubility curves are used to classify solutions as saturated, unsaturated, or supersaturated.

Temperature Dependance of Solubility of Gases in Water

Solubility is generally reported as moles of solute that will dissolve in 1 L of solution.

Because most gases are nonpolar, solubilities are generally lower than ionic compounds or

polar covalent compounds.

Solubility of gases decreases as temperature increases.

Temperature Dependence of Gas Solubilty in Water

Pressure Dependance of Solubility of Gases in Water - Henry’s Law

“The solubility of a gas is directly

proportional to its partial pressure.”

SH = kHPgas

kH = Henry’s constant

gas kH (M/atm)

O2 1.3 x 10-3

N2 6.1 x 10-4

CO2 3.4 x 10-3

NH3 5.8 x 101

He 3.7 x 10-4

Henry’s Law

Henry’s Law

Pressure Dependance of Solubility of Gases in Water

Concentration of Solutions

Solutions have variable composition.

Therefore, descriptions of solutions must include the components and relative

quantities.

The terms dilute and concentrated are sometimes used, qualitatively.

In general, concentration refers to the amount of solute in a given amount of

solvent.

Concentration Units

Molarity

Molality

Mole Fraction

Mass Percent

Molarity (M) and Dissociation

The molarity of an ionic compound allows you to determine the molarity of dissolved ions .

CaCl2 (aq) = Ca2+ (aq) + 2 Cl- (aq)

A 1 M solution of CaCl2 contains:

1 mol of Ca2+ / liter2 mol of Cl- / liter

3 mol total of ions/liter

Molality (m)

Moles of solute/1 kg of solvent

Note: The definition is in terms of amount of solvent, not solution.

Does not vary with temperature, since it is based on mass and not volume.

Percent Concentration

Mole Fraction (X)

Mole fraction = the fraction of the moles of one component in the total moles of all

components in a solution

total of all the mole fractions = 1

Mole percentage = percentage of the moles of one component in the total moles

of all components in a solution

Raoult’s Law

“The vapor pressure of a volatile solvent above a solution is equal to its mole fraction of its

normal vapor pressure, Po.”

Psolvent in solution = (XA)(Po)

Psolvent in solution is always less than Po.

For a solution of a nonvolatile substance, such as salt, the boiling point of the

solution is always higher than that of the pure solution.

The Equilibrium Vapor Pressure of a Solution

The Equilibrium Vapor Pressure of a Pure Solvent

Raoult’s Law

Dissolved Solids and Vapor Pressure

The effect of a dissolved solute depends on the number of solute particles.

When ionic compounds dissolve, they dissociate, creating more particles.

When molecular compounds dissolve, they remain as individual particles in solution.

For example a 1 M solution of NaCl in water contains twice as many dissolved

particles as a 1 M solution of glucose, C6H12O6.

Solutions of Volatile SolutesThe “Ideal” Case

When both the solvent and the solute can evaporate, both will be found in the vapor phase.

Ptotal = Psolvent + Psolute

The total vapor pressure will be the sum of the vapor pressures of the solvent and solute.

The solute decreases the solvent’s vapor pressure and the solvent decreases the

solute’s vapor pressure.

Psolvent = (Xsolvent) (P0solvent) Psolute = (Xsolute) (P0solute)

Raoult’s Law: Ideal Solutions

In ideal solutions, the solute-solvent interactions in the solution are equal to the solvent-solvent and solute-solute

interactions broken in the creation of the solution.

Raoult’s Law

Raoult’s Law - Deviations

If the solvent-solute interactions are weaker or stronger than those in the broken interactions in the pure substances, deviations from Raoult’s law occur.

Raoult’s Law: Non-Ideal Solutions

Colligative Properties of Solutions

Vapor Pressure Lowering

Boiling Point Elevation (as a result of reduced vapor pressure)

Freezing Point Lowering

Osmotic Pressure Changes

(a result of nonvolatile solute particles)

Boiling Point Elevation

For a nonvolatile solute, the boiling point of a solution is higher than that of the pure solvent.

BPsolution - BPsolvent = ∆Tb = (m)(Kb)

The difference between the boiling point of the solution and that of the pure solvent is

directly proportional to the molal concentration of solute particles.

Kb = boiling point elevation constant (units are ºC/m)

(the constant is solvent dependent)

Freezing Point Depression For a nonvolatile solute, the freezing point (or

melting point) of a solution is lower than that of the pure solvent.

FPsolvent - FPsolution = ∆Tf = (m)(Kf)

The difference between the freezing point of the solution and that of the pure solvent is

directly proportional to the molal concentration of solute particles.

Kf = freezing point depression constant (units are ºC/m)

(the constant is solvent dependent)

Kb and Kf for Some Common Substances

Substance Kb Kf

Benzene 2.53 5.12

Camphor 5.95 37.7

Chloroform 3.63 4.70

Ether 2.02 1.79

Ethyl alcohol 1.22 1.99

Water 0.51 1.86

Phase diagrams of solvent and solution.

A solute changes the temperature range over which a liquid remains in the liquid state.

Osmosis Osmosis - the flow of solvent through a

semipermeable membrane from a solution of low concentration to a solution of high concentration

∏ = (M)(R)(T)

Osmotic Pressure - the amount of pressure needed to prevent osmotic flow from taking place

Osmotic Pressure is directly proportional to the molarity of the solute particles.

More opportunities exist for water molecules to cross the

membrane.

Equal opportunities exist for water molecules to cross the

membrane from both directions.

isotonic solutions

∏

∏ = (M)(R)(T)∏ =(i)(M)(R)(T)

Jacobus Henricus van 't Hoff

Solute i expected i measured

non-electrolyte

1 1

NaCl 2 1.9

MgSO4 2 1.3

MgCl2 3 2.7

K2SO4 3 2.6

FeCl3 4 3.4