Chapter 12 Physical Properties of Solutions General Chemistry: An Integrated Approach Hill, Petrucci, 4 th Edition Mark P. Heitz State University of New

Chapter 12Physical Properties of Solutions

General Chemistry: An Integrated Approach

Hill, Petrucci, 4th Edition

Mark P. HeitzState University of New York at Brockport

© 2005, Prentice Hall, Inc.

Chapter 12: Physical Properties of Solutions 2

Some Types of Solutions

EOS


Solution ConcentrationMolarity is the amount of solute dissolved into a volume of solvent – the ratio is moles solute to liters solution = mol L–1

Percent by mass is the mass ratio of solute to solution multiplied by 100

Percent by volume is the volume ratio of solute to solution multiplied by 100

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Mass/volume percent is the mass of solute divided by the volume of solution multiplied by 100


Solutions by “Parts”

Parts per billion is the number of particles of solute per one billion particles of solution.

1 ppb = 1 g/L

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Parts per trillion is the number of particles of solute per one trillion particles of solution.

1 ppt = 1 ng/L

Parts per million is the number of particles of solute per one million particles of solution

units are 1 ppm = 1 mg/L


Molality

Molality (m) is the number of moles of solute per one kilogram of solvent (not solution!)

EOS

To be independent of temperature, a concentration unit must be based on mass only, not volume

Molarity (M) varies with temperature due to the expansion or contraction in the volume of the solution


Mole Fraction and Mole PercentThe mole fraction (xi) of a solution component i is the fraction of all the molecules in the solution that are molecules of i

total

( ) ii

nmol ix

total mol n

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The mole percent of a solution component is its mole fraction multiplied by 100

Sum of xi must equal 1


Enthalpy of Solution

Solution formation can be considered to take place in three steps

Move the molecules of solvent apart to make room for the solute molecules. H1 > 0

Separate the molecules of solute to the distances found between them in the solution. H2 > 0

Allow the separated solute and solvent molecules to mix randomly. H3 > 0

EOS

Hsoln = H1 + H2 + H3


Visualizing Enthalpy of Solution

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Intermolecular Forcesin Solution Formation

If all intermolecular forces are of comparable strength, this type of solution is called an ideal solution and Hsoln = 0

If the intermolecular forces between solute and solvent molecules are stronger than other intermolecular forces, Hsoln < 0

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For solute/solvent forces that are weaker than other intermolecular forces, Hsoln > 0


Non-ideal Solutions

Ethanol and water have strong attractions – when mixed, the volume of 50 mL H2O + 50 mL EtOH is less than 100 mL

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Aqueous Solutions of Ionic Compounds

The forces causing an ionic solid to dissolve in water are ion–dipole forces, the attraction of water dipoles for cations and anions.

EOS

The attractions of water dipoles for ions pulls the ions out of the crystalline lattice and into aqueous solution


When there is a dynamic equilibrium between an undissolved solute and a solution, the solution is saturated

Some Solubility TermsLiquids that mix in all proportions are called miscible

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A solution which contains less solute than can be held at equilibrium is unsaturated


SolubilityThe concentration of the solute in a saturated solution is the solubility of the solute

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About 95% of all ionic compounds have aqueous solubilities that increase with increasing temperature


SupersaturationA supersaturated solution is created when a warm, saturated solution is allowed to cool without the precipitation of the excess solute

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Selective Crystallization

When KNO3(s) is crystallized from an aqueous solution of KNO3 containing CuSO4 as an impurity, CuSO4 (blue) remains in the solution

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The Solubilities of Gases

Most gases become less soluble in liquids as the temperature increases

a common example is carbonated beverages

At a constant temperature, the solubility (S) of a gas is directly proportional to the pressure of the gas (Pgas) in equilibrium with the solution.

S = k Pgas

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The effect of pressure on the solubility of a gas is known as Henry’s law


Temperature and Solubility of Gases

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Pressure and Solubility of Gases

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The concentration of the solution increases

Gas compressed into a smaller volume increases the number of molecules per unit volume


Solubility and Gas Pressure

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Vapor Pressures of Solutions

Raoult’s law states that the vapor pressure of the solvent above a solution (Psolv) is the product of the vapor pressure of the pure solvent (Po

solv) and the mole fraction of the solvent in the solution (xsolv)

EOS

Psolv = xsolv . Posolv


Solution SeparationThe vapor in equilibrium with an ideal solution of two volatile components has a higher mole fraction of the more volatile component than is found in the liquid

EOS

Separation can be achieved by fractional distillation


Colligative PropertiesFreezing Point Depression (FPD)

Boiling Point Elevation (BPE)

Consider situations with a volatile solvent and a solute that is nonvolatile, nonelectrolytic, and soluble in the liquid solvent, but not the solid solvent

EOS

The vapor pressure of the solution is that of the solvent in the solution, and at all temperatures this vapor pressure is lower than that of the pure solvent


FPD and BPE

The presence of the solute lowers (depresses) the freezing point of the solvent (Tf) and increases (elevates) the boiling point of the solvent (Tb)

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Example: adding salt to water allows the water temperature to exceed 100 oC, thereby cooking food faster


Vapor Pressure Lowering

In a solution, the solvent vapor pressure is lowered and the fusion curve is displaced to lower temperatures

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FPD and BPE Constants

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Osmosis

Semipermeable membranes are usually films of a material containing a network of microscopic pores through which small solvent molecules can pass, but larger solute molecules cannot

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Osmosis is the net flow of solvent molecules from pure solvent through a semipermeable membrane into a solution


Osmotic PressureThe pressure required to stop osmosis is called the osmotic pressure of the solution = (n/V)RT = M RT

A solution (green) is separated from pure water by a membrane permeable to H2O molecules but not to solute particles

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When the flow of water is at equilibrium, the hydrostatic pressure is now called the osmotic pressure, .


Osmosis Applications

Water purification:

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Reverse osmosis is the process of reversing the normal net flow of solvent molecules through a semipermeable membrane by applying to the solution a pressure exceeding the osmotic pressure


Solutions of Electrolytes

The van’t Hoff factor (i) is used to modify the equations for colligative properties

FPD: Tf = –i Kfm BPE: Tb = i Kbm OP: = i M RT

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i is dependent on solution molality


Colloids

A colloid is a dispersion in an appropriate medium for particles ranging in size from about 1 to 1000 nm

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The scattering of a light beam through a colloidal material is known as the Tyndall effect


Common Colloids

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Formation and Coagulationof a Colloid

A high concentration of an electrolyte can cause a colloid to coagulate, or precipitate

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A Suspension and a Colloid

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Summary of Concepts

• Molarity (M) is expressed as moles per liter and molality (m) is expressed as moles of solute per kilogram of solvent

• Units used for very dilute concentrations of solutes include parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt)

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• The type and magnitude of intermolecular forces are important in solution composition


Summary of Concepts

• The solubility of a solute is its concentration in a saturated solution

• The solubility of gases in solutions decreases with an increase in temperature but increases with an increase in pressure

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• The presence of solutes lowers the vapor pressure of the solution and causes both a freezing point depression and a boiling point elevation of the solvent


Summary of Concepts

• Colligative properties depend on the particular solvent and the number of solute particles present

• Colloids are dispersions in an appropriate medium of particles ranging in size from 1 nm to 1000 nm

EOS

Documents

Chapter 12 Physical Properties of Solutions General Chemistry: An Integrated Approach Hill, Petrucci, 4 th Edition Mark P. Heitz State University of New