properties of matter and solution

7/27/2019 Properties of Matter and Solution

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Chapter 13Properties of Solutions

Learning goals and key skills:

Describe how enthalpy and entropy changes affect solution formation Describe the relationship between intermolecular forces and solubility, like dissolves like

Describe the role of equilibrium in the solution process and relationship to the solubility of a

solute

Describe the effect of temperature on solubility of solids and gases

Describe the relationship between partial pressure of a gas and solubility

Calculate the concentration of a solution in terms of molarity, molality, mole fraction, percent

composition, and ppm and be able to interconvert between them.

Describe what a colligative property is and explain the difference between the effects of

nonelectrolytes and electrolytes on colligative properties.

Calculate the vapor pressure of a solvent over a solution

Be able to calculate the boiling point elevation and freezing point depression of a solution

Calculate the osmotic pressure of a solution

Explain the difference between a solution and a colloid

Classificationof Matter

Classificationof Matter

MixturesMixtures

Mixture Have variable composition and can be separated

into component parts by physical methods. Mixtures containmore than one kind of molecule, and their properties

depend on the relative amount of each component

presentin the mixture.

Homogeneous Mixture (solution) Uniform composition.

Gaseous solution air (N2, O2, CO2, etc)

Liquid solution seawater (H2O, salts, etc)

Solid solution brass (Cu and Zn)


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SolutionsSolutions

Solutions are homogeneous mixtures

consisting of a solvent and one or moresolutes.

In a solution, the solute is disperseduniformly throughout the solvent.

SolutionsSolutions

As a solution forms, the solvent pulls soluteparticles apart and surrounds, or solvates, them.

The solute-solvent interactions compete with thesolute-solute and solvent-solvent interactions.

Aqueous solutionsAqueous solutions

Aqueous solutions made fromionic salts have ion-dipoleinteractions that are strongenough to overcome thelattice energyof the saltcrystal.

For aqueous solutions, solute(H2O)-solvent interactions arereferred to as hydration.


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Energetics of solutionsEnergetics of solutions

Energetics of solutionsEnergetics of solutions

Hsoln = Hsolute + Hsolvent + Hmix

Hsoln for a solid dissolving in a liquid is usually slightly endothermic

EntropyEntropy

The reason is that increasingthe entropy (i.e., disorder orrandomness) of a system tendsto lower the energy (notenthalpy) of the system.


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Physical dissolution vschemical reaction

Physical dissolution vschemical reaction

Here is a single displacement/redox reaction:

Ni (s) + 2 HCl (aq) NiCl2 (aq) + H2 (g)

We cant get back the original Ni (or HCl) byphysical methods, so this is NOT physicaldissolution it is a chemical reaction.

Saturated vs unsaturatedsolutions

Saturated vs unsaturatedsolutions

Saturated Solvent holds as much

solute as is possible at

that temperature.

Dissolved solute is indynamic equilibrium with

solid solute particles.

Unsaturated Less than the maximum

amount of solute is

dissolved in the solvent atthat temperature.

Supersaturated solutionsSupersaturated solutions

Solvent holds more solute than is normallypossible at that temperature.

These solutions are unstable; crystallizationcan usually be stimulated by adding a seedcrystal or scratching the side of the flask.


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Like dissolves likeLike dissolves like

Polar substances tend todissolve in polar solvents.

Nonpolar substances tend todissolve in nonpolar solvents.

The more similar the

intermolecular

attractions, the morelikely one substance is tobe soluble in another.

The more similar the

intermolecular

attractions, the morelikely one substance is tobe soluble in another.

Structure andSolubility

Structure andSolubility

Gases in solutionGases in solution

In general, thesolubility of gases inwater increases with

increasing mass.

Larger molecules havestronger Londondispersion forces.


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Gases in solutionGases in solution

The solubility of liquids and solids does notchange appreciably with pressure.

The solubility of a gas in a liquid is directlyproportional to its pressure.

Pressure and solubility:Henrys Law

Pressure and solubility:Henrys Law

Sg = kPg

Sg is the solubility of thegas

kis the Henrys lawconstant for that gas inthat solvent

Pg is the partial pressureof the gas above the

liquid.

Temperature and solubilityTemperature and solubility

Generally, the

solubility ofsolid solutes inliquid solventsincreases withincreasingtemperature.


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Temperature and solubilityTemperature and solubility

The opposite istrue of gases:

Carbonated softdrinks are more

bubbly if storedin the refrigerator.

Warm lakes haveless O2 dissolvedin them than coollakes.

Colligative propertiesColligative properties

colligative properties depend only on thenumberof solute particles present, noton the identityof the solute particles.

Four important colligative properties are

Vapor pressure lowering

Boiling point elevation

Melting point depression

Osmotic pressure forms/increases

Expressing concentrationsExpressing concentrations

Mass % of A =mass of A in solution

total mass of solution 100

ppm =mass of A in solution


Parts per Million (ppm)

Parts per Billion (ppb)

ppb =mass of A in solution



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moles of Atotal moles in solution

Mole Fraction XA =

mol of solute

L of solutionMolarity M=

mol of solutekg of solvent

Molality m=

Changing Molarity to MolalityChanging Molarity to Molality

If we knowthe density ofthe solution,we cancalculate themolality fromthe molarity,and viceversa.

Example 1Example 1

Dissolve 62.1 g (1.00 mol) of ethylene glycol,C2H6O2, in 250. g H2O. Calculate the masspercentage of ethylene glycol, mole fractionof ethylene glycol, and molality.


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Example 2Example 2

A saturated solution of manganese (II) chloride

(FW = 125.84 g/mol) in H2O (MW = 18.02 g/mol)is 43.6% MnCl2 by weight. Calculate the molalityof the saturated solution.

Example 3Example 3

A solution is made from dissolving lithiumbromide (FW = 86.845 g/mol) in acetonitrile(CH3CN, 41.05 g/mol). Calculate the molality ifthe 1.80 molar solution has a density of 0.826g/mL.

Vapor PressureVapor Pressure

Because of solute-solvent intermolecularattraction, higherconcentrations of

nonvolatile solutesmake it harder forsolvent to escape tothe vapor phase.

Therefore, the vaporpressure of a solutionis lower than that ofthe pure solvent.


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Vapor pressure: Raoults LawVapor pressure: Raoults Law

Psolution =XsolventPsolvent

Xis the mole fraction of the SOLVENT

P is the normal vapor pressure ofSOLVENTat that temperature

The extent to which a nonvolatile solute lowers thevapor pressure is proportional to its concentration.

Example 4Example 4

At 20 C the vapor pressure of water is 17.5 torr.If we add enough glucose, C6H12O6, to obtain

XH2O = 0.800

XC6H12O6 = 0.200

What is the vapor pressure?

Boiling Point Elevation andFreezing Point DepressionBoiling Point Elevation andFreezing Point Depression


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Boiling point elevationBoiling point elevation

Tb = Kb m

Kb is the molal boiling point elevation constant,a solvent dependent property.

Tbis added tothe normal boiling point of the solvent.

Freezing point depressionFreezing point depression

Tf= Kf m

Kf is the molal freezing point depression constant ofthe solvent.

Tfis subtracted fromthe normal freezing point ofthe solvent.

Example 5Example 5

Antifreeze consists of ethylene glycol, C2H6O2, anonvolatile nonelectrolyte. Calculate the boiling pointand freezing point of a 25.0% (weight) aqueoussolution. K

b,H2O= 0.52 C/m, K

f,H2O= 1.86 C/m


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Colligative Properties ofElectrolytes

Colligative Properties ofElectrolytes

Since these properties depend on the

number of particles dissolved, solutionsof electrolytes (which dissociate insolution) should show greater changesthan those of nonelectrolytes.

vant Hoff factorvant Hoff factor

Tb = Kb m i

Tf= Kf m i

for dilute solutions, i whole numberNote: The vant Hoff factor can also be used in other

colligative properties.

Example 6Example 6

Arrange the following aqueous solutions in order ofdecreasing freezing point.

(a) 0.20 m ethylene glycol

(b) 0.12 m potassium sulfate

(c) 0.10 m magnesium chloride

(d) 0.12 m potassium bromide


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OsmosisOsmosis

Some substances form semipermeable

membranes, allowing some smaller particlesto pass through, but blocking other largerparticles.

In biological systems, most semi-permeablemembranes allow water to pass through, butsolutes are not free to do so.

In osmosis, there is net movement of solventfrom the area ofhigher solventconcentration (lower solute concentration) tothe area oflower solvent concentration(higher solute concentration).

OsmosisOsmosis

Osmotic pressureOsmotic pressure

The pressure required to stop osmosis,known as osmotic pressure, , is

nV

=( )RT = MRTwhere Mis the molarity of the solution

If the osmotic pressure is the same on both sidesof a membrane (i.e., the concentrations are thesame), the solutions are isotonic.


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Osmosis in cellsOsmosis in cells

Osmosis and cellsOsmosis and cells

Example 7Example 7

3.50 mg of a protein is dissolved in water to form a5.00 mL solution. The osmotic pressure was foundto be 1.54 torr at 25 C. Calculate the molar massof the protein.


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Reverse osmosiswater desalinationReverse osmosis

water desalination

Water desalination plant in Tampa

Colloidal dispersions or colloidsColloidal dispersions or colloids

Suspensions of particles larger than individual ions ormolecules, but too small to be settled out by gravity.

You can think of them as somewhere in betweenhomogeneous and heterogeneous mixtures.

Tyndall effectTyndall effect

Colloid particles arelarge enough toscatter light.

Most colloids appear

cloudy or opaque.


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Colloids in biological systemsColloids in biological systems

Some molecules

have a polar,hydrophilic (water-loving) end and anonpolar,hydrophobic (water-hating) end.

Colloids in biological systemsColloids in biological systems

These molecules can aid in the emulsification offats and oils in aqueous solutions.

properties of matter and solution

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