Properties of SolutionsProperties of Solutions

Chapter 11Chapter 11

Composition of SolutionsComposition of Solutions

Solutions = homogeneous mixtures, Solutions = homogeneous mixtures, any state of matter any state of matter

(mol solute*cation charge)

L solution

Mass Percent

Molarity

How to Calculate ConcentrationHow to Calculate Concentration

solutionLsolutemoles

100xsolutionmasssolutemass

BA

A

nnn

Molality

Normality

Mole Fraction

solventkgsolutemol

Energies of SolutionsEnergies of Solutions

““Like Dissolves Like”Like Dissolves Like”• Polar solvents dissolve polar solutesPolar solvents dissolve polar solutes• Non-Polar solvents dissolve non-polar solutesNon-Polar solvents dissolve non-polar solutes

WHY?WHY?

ENTHALPY OF SOLUTIONENTHALPY OF SOLUTION

3 part process3 part process1.1. Solute components separate (endo)Solute components separate (endo)

2.2. Overcome IMF’s in solvent (endo)Overcome IMF’s in solvent (endo)

3.3. Solvent and Solute interact (exo)Solvent and Solute interact (exo)

Enthalpy of SolutionEnthalpy of Solution

HHsolnsoln = = HH11 + + HH22 + + HH33 Heat of solution can either be + or –Heat of solution can either be + or –

is expected to be small for non-polar molecules except is expected to be small for non-polar molecules except for when they are very large molecules.for when they are very large molecules.

Hsoln is large and +

This rarely occurs

Hsoln is large and -

This usually occurs

Hsoln is close to zero

Exceptions, becauseSpontaneity does not

Depend solely on enthalpy

Try Me!Try Me!

Decide whether liquid hexane (CDecide whether liquid hexane (C66HH1414) or ) or

liquid methanol (CHliquid methanol (CH33OH) is more OH) is more

appropriate solvent for substances, grease appropriate solvent for substances, grease (C(C2020HH4242) and potassium iodide (KI).) and potassium iodide (KI).

Factors Affecting SolubilityFactors Affecting Solubility Structure---PolarityStructure---Polarity

Non polar are hydrophobicNon polar are hydrophobic Polar are hydrophillicPolar are hydrophillic Some are a combo with different ends.Some are a combo with different ends. Vitamin C and scurvyVitamin C and scurvy

Pressure---works only on gasesPressure---works only on gases C = kP (Henry’s Law)C = kP (Henry’s Law) [gas] [gas] P Pvaporvapor

Temperature---not always a direct relationshipTemperature---not always a direct relationship Gas in water; as T increases, solubility decreasesGas in water; as T increases, solubility decreases See solubility curves for individual compound reactions See solubility curves for individual compound reactions

to temperature.to temperature.

Colligative PropertiesColligative Properties

A property of a solution that depends only A property of a solution that depends only on the number, and not on the identity of on the number, and not on the identity of the solute particles present.the solute particles present. Vapor PressureVapor Pressure Boiling Point ElevationBoiling Point Elevation Freezing Point DepressionFreezing Point Depression Osmotic PressureOsmotic Pressure

Vapor PressureVapor Pressure

Non-volatile Solute added to solventNon-volatile Solute added to solvent Reduces the tendency of the solvent Reduces the tendency of the solvent

molecules to escape (evaporate)molecules to escape (evaporate) Equilibrium pressure of the solvent is higher Equilibrium pressure of the solvent is higher

than equilibrium pressure of solution. More of than equilibrium pressure of solution. More of the solvent (pure) has to evaporate to be at the solvent (pure) has to evaporate to be at equilibrium than is the case in a beaker full of equilibrium than is the case in a beaker full of solution.solution.

François Raoult and his LawFrançois Raoult and his Law

Vapor pressure of a solution is directly Vapor pressure of a solution is directly proportional to the mole fraction of the proportional to the mole fraction of the solvent present.solvent present. Psoln = XsolventPo

solvent

Just what I was thinking!!

Me Too!I agree!

Try Me!Try Me!

Calculate the expected vapor pressure at Calculate the expected vapor pressure at 2525ooC for a solution prepared by dissolving C for a solution prepared by dissolving 85.58 g of table sugar (sucrose molar 85.58 g of table sugar (sucrose molar mass = 342.3 g/mol) in 320.5 cmmass = 342.3 g/mol) in 320.5 cm33 of of water. The vapor pressure of water is water. The vapor pressure of water is 23.76 torr at this temperature.23.76 torr at this temperature.

AnswerAnswer

23.431 torr 23.431 torr

More solute, lower vapor pressureMore solute, lower vapor pressure

Predict the vapor pressure of a solution Predict the vapor pressure of a solution prepared by mixing 35.0 g solid Na2SO4 prepared by mixing 35.0 g solid Na2SO4 with 175 g water at 25oC. The vapor with 175 g water at 25oC. The vapor pressure of water is still 23.76 torr.pressure of water is still 23.76 torr.

Volatile solutesVolatile solutes

Raoult’s Law is modified to account for the Raoult’s Law is modified to account for the vapor pressure of both solute.vapor pressure of both solute.

PPtotaltotal = P = Paa + P + Pbb = X = XaaPPaaoo + X + XbbPPbb

oo

Always some degree of deviation from Always some degree of deviation from ideal…more similar the solute is to the ideal…more similar the solute is to the solvent the more ideal it behaves.solvent the more ideal it behaves.

Boiling Point ElevationBoiling Point Elevation

When a solute is added, vapor pressure is When a solute is added, vapor pressure is lowered.lowered.

Must be heated to higher temperature to Must be heated to higher temperature to give reach normal boiling point.give reach normal boiling point.

Boiling point is higher than usual.Boiling point is higher than usual. T = KT = Kbbmmsolutesolute

Freezing Point DepressionFreezing Point Depression

When a solute is added, the vapor When a solute is added, the vapor pressure is lowered. pressure is lowered.

Water in the solution has a lower vapor Water in the solution has a lower vapor pressure than that of pure ice.pressure than that of pure ice.

Ice will not form.Ice will not form. Freezing point has been lowered.Freezing point has been lowered. T = KT = Kffmmsolutesolute

Try Me!Try Me!

A chemist is trying to identify a human A chemist is trying to identify a human hormone, which controls metabolism by hormone, which controls metabolism by determining its molar mass. A sample determining its molar mass. A sample weighing 0.546 g was dissolved in 15.0 g weighing 0.546 g was dissolved in 15.0 g benzene, and the freezing point benzene, and the freezing point depression was determined was depression was determined was determined to be 0.240determined to be 0.240ooC. Calculate the C. Calculate the molar mass of the hormone.molar mass of the hormone.

Osmotic PressureOsmotic Pressure

Solvent, but not solute can pass through a Solvent, but not solute can pass through a semi-permeable membrane via osmosis.semi-permeable membrane via osmosis.

When pure solvent and solution volumes When pure solvent and solution volumes are stabilized (the system is at equilibrium) are stabilized (the system is at equilibrium) there is a greater pressure on the solution there is a greater pressure on the solution side.side.

The difference in pressure is the Osmotic The difference in pressure is the Osmotic Pressure.Pressure.

= MRT= MRT

Used to determine osmotic pressure.Used to determine osmotic pressure.• P is osmotic pressureP is osmotic pressure• M is molarity of solutionM is molarity of solution• R is the gas law constant (0.0821 Latm/Kmol)R is the gas law constant (0.0821 Latm/Kmol)• T is the temperature in KelvinT is the temperature in Kelvin

Colligative Properties of Electrolyte Colligative Properties of Electrolyte SolutionsSolutions

The type of solute DOES matter The type of solute DOES matter sometimessometimes

Only in ability to dissociateOnly in ability to dissociate

Compare: 0.1 m glucose to 0.1m NaClCompare: 0.1 m glucose to 0.1m NaCl

Van’t Hoff FactorVan’t Hoff Factor

Van’t Hoff Factor expresses the Van’t Hoff Factor expresses the relationship between moles of solute and relationship between moles of solute and solution:solution:

This is a multiplier number that describes This is a multiplier number that describes the ratio of elevation or depression.the ratio of elevation or depression.

i = moles particles moles solute

i = moles particles moles solute

T = iKbm T = iKfm = iMRT

Ion PairingIon Pairing

Van’t Hoff factor is fallible. Van’t Hoff factor is fallible. Ion Pairing: at a certain instant, some ions Ion Pairing: at a certain instant, some ions

may not be dissociated, and thus count as may not be dissociated, and thus count as a single particle.a single particle.

Still use equations, but keep in mind that Still use equations, but keep in mind that there can be some deviation.there can be some deviation.

ColloidsColloids

Suspension of tiny particles in a medium Suspension of tiny particles in a medium caused by electrostatic repulsion.caused by electrostatic repulsion.

Tyndall Effect: Scattering of light effect by Tyndall Effect: Scattering of light effect by suspended particles.suspended particles.