Download - Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Properties of Solutions Chapter 11

Copyright©2000 by Houghton Mifflin Company. All rights reserved.

1

Properties of Solutions

Chapter 11


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Overview

Introduce student to solution composition and energy of solution formation.

Factor affecting solubilities will be discussed: structure, pressure and temperature effects.

Vapor pressure of solutions, boiling point, freezing point affected by solute addition.

Colligative properties of electrolyte solutions and colloids.


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A solution is a homogenous mixture of 2 or more substances

The solute is(are) the substance(s) present in the smaller amount(s)

The solvent is the substance present in the larger amount


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A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature.

An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature.

A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature.

Sodium acetate crystals rapidly form when a seed crystal isadded to a supersaturated solution of sodium acetate.


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Solution Composition

1. Molarity (M) =

2. Mass (weight) percent =

3. Mole fraction (A) =

4. Molality (m) =

moles of soluteliters of solution

mass of solutemass of solution

100%

molestotal moles in solution

A

moles of solutekilograms of solvent


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Concentration UnitsThe concentration of a solution is the amount of solute present in a given quantity of solvent or solution.

Percent by Mass

% by mass = x 100%mass of solutemass of solute + mass of solvent

= x 100%mass of solutemass of solution

Mole Fraction (X)

XA = moles of A

sum of moles of all components


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Concentration Units Continued

M =moles of solute

liters of solution

Molarity (M)

Molality (m)

m =moles of solute

mass of solvent (kg)


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N =Number of equivalent

liters of solution

Normality (N)

Acid-Base reaction: is the amount needed to accept one mole of H+

Number of equivalent

=mass

Equivalent massx

1

V

Equivalent mass of H2SO4 = MM

2

Equivalent mass of Ca(OH)2 = MM

2


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Redox reaction : is the amount needed to accept exactly one mole e-

MnO4- + 5e- + 8H+ Mn2+ + 4H2O

Equivalent mass of KMnO4 = MM

5


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What is the molality of a 5.86 M ethanol (C2H5OH) solution whose density is 0.927 g/mL?

m =moles of solute

mass of solvent (kg)M =

moles of solute

liters of solution

Assume 1 L of solution:5.86 moles ethanol = 270 g ethanol927 g of solution (1000 mL x 0.927 g/mL)

mass of solvent = mass of solution – mass of solute

= 927 g – 270 g = 657 g = 0.657 kg

m =moles of solute

mass of solvent (kg)=

5.86 moles C2H5OH

0.657 kg solvent= 8.92 m


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Steps in Solution Formation

Step 1 - Expanding the solute (endothermic)

Step 2 - Expanding the solvent (endothermic)

Step 3 - Allowing the solute and solvent to interact to form a solution (exothermic)

Hsoln = Hstep 1 + Hstep 2 + Hstep 3


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The formation of a liquid solution can be divided into three steps: (1) expanding the solute, (2) expanding the solvent, and (3)

combining the expanded solute and solvent to form the solution.


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H < 0 ExothermicSolution will occur


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The driving factor that favor a process of solution formation is an increase in disorder


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Factor Affecting Solubility

1. Structure

2. Pressure

3. Temperature


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“like dissolves like”

Two substances with similar intermolecular forces are likely to be soluble in each other.

• non-polar molecules are soluble in non-polar solvents

CCl4 in C6H6

• polar molecules are soluble in polar solvents

C2H5OH in H2O

• ionic compounds are more soluble in polar solvents

NaCl in H2O or NH3 (l)


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The molecular structures of (a) vitamin A (nonpolar, fat-soluble) and (b) vitamin C (polar, water-soluble). The circles in the

structural formulas indicate polar bonds. Note that vitamin C contains far more polar bonds than vitamin A.

Fat Soluble

Hydrophobic

Water Soluble

Hydrophilic


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Pressure Effects

• Pressure has little effect on solids and liquids.

• It increases the solubility of gases.


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(a) A gaseous solute in equilibrium with a solution. (b) The piston is pushed in, increasing the pressure of the gas and number of gas molecules per unit volume. This causes an increase in the rate at which the gas enters the solution, so the concentration of dissolved gas increases. (c) The greater gas concentration in the solution causes an

increase in the rate of escape. A new equilibrium is reached.


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Henry’s Law

C = kPC = concentration of dissolved gas

P = partial pressure of gaseous solute above the solution

k = a constant

The amount of a gas dissolved in a solution is The amount of a gas dissolved in a solution is directly proportional to the pressure of the gas directly proportional to the pressure of the gas above the solution.above the solution.


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Henry’s Law Applied for

• Dilute solutions

• Gases that do not dissociate or react with solvent:– O2/water Applied

– HCl/water Is not applied


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Temperature Effect on Gases

The solubilities of several gases in water as a function of temperature at a constant pressure of 1 atm of gas above the solution.


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The solubilities of several solids as a function of temperature.

Temperature Effect on Gases


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The Vapor Pressure of Solutions

• Solutions have different physical properties from pure solvent.

• Solutions of nonvolatile solutes differ from solutions of volatile solvents.


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An aqueous solution and pure water in a closed environment. (a) Initial stage. (b) After a period of time,

the water is transferred to the solution.

Vapor pressure of pure water is higher VP of solution is lower


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Raoult’s Law

Psoln = solvent Psolvent

Psoln = vapor pressure of the solution

solvent = mole fraction of the solvent

Psolvent = vapor pressure of the pure solvent

The presence of a The presence of a nonvolatilenonvolatile solute solute lowerslowers the vapor pressure of a solvent.the vapor pressure of a solvent.

solvent =Moles of solvents

Moles of solvent + Moles of solute


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Note

• Solutions that obey Raoult’s Law are called Ideal Solutions.

• Can be used to determine the molar mass of unknown.

• For ionic compounds you should multiply by the total number of ions per molecule

• e.g. Na2SO4 n = 3 x Na2SO4


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For a solution that obeys Raoult's law, a plot of Psoln versus xsolvent

gives a straight line.


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Non-Ideal Solutions

When a solution contains two volatile components, both contribute to the total vapor pressure.


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PA = XA P A0

PB = XB P B0

PT = PA + PB

PT = XA P A0 + XB P B

0


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(a) ideal liquid-liquid solution by Raoult's law. (b) This solution shows a positive deviation from Raoult's law. (c) This solution

shows a negative deviation from Raoult's law.


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PT is greater thanpredicted by Raoults’s law

PT is less thanpredicted by Raoults’s law

ForceA-B

ForceA-A

ForceB-B< &

ForceA-B

ForceA-A

ForceB-B> &


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Colligative Properties of Non-Electrolyte Solutions

Depend only on the number, not on the identity, of the solute particles in an ideal solution.

Boiling point elevation Freezing point depression Osmotic pressure


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Boiling Point Elevation

A nonvolatile solute elevates the boiling point of the solvent.

T = Kbmsolute

Kb = molal boiling point elevation constant

m = molality of the solute


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Boiling-Point Elevation

Tb = Tb – T b0

Tb > T b0 Tb > 0

T b is the boiling point of the pure solvent

0

T b is the boiling point of the solution

Tb = Kb m

m is the molality of the solution

Kb is the molal boiling-point elevation constant (0C/m)


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Freezing Point Depression

A nonvolatile solute depresses the freezing point of the solvent.

T = Kfmsolute

Kf = molal freezing point depression constant

m = molality of the solute


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Freezing-Point Depression

Tf = T f – Tf0

T f > Tf0 Tf > 0

T f is the freezing point of the pure solvent

0

T f is the freezing point of the solution

Tf = Kf m

m is the molality of the solution

Kf is the molal freezing-point depression constant (0C/m)


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Osmotic Pressure

Osmosis: The flow of solvent into the solution through the semipermeable membrane.

Osmotic Pressure: The excess hydrostatic pressure on the solution compared to the pure solvent.


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Osmotic Pressure ()

Osmosis is the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one.

A semipermeable membrane allows the passage of solvent molecules but blocks the passage of solute molecules.

Osmotic pressure () is the pressure required to stop osmosis.

= MRT

M is the molarity of the solution

R is the gas constant

T is the temperature (in K)


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A tube with a bulb on the end that is covered by a semipermeable membrane.


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The normal flow of solvent into the solution (osmosis) can be prevented by applying an external pressure to the solution. The minimum pressure required to stop the osmosis is equal to the osmotic pressure of the solution.


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(a) A pure solvent and its solution (containing a nonvolatile solute) are separated by a semipermeable membrane through which solvent molecules

(blue) can pass but solute molecules (green) cannot. The rate of solvent transfer is greater from solvent to solution than from solution to solvent. (b) The system at equilibrium, where the rate of solvent transfer is the same in both directions.


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If the external pressure is larger than the osmotic pressure, reverse osmosis occurs.

One application is desalination of seawater.


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Reverse osmosis.


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A cell in an:

isotonicSolution

(identical )

hypotonicSolution

(water in)

hypertonicSolution

(water out)


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Colligative Properties of Electrolyte Solutions

T = imK

= iMRT

i = moles of particles in solution

moles of solute dissolved

van’t Hoff factor, “van’t Hoff factor, “ii”, relates to the number of ”, relates to the number of ions per formula unit.ions per formula unit.

NaCl = 2, KNaCl = 2, K22SOSO44 = 3 = 3


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Boiling-Point Elevation Tb = i Kb m

Freezing-Point Depression Tf = i Kf m

Osmotic Pressure () = iMRT

Colligative Properties of Electrolyte Solutions


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In an aqueous solution a few ions aggregate, forming ion pairs that

behave as a unit: Ion Pairing


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Colloids

Colloidal Dispersion (colloid): A suspension of tiny particles in some medium.

aerosols, foams, emulsions, sols

Coagulation: The addition of an electrolyte, causing destruction of a colloid.


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A representation of two colloidal particles.

The opposing charge repel and prevent precipitation

The destruction of colloids is called coagulation, achieved by1. Increasing temperature where colliding at higher veloicities

to penetrate the ion barriers and aggregate.2. Adding electrolytes to neutralize ion layers, this is why clay

deposits where rivers reach the oceans.


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A colloid is a dispersion of particles of one substance throughout a dispersing medium of another substance.

Colloid versus solution

• colloidal particles are much larger than solute molecules

• colloidal suspension is not as homogeneous as a solution


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The Cleansing Action of Soap


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The Cottrell precipitator installed in a smokestack. The charged plates attract

the colloidal particles because of their ion layers and thus remove them from the smoke.

Download - Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Properties of Solutions Chapter 11

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