Blb12 Ch13 Lecture

8/12/2019 Blb12 Ch13 Lecture

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Chapter 13

Properties ofSolutions

Lecture Presentation

John D. BookstaverSt. Charles Community College

Cottleville, MO 2012 Pearson Education, Inc.


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Solutions

Solutions

Solutions arehomogeneous

mixtures of two ormore puresubstances.

In a solution, thesolute is disperseduniformly throughoutthe solvent .

2012 Pearson Education, Inc.


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Solutions

Solutions

The intermolecular forces between solute and

solvent particles must be strong enough tocompete with those between solute particlesand those between solvent particles.



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Solutions

How Does a Solution Form?

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



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Solutions

How Does a Solution Form?

If an ionic salt issoluble in water, it is

because the ion dipole interactionsare strong enoughto overcome the

lattice energy of thesalt crystal.



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Solutions

Energy Changes in Solution

Simply put, threeprocesses affect theenergetics of solution:

Separation of soluteparticles,

Separation of solventparticles,

New interactionsbetween solute andsolvent.



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Solutions

Energy Changes in Solution

The enthalpychange of the

overall processdepends on H foreach of these steps.



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Solutions

Why Do EndothermicProcesses Occur?

Things do not tend tooccur spontaneously

(i.e., without outsideintervention) unlessthe energy of thesystem is lowered.



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Solutions

Enthalpy Is Only Part of the Picture

The reason is thatincreasing the disorder

or randomness (knownas entropy ) of asystem tends to lowerthe energy of the

system.



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Solutions

Enthalpy Is Only Part of the Picture

So even thoughenthalpy may increase,

the overall energy ofthe system can stilldecrease if the systembecomes more

disordered.



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Solutions

Student, Beware!

Just because a substance disappears when itcomes in contact with a solvent, it doesntmean the substance dissolved. It may havereacted.



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Solutions

Student, Beware!

Dissolution is a physical change you can get back theoriginal solute by evaporating the solvent.

If you cant get it back, the substance didnt dissolve, itreacted.



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Solutions

Types of Solutions

Saturated In a saturated

solution, the solventholds as much soluteas is possible at thattemperature.

Dissolved solute is indynamic equilibriumwith solid soluteparticles.



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Solutions

Types of Solutions

Unsaturated If a solution is

unsaturated , lesssolute than candissolve in thesolvent at thattemperature is

dissolved in thesolvent.



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Solutions

Types of Solutions

Supersaturated In supersaturated solutions, the solvent holds

more solute than is normally possible at thattemperature. These solutions are unstable; crystallization can

usually be stimulated by adding a seed crystal orscratching the side of the flask.



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Solutions

Factors Affecting Solubility

Chemists use the axiom like dissolves like. Polar substances tend to dissolve in polar solvents. Nonpolar substances tend to dissolve in nonpolar

solvents.



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Solutions


The more similar theintermolecular

attractions, the morelikely one substanceis to be soluble inanother.



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Solutions


Glucose (which hashydrogen bonding)

is very soluble inwater, whilecyclohexane (whichonly has dispersion

forces) is not.



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Solutions


Vitamin A is soluble in nonpolar compounds(like fats).

Vitamin C is soluble in water.



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Solutions

Gases in Solution

In general, thesolubility of gases in

water increases withincreasing mass. Larger molecules

have strongerdispersion forces.



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Solutions

Gases in Solution

The solubility ofliquids and solidsdoes not change

appreciably withpressure.

But the solubility ofa gas in a liquid isdirectly proportionalto its pressure.



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Solutions

Henrys Law

S g = kP g

where

S g is the solubility ofthe gas,

k is the Henrys Lawconstant for that gas in

that solvent, and P g is the partialpressure of the gasabove the liquid.



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Solutions

Temperature

Generally, thesolubility of solid

solutes in liquidsolvents increaseswith increasingtemperature.



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Solutions

Temperature

The opposite is trueof gases.

Carbonated softdrinks are morebubbly if stored inthe refrigerator.

Warm lakes haveless O 2 dissolved inthem than cool lakes.



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Solutions

Ways of

ExpressingConcentrationsof Solutions



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Solutions

Mass Percentage

Mass % of A = mass of A in solutiontotal mass of solution 100



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Solutions

Parts per Million andParts per Billion

ppm =mass of A in solutiontotal mass of solution 10

6

Parts per million (ppm)

Parts per billion (ppb)

ppb =mass of A in solutiontotal mass of solution 10

9



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Solutions

moles of Atotal moles of all components X A =

Mole Fraction ( X )

In some applications, one needs themole fraction of solvent , not solute make sure you find the quantity youneed!



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Solutions

moles of soluteliters of solutionM =

Molarity ( M )

You will recall this concentrationmeasure from Chapter 4.

Since volume is temperature-dependent, molarity can change withtemperature.



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Solutions

moles of solutekilograms of solventm =

Molality ( m)

Since both moles and mass do notchange with temperature, molality(unlike molarity) is not temperature-dependent.



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Solutions

Colligative Properties

Changes in colligative properties depend only on the number of soluteparticles present, not on the identity of

the solute particles. Among colligative properties are

Vapor-pressure lowering

Boiling-point elevation Melting-point depression Osmotic pressure



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Solutions

Vapor Pressure

Because of solute solvent intermolecularattraction, higher concentrations ofnonvolatile solutes make it harder for solventto escape to the vapor phase.



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Solutions

Vapor Pressure

Therefore, the vapor pressure of a solution islower than that of the pure solvent.



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Solutions

Raoults Law

P A = X AP A where

X A is the mole fraction of compound A, and P A is the normal vapor pressure of A at

that temperature.

Note: This is one of those times when youwant to make sure you have the vaporpressure of the solvent .



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Solutions

Boiling-Point Elevation

The change in boilingpoint is proportional tothe molality of thesolution:

T b = K b m

where K b is the molal

boiling-point elevationconstant, a property ofthe solvent.T b is added to the normal

boiling point of the solvent. 2012 Pearson Education, Inc.


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B ili P i El i d


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Solutions

Boiling-Point Elevation andFreezing-Point Depression

Note that in bothequations, T does

not depend on whatthe solute is , butonly on how many

particles are

dissolved.

T b = K b m

T f = K f m


C lli i P i f


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Solutions

Colligative Properties ofElectrolytes

Since the colligative properties of electrolytes dependon the number of particles dissolved, solutions ofelectrolytes (which dissociate in solution) should showgreater changes than those of nonelectrolytes.


C lli ti P ti f


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Solutions

Colligative Properties ofElectrolytes

However, a 1 M solution of NaCl does not showtwice the change in freezing point that a 1 M solution of methanol does.



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Solutions

vant Hoff Factor

One mole of NaCl inwater does not

really give rise totwo moles of ions.



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Solutions

vant Hoff Factor

Some Na + and Cl reassociate for ashort time, so thetrue concentration ofparticles issomewhat less thantwo times theconcentration ofNaCl.



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Solutions

vant Hoff Factor

Reassociation ismore likely at higherconcentration.

Therefore, thenumber of particlespresent is

concentration-dependent.



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Solutions

Osmosis

Some substances form semipermeablemembranes , allowing some smallerparticles to pass through, but blockingother larger particles.

In biological systems, mostsemipermeable membranes allow waterto pass through, but solutes are not freeto do so.



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Solutions

Osmosis

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



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Solutions

Osmotic Pressure

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

= ( )RT = MRTnVwhere M is 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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Solutions

Osmosis in Blood Cells

If the soluteconcentration outsidethe cell is greater thanthat inside the cell, thesolution is hypertonic .

Water will flow out ofthe cell, and crenation results.



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Solutions

Osmosis in Cells

If the soluteconcentration outsidethe cell is less than

that inside the cell, thesolution is hypotonic .

Water will flow into thecell, and hemolysis results.



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Solutions

Colloids

Suspensions of particles larger thanindividual ions or molecules, but too small tobe settled out by gravity, are called colloids .



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Solutions

Tyndall Effect

Colloidal suspensionscan scatter rays of light.

This phenomenon is

known as the Tyndalleffect .



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Solutions

Colloids in Biological Systems

Some molecules havea polar, hydrophilic (water-loving ) end anda nonpolar,hydrophobic (water-hating ) end.



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Solutions

Colloids in Biological Systems

These moleculescan aid in theemulsification of fatsand oils in aqueoussolutions.

Blb12 Ch13 Lecture

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