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General Chemistry CourseCourse 5-6

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Disperse systems

Definition

A system in which one substance (particulate matter), the disperse phase, isdispersed as particles throughout another, the dispersion medium or thecontinuous phase.

1. Molecular dispersions

2. Colloidal dispersions

3. Coarse dispersions

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Definitions

A solution is a homogeneous mixture A solute is dissolved in a solvent.

solute is the substance being dissolved

solvent is the liquid in which the solute is dissolved

an aqueoussolution has water as solvent A saturated solution is one where the concentration is at

a maximum - no more solute is able to dissolve.

A saturated solution represents an equilibrium: therate of dissolving is equal to the rate ofcrystallization. The salt continues to dissolve, butcrystallizes at the same rate so that there appearsto be nothing happening.

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Dissolution of Solid Solute

What are the driving forces which cause solutes to dissolve

to form solutions?1. Covalent solutes dissolve by H-bonding to water or by

LDF

2. Ionic solutes dissolve by dissociation into their ions.

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When a solution is diluted,solvent is added to lower itsconcentration.

The amount of solute remainsconstant before and after thedilution:

moles BEFORE = moles AFTER

C1V1 = C2V2

Suppose you have 0.500

M sucrose stock solution.

How do you prepare 250

mL of 0.348 M sucrose

solution ?

Concentration

0.500 M

Sucrose

250 mL of 0.348 M sucrose

Dilution

A bottle of 0.500 M standardsucrose stock solution is in the

lab.

Give precise instructions toyour assistant on how to usethe stock solution to prepare

250.0 mL of a 0.348 M

sucrose solution.

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3 Stages of Solution Process

Separation of Solute must overcome IMF or ion-ion attractions in solute

requires energy, ENDOTHERMIC ( + DH)

Separation of Solvent

must overcome IMF of solvent particles

requires energy, ENDOTHERMIC (+ DH)

Interaction of Solute & Solvent

attractive bonds form between solute particles andsolvent particles

Solvation or Hydration (where water = solvent) releases energy, EXOTHERMIC (-DH)

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Dissolution at the molecular level?

Consider the dissolution of NaOH in H2O

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Factors Affecting Solubility

1. Nature of Solute / Solvent. - Like dissolves like (IMF)

2. Temperature -i) Solids/Liquids- Solubility increases with Temperature

Increase K.E. increases motion and collision between solute/ solvent.

ii) gas - Solubility decreases with TemperatureIncrease K.E. result in gas escaping to atmosphere.

3. Pressure Factor -

i) Solids/Liquids - Very little effect

Solids and Liquids are already close together, extra

pressure will not increase solubility.ii) gas - Solubility increases with Pressure.

Increase pressure squeezes gas solute into solvent.

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Solubilities of Solids vs Temperature

Solubilities of severalionic solid as a functionof temperature. MOSTsalts have greatersolubility in hot water.

A few salts havenegative heat ofsolution, (exothermicprocess) and theybecome less soluble with

increasing temperature.

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Temperature & the Solubility of GasesThe solubility of gases DECREASES at higher temperatures

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Ways of Expressing

Concentrations of Solutions

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Mass Percentage

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

100

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% Concentration: % Mass Example

3.5 g of CoCl2 is dissolved in100mL water.

Assuming the density of thesolution is 1.0 g/mL, what is

concentration of the solutionin % mass?

%m = 3.5 g CoCl2

100g H2O

= 3.5% (m/m)

P t Mi i

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Parts per Mi ion anParts per Billion

ppm = mass of A in solutiontotal mass of solution

106

Parts per Million (ppm)

Parts per Billion (ppb)

ppb =

mass of A in solution

total mass of solution

109

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moles of A

total moles in solutionXA =

Mole Fraction (X)

In some applications, one needs the mole fraction ofsolvent, not solutemake sure you find the quantity youneed!

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mol of solute

L of solutionM=

Molarity (M)

Because volume is temperature dependent, molarity canchange with temperature.

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Concentration: Molarity Example

If 0.435 g of KMnO4 is dissolved in enough water to give

250. mL of solution, what is the molarity of KMnO4?

Now that the number of moles of substance is known, thiscan be combined with the volume of solution which must bein liters to give the molarity. Because 250. mL isequivalent to 0.250 L .

As is almost always the case,the first step is to convertthe mass of material to

moles.

0.435 g KMnO4 1 mol KMnO4 = 0.00275 mol KMnO4158.0 g KMnO4

Molarity KMnO4 = 0.00275 mol KMnO4 = 0.0110 M

0.250 L solution

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mol of solute

kg of solventm =

Molality (m)

Because neither moles nor mass change withtemperature, molality (unlike molarity) is not

temperature dependent.

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SAMPLE EXERCISE Calculation of Mass-Related Concentrations

(a) A solution is made by dissolving 13.5 g of glucose (C6H12O6) in 0.100 kg ofwater. What is the mass percentage of solute in this solution? (b) A 2.5-g

sample of groundwater was found to contain 5.4 g of Zn2+ What is theconcentration of Zn2+ in parts per million?

PRACTICE EXERCISE(a) Calculate the mass percentage of NaCl in a solution containing 1.50 g ofNaCl in 50.0 g of water. (b) A commercial bleaching solution contains 3.62 mass

% sodium hypochlorite, NaOCl. What is the mass of NaOCl in a bottlecontaining 2500 g of bleaching solution?

PRACTICE EXERCISE

A commercial bleach solution contains 3.62 mass % NaOCl in water. Calculate(a) the molality and (b) the mole fraction of NaOCl in the solution.

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Degree of saturation

Supersaturated

Solvent holds moresolute than is normally possible atthat temperature.

These solutions are unstable; crystallization can often bestimulated by adding a seed crystal or scratching theside of the flask.

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Degree of saturation

Unsaturated, Saturated or Supersaturated?

How much solute can be dissolved in a solution?

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Properties of Solutions

Vapor Pressure

Ideal and non-ideal solutions

Raoults Law and Henrys Law

Colligative Properties Vapor Pressure Lowering

Freezing point depression (cryoscopy)

Boiling point elevation (ebullioscopy)

Osmotic Pressure (Osmosis)

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At pressure of few atmosphere or less, solubility of gas solutefollows Henry Law which states that the amount of solute gasdissolved in solution is directly proportional to the amount ofpressure above the solution.

c = k P

c = solubility of the gas (M)k = Henrys Law ConstantP = partial pressure of gas

Henrys Law Constants (25C), kN2 8.42 10

-7 M/mmHg

O2 1.66 10-6 M/mmHg

CO2 4.4810-5 M/mmHg

Henrys Law

The effect of partial pressure on solubility of gases

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Henrys Law & Soft Drinks

Soft drinks contain carbonatedwater water with dissolvedcarbon dioxide gas.

The drinks are bottled with a CO2

pressure greater than 1 atm. When the bottle is opened, the

pressure of CO2 decreases and thesolubility of CO2 also decreases,according to Henrys Law.

Therefore, bubbles of CO2 escapefrom solution.

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Henrys Law Application

The solubility of pure N2 (g) at 25oC and 1.00 atmpressure is 6.8 x 10-4 mol/L. What is the solubility of

N2 under atmospheric conditions if the partial pressure ofN2 is 0.78 atm?

Step 1: Use the first set of data to find k for N2 at25C

Step 2: Use this constant to find the solubility(concentration) when P is 0.78 atm:

44 16.8 10 6.8 10

1.00

c x Mk x M atm

P atm

4 1 4(6.8 10 )(0.78 ) 5.3 10c kP x M atm atm x M

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Raoults Law

Describes vapor pressure lowering mathematically.

The lowering of the vapour pressure when a non-volatilesolute is dissolved in a volatile solvent (A) can bedescribed by Raoults Law:

PA

= cAP

A

PA = vapour pressure of solvent A above the solution

cA = mole fraction of the solvent A in the solution

PA = vapour pressure of pure solvent A

only the solvent (A) contributes to

the vapour pressure of the solution

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What is the vapor pressure of water above a sucrose(MW=342.3 g/mol) solution prepared by dissolving 158.0 gof sucrose in 641.6 g of water at 25 C?The vapor pressure of pure water at 25 C is 23.76

mmHg.

mol sucrose = (158.0 g)/(342.3 g/mol) = 0.462 mol

mol water = (641.6 g)/(18 g/mol) = 35.6 mol

Xwater =mol water

(mol water)+(mol sucrose)=

35.6

35.6+0.462= 0.98

Psoln = XwaterPwater = (0.987)(23.76 mm Hg)

= 23.5 mm Hg

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The following graph shows the vapor pressure for water (solvent) at90oC as a function of mole fraction of water in several solutionscontaining sucrose (a non-volatile solute). Note that the vaporpressure of water decreases as the concentration of sucroseincreases.

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Raoults Law: Mixing Two Volatile Liquids

Since BOTH liquids are volatile and contribute to thevapour, the total vapor pressure can be represented usingDaltons Law:

PT= PA + PB

The vapor pressure from each component follows RaoultsLaw:

PT= cAPA + cBPB

Also, cA + cB = 1 (since there are 2 components)

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Mixtures of Volatile Liquids

Both liquids evaporate & contribute to the vapor pressure

Colligative Properties

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Colligative Properties

Dissolving solute in pure liquid will change all physical

properties of liquid, Density, Vapor Pressure, Boiling Point,Freezing Point, Osmotic Pressure

Colligative Properties are properties of a liquid that changewhen a solute is added.

The magnitude of the change depends on the number ofsolute particles in the solution, NOT on the identity of thesolute particles.

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Vapor Pressure Lowering for a Solution

The diagram below shows how a phase diagram isaffected by dissolving a solute in a solvent.

The black curve represents the pure liquid and the bluecurve represents the solution.

Notice the changes in the freezing & boiling points.

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Vapor Pressure Lowering

The presence of a non-volatile solute means that fewersolvent particles are at the solutions surface, so less

solvent evaporates!

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Application of Vapor Pressure Lowering

Describe what is happening in the pictures below.

Use the concept of vapor pressure lowering to explain

this phenomenon.

N l B ili P

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Normal Boiling Process

Extension of vapor pressure concept:

Normal Boiling Point: BP of Substance @ 1atm

When solute is added, BP > Normal BPBoiling point is elevated when solute inhibits solvent fromescaping.

Elevation of B. pt.

Express by Boiling

point Elevation

equation

B ili P i El i

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Boiling Point Elevation

DTb = (Tb -Tb) = i m kbWhere, DTb = BP. ElevationTb = BP of solvent in solution

Tb = BP of pure solventm= molality , kb = BP Constant

Some Boiling Point Elevation and Freezing Point Depression Constants

Normal bp (C) Kb Normal fp (C) Kf

Solvent pure solvent (C/m) pure solvent (C/m)

Water 100.00 +0.5121 0.0 1.86

Benzene 80.10 +2.53 5.50 4.90

Camphor 207 +5.611 179.75 39.7

Chloroform 61.70 +3.63 - 63.5 4.70(CH3Cl)

F i P i t D i

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When solution freezes the solid form isalmost always pure.

Solute particles does not fit into thecrystal lattice of the solvent because ofthe differences in size. The soluteessentially remains in solution and blocksother solvent from fitting into the crystal

lattice during the freezing process.

Freezing Point Depression

Normal Freezing Point: FP of Substance @ 1atm

When solute is added, FP < Normal FP

FP is depressed when solute inhibits solvent fromcrystallizing.

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Osmotic pressure

Osmosis is the spontaneous movement of water across asemi-permeable membrane from an area of low solute

concentration to an area of high solute concentration Osmotic Pressure - The Pressure that must be applied tostop osmosis

P = i CRTwhere P = osmotic pressure

i = vant Hoff factorC = molarityR = ideal gas constantT = Kelvin temperature

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Osmosis and Blood Cells

(a) A cell placed in an isotonic solution. The net movement ofwater in and out of the cell is zero because the concentration of

solutes inside and outside the cell is the same.(b) In a hypertonic solution, the concentration of solutes outsidethe cell is greater than that inside. There is a net flow of waterout of the cell, causing the cell to dehydrate, shrink, andperhaps die.

(c) In a hypotonic solution, the concentration of solutes outsideof the cell is less than that inside. There is a net flow of waterinto the cell, causing the cell to swell and perhaps to burst.

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Microscopic mechanism of solution (energetics) Physical factors affecting solubility

Temperature

Pressure (Henrys law)

Ideal and nonideal solutions Raoults law

Colligative properties (nonelectrolytes) boiling point elevation

freezing point depression osmotic pressure

electrolytes and vant Hoff factor

Summary

Types of disperse systems

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A. On the basis of particle size

T f di t

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B- On the basis of the physical state

Disp s d ph s :

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Dispersed phase:

Stability of disperse systems

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Stability of disperse systems

Colloidal dispersions are more stablethan suspensions and emulsions, dueto:

- Smaller particle size

- Brownian movement

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Size of colloidal particles

Large area to volume ratio Specific surface area:

Surface area / unit weight, or Surface area / unit volume

Properties due to large specific surface area:

1- Catalysis (adsorption): e.g.: platinum black2- Colour

3- Dialysis: separation from molecular and ionic particles

Sh f ll id l ti l

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Shape of colloidal particle:

Extended particles: interaction with dispersion medium

Rolled particles: poor interactions with dispersion medium

Types of colloids:

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Types of colloids:

A- On the basis of the interaction of the dispersed particleswith the dispersion medium.

1-Lyophilic colloids : (hydrophilic)- Solvent loving solvent sheath- Mostly organic molecules e.g. :

Acacia, tragacanth and insulin in water

Rubber and polystyrene in benzene.

2-Lyophobic colloids : (hydrophobic)- Solvent - hating- Mostly inorganic particles e.g.: gold, silver, sulfur and silver

iodide.

3-Association colloids : amphiphilic- Surfactant micelles

Preparation of colloids:

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Preparation of colloids:

Lyophilic colloidsSpontaneous, e.g.: gelatin soaked in water Lyophobic colloidsA- Dispersion methods: breakdown of coarse particle

Colloid mill Electric dispersion Ultrasonic irradiation

Peptisation: addition of preferentially adsorbedions.

B Condensation method: aggregation off subcolloidal particles1- Chemical reaction

Reduction: e.g.: colloidal Ag Oxidation: e.g.: H2S S

Hydrolysis: Fe2O3 Double decomposition: colloidal AgI2- Change of solvent:Precipitation of colloidal S from alcoholic solution by addition

of water

Purification of colloids

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1. Dialysis

Ions Stirring or renewal of the outer liquidhasten dialysis

Semipermeable membrane

Colloidal particles

Applications:

- Membrane filters- Membrane diffusion- Study of drug/protein binding- Heamodyalysis


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2. Electrodialysis:

Application of an electric potential across the semipermeablemembrane

3. Electrodecantation:

Concentration of charged colloidal particles at one side andat the base of the membrane

4. Ultrafiltration:

Application of a pressure or suction across a filter

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