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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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Types of disperse systems
A. On the basis of particle size
T f di t
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Types of disperse systems
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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Purification of colloids
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
Purification of colloids
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Purification of colloids
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