chapter 14 – mixtures and solutions 14.1types of mixtures 14.2 solution concentration 14.3factors...
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Chapter 14 – Mixtures and Solutions14.1 Types of Mixtures14.2 Solution Concentration14.3 Factors Affecting Solvation14.4 Colligative Properties of Solutions
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Section 14.1 Types of Mixtures
• Compare the properties of suspensions, colloids, and solutions and the methods used to separate out their particles.
• Identify types of colloids (particularly sols, emulsions and both types of aerosols) and types of solutions.
• Describe the electrostatic forces in colloids and their role in colloid stabilization.
• Describe and give examples of shear-thickening and shear-thinning suspensions or colloids.
Mixtures can be either heterogeneous or homogeneous
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Section 14.1 Types of Mixtures
• Define and give examples of Brownian motion and the Tyndall effect.
• Describe the role of the emulsifying agent in forming an emulsion.
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Section 14.1 Types of MixturesKey Concepts
• The individual substances in a heterogeneous mixture remain distinct.
• Two types of heterogeneous mixtures are suspensions and colloids; these differ in the sizes of the particles.
• Brownian motion is the erratic movement of colloid particles due to random impacts of the dispersing medium.
• Colloids and suspensions exhibit the Tyndall effect.
• A solution can exist as a gas, a liquid, or a solid, depending on the solvent.
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Section 14.1 Types of Mixtures
Key Concepts• Solutes in a solution can be gases, liquids, or solids.
• Salt solutions (electrolytes) can destabilize colloids and cause the particles to aggregate (stick together) and then flocculate (settle out).
• When emulsions are formed, and emulsifying agent can stabilize it because it contains both polar and non-polar regions which allow the agent to collect on the particle surface and interact with the dispersing medium.
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Section 14.1 Types of Mixtures
Key Concepts
• The rheological (flow) properties of suspensions and colloids are more complicated than that of pure liquids or solutions. This is because the particles interact with each other in a way that can alter the flow.
• Some suspensions and colloids are shear thinning – their viscosity drops as they are stirred or shaken. Categories of shear thinning materials include pseudoplastic, and thixotropic materials.
• Some suspensions and colloids are shear thickening – their viscosity increases as they are stirred or shaken. The major shear thickening category is dilatant materials.
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Heterogeneous Mixtures
Non-uniform composition; individual substances remain distinct
Suspension – heterogeneous mixture containing particles that settle out if left undisturbed
Colloid - heterogeneous mixture of intermediate sized particles (between 1 nm and 1000 nm) that do not settle out
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A mixture containing particles that settle out upon standing
• Cornstarch in water• Muddy water
Can be filtered out by simple filters
>1000 nm (1 micron, ) diameter • Large compared to solvent molecules• Gravity can act on particles
Suspensions
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Particle Model for
Suspension
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Suspension – Cornstarch in Water
Settling
Filtration
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Filtration
In past, only suspensions could be successfully filtered
No longer true – lots of filtration technology now available
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Intermediate size between solution particles (molecular/atomic/ionic) and suspensions (1000 nm and up)1 nm to 1000 nm particle sizeParticles in dispersing medium (not solvent)Milk – particles can’t be filtered by ordinary filtration but can by ultra methods
Heterogeneous Mixtures - Colloids
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Properties of Solutions, Colloids, & Suspensions
Property Solution Colloid Suspension
Particle Size 0.1-1.0 nm
1-1000 nm >1000 nm
Settles on Standing?
No No Yes
Filter with Paper? No No Yes
Separate by Dialysis?
No Yes Yes
Homogeneous? Yes Borderline No
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Category Medium Particles Example
Solid Aerosol G S Smoke, dust in air
Liquid Aerosol G L Fog, cloud
Foam L G Whipped cream
Emulsion L L Mayonnaise, milk
Sol L S Blood, paint, milk of magnesia, mud
Solid Foam(Aerogel)
S G Marshmallow, exp. polystyrene
Solid Emulsion(Gel)
S L Butter, cheese, jello, jelly
Solid sol S S Colored Gems
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Separation
of Solution,
Colloid, and Suspension
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Suspension versus Colloid
Suspended SiO2 (sand) settles very quickly.
Each colloidal particle of SiO2 (Ludox®) attains (–) charge, which
repels other colloidal particles
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Food EmulsionsOil and water don’t mix!
However, many foods exist that are emulsions made by mixing oil & water: salad dressing, butter, ice cream, & milk
Emulsion of liquid butterfat globules dispersed within a water-based liquid
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Emulsions (type of colloid)Liquids that do not normally mix are spread throughout each other
Emulsify by slowly adding one ingredient to another while mixing
Process disperses and suspends tiny droplets of one liquid (dispersed phase) through another (continuous phase)
Oil
Water
ContinuousPhase
DispersedPhase
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Emulsions & Emulsifying Agents
Emulsifying agents (emulsifiers) are materials that help stabilize an emulsion
Surfactants (surface active agents) are a class of emulsifying agents
Detergents are a category of surfactants
Surfactants and detergents are amiphiphilic – they are partly hydrophilic (polar) and partly hydrophobic (non-polar)
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Emulsifier (Emulsifying Agent)Prevents mixture from separating
Attracted to both oil & water, allowing both to mix
Functions by surrounding oil droplets to form protective coat - holds oil droplets in suspension
One end (polar) of emulsifier molecule soluble in water; other end (nonpolar) soluble in oil
A to C: no emulsifier
D: with emulsifier
Oil
Water
Non-polarendPolar
end
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Mayonnaise - Emulsifier
Egg yolk contains lecithin (phosphatidyl choline) - naturally occurring emulsifier
Lecithin functions by surrounding oil droplets to form protective coat which holds oil droplets in suspension
Polar End
Non-Polar End
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Soap and MicellesSoap molecule has hydrophilic (polar/ionic) “head” and hydrophobic (nonpolar) “tail”
Head attracted to water, rest isn’t
Nonpolar tails tend to coagulate together to form spherical structure called a micelle
Some emulsions have dispersed phase in form of micelles
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Model of Sodium Lauryl Sulfate
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Micelle Formation in Na Stearate
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Stearate Micelle in Water Solution
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Polar heads of micelles attract them to water, and simultaneously repel other micelles – won’t coalesce and settle out
Soap Action
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Cleansing Action of Soap
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Cleansing Action of Soap
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FoamsColloid in which very tiny particles of gas are dispersed in liquid or solid substance
Examples: ice cream, whipped cream, foamed milk, marshmallows, and beaten egg white
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Gels – Liquids Dispersed in Solids
More-or-less rigid systems
Solid particles form network structure which traps liquid; alginate, starch, gelatin & pectin are typical gelling agents
Often formed by proteins of eggs or flour in puddings, batters, and doughs
Gelatin - type of protein found in bone and skin tissue of animals
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Colloidal particles have erratic movement caused by collisions of molecules of the dispersing medium
Brownian Motion
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Dispersed particles don’t settle due to electrostatic interactions between medium and particle
Can break colloid structure by adding an electrolyte (alters interactions)
Colloid Stabilization
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Colloidal Particles with Cation ShellColloid Stabilized by Electrostatic Interaction
Like charges repel – keep particles separated
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Unstable colloidal dispersions form flocs as particles aggregate due to interparticle attractions
Flocculation can be induced by removal of electrostatic barrier that prevents particle aggregation by addition of salt (electrolyte) or by changing pH
The addition neutralizes or "screens" surface charge of particle - removes repulsive forces that keep colloidal particles separate and allows for coagulation due to van der Waals forces
Colloid Destablilization - Flocculation
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Formation and Coagulationof ColloidWhen strong electrolyte
added to colloidal iron oxide, charge on surface
of each particle is partially neutralized …
… and colloidal particles coalesce into suspension
that quickly settles
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Colloidal particle large enough to scatter light
Suspensions also scatter (tend to be more opaque )
Solution particles are too small to scatter
Tyndall Effect
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Tyndall Effect – Light scattering
Solution Colloid
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Rheology of Colloids & Suspensions
Because these are heterogeneous mixtures, the flow properties of these materials can be unusual, especially for solid particles dispersed/suspended in a liquid medium
Flow properties are the subject of the field of study called rheology
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RheologyStudy of the flow of matter, primarily in liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force.
Applies to substances which have a complex structure, such as muds, sludges, suspensions, polymers and other glass formers, as well as many foods and additives, bodily fluids (e.g. blood) and other biological materials.
http://en.wikipedia.org/wiki/Rheology
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Rheology – Shear StressForce applied to overcome fluid’s resistance to flow, divided by area that force is acting upon (units of pressure but unlike pressure, force is parallel to surface)
A
F
shear stress, N/m2 = PaF force applied, N
A surface area subjected to stress, m2
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Rheology – Shear RateRelative velocity of fluid layers divided by their normal separation distance (fluid not moving next to bottom plate)
d
U
shear rate, sec-1 U velocity, m/sec
d plate distance, m
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Rheology - ViscosityRatio of shear stress to shear rate; indicates resistance material has to a change in form
Property can be thought of as an internal friction (measure of how difficult it is to slide one layer of fluid past an adjacent layer)
viscosity, Pa s shear stress, Pa shear rate, sec-1
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Rheology – Newtonian FluidNewtonian fluid has a fixed viscosity that is independent of time and of shear stress
= shear stress/shear rate
constant slope
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Rheology – Non-Newtonian Fluids
For our purposes, will focus on following types of non-Newtonian fluids:
Dilatant: shear-thickening
Pseudoplastic: shear-thinning
Thixotropic: time-dependent pseudoplastic flow; at constant applied shear rate, viscosity gradually decreases
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Non-Newtonian
Liquids
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Shear-Thickening Behavior
Dilatant mixture – “liquid-like” mixture that solidifies when shaken or stirred
Alternative statement: viscosity increases with rate of shear strain
All are stabilized suspensions with high volume fraction of solid
Examples: Silly Putty, oobleck (corn starch & water), silica in polyethylene glycol (liquid body armor), quicksand
http://en.wikipedia.org/wiki/Dilatant
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Shear-Thickening Dilatant
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Shear-Thinning Behavior
Psuedoplastic & Thixotropic mixtures –thick or solid-like mixture that flows more easily when stirred or agitated (shear is applied); thixotropic is time-dependence while psuedoplastic is not (won’t worry about distinction for examples)
Examples: toothpaste, certain paints and inks, catsup, clay soils that liquefy in an earthquake, mayonaisse
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Shear-Thinning Psuedoplastic
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Rheological Properties
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Why Shear Thinning Occurs
Random coilpolymers elongate & break
Unsheared
Anisotropic Particles alignwith Flow Streamlines
Sheared
Aggregatesbreak up
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Homogeneous Mixture - Solution
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Solution Characteristics
Homogeneous mixtures
Solute = substance that dissolves• minor component
Solvent = dissolving medium• major component
By visual examination (even under magnification), not possible to distinguish solute from solvent
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Types and Examples of SolutionsPhase Solute Solvent Example
G Gas Gas Air – N2, O2, Ar
L Gas Liquid Soda – CO2 / H2O
L Liquid Liquid Antifreeze Glycol / H2O
L Solid Liquid Salt water
S Gas Solid H2 in Pd or Pt
S Liquid Solid Dental Amalgam (Hg/Ag)
S Solid Solid Brass (Cu/Zn alloy)
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Solution Terminology
Soluble• Substance dissolves in solvent• If solvent and solute are liquids, also
said to be miscible
Insoluble• Substance doesn’t dissolve• If solvent and solute are liquids, also
said to be immiscible - oil and water
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Chapter 14 – Mixtures and Solutions
14.1 Types of Mixtures14.2 Solution Concentration14.3 Factors Affecting Solvation14.4 Colligative Properties of Solutions
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Section 14.2 Solution Concentration
• Describe concentration using different units.
• Determine the concentrations of solutions.
• Calculate the molarity of a solution.
Concentration can be expressed in terms of percent or in terms of moles.
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Section 14.2 Solution Concentration
Key Concepts• Concentrations can be measured qualitatively and
quantitatively.
• Molarity is the number of moles of solute dissolved per liter of solution.
• Molality is the ratio of the number of moles of solute dissolved in 1 kg of solvent.
• The number of moles of solute does not change during a dilution.
M1V1 = M2V2
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Measure of how much solute is dissolved in a specific amount of solvent or solution
General terms• Dilute – small amount of solute• Concentrated – large amount of solute
Solution Concentration
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See table 14-3, page 480
Percent by mass
Percent by volume
Molarity
Molality
Mole Fraction
ppm (not in book)
Concentration Units Used
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% by mass = 100 mass of solute mass of solution
Example problem 14.1, page 481
Dissolve 3.6 g NaCl in 100.0 g H2O
% by mass = 100 3.6 103.6
= 3.5 %
Percent by Mass
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Have 14.0% by mass KI solution in H2ODensity = d = 1.208 g/mLGrams KI in 25.0 mL of solution?Answerd = m/V m = V dm = 25.0 mL 1.208 g solution/mL m = 30.2 g solution0.140 g KI/g solution 30.2 g solution = 4.23 g KI
Percent by Mass Using Density
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Practice (% by mass)
Problems 9 – 12, page 481
Problems 67 – 68, page 508
Problems 1 – 2, 7, page 985
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Volumetric Measurements
Variety of laboratory containers available in which volume can be measured
Need to choose one appropriate for task at hand
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Beaker
Volume graduations should be used only for "ballpark" estimates
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Erlenmeyer flaskShape constructed to facilitate swirling or mixing of reactantsNOT precise-250 mL flask typically holds ~ 270 mL Use only for approximate measurements
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Volumetric FlasksUsed to prepare solutions of exact concentrations of solutionsHave precise graduation line in neck of flask
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Volumetric & Graduated PipettesUse bulb/pipette pump/pipette filler to fillConvenient way to accurately transfer small volume of liquidVolumetric pipette -used to measure one volume onlyGraduated pipette - lots of lines so you can measure many different amounts
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Volumetric Glassware
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Accurate/Precise Volumetric Glassware
Burette (or Buret)
Volumetric Flask
Volumetric (a) and Graduated (b) Pipettes (or pipets)
Beaker
Graduated Cylinder
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% by volume = 100 volume of solute volume of solutionAlthough book and your own experience tell you that
Vsolution = Vsolute + Vsolvent
Additivity of volumes does not always occur!
For purposes of doing calculations in this course, we will assume that volumes add, but won’t when describe lab methods of preparation of solutions
Percent by Volume
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Volumes Don’t Always Add
50 mL of
ethanol …
… and 50 mL of water …
… when mixed,
give less than 100
mL of solution
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Preparation of 10% Ethanol Solution by
Volume
10.00 mL C2H5OH
100.0 mL
Volumetric pipette
Volumetric flask
Dispense ethanol into flask, slowly add water with swirling
When well mixed, top off to mark with water
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Practice (% by volume)
Problems 13 – 15, page 482
Problems 69 – 70, page 508
Problems 3 – 4, page 985
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Molarity = M = moles of solute liters of solution
Sample problem 14.2 page 483
100.5 mL of solution contains 5.10 g glucose (180.16 g/mol) Molarity?
5.10 g gluc. 2.83x10-2 mol gluc.
M = 2.83x10-2 mol/ 0.1005 L = 0.282 M
Molarity
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Practice (molarity)
Problems 16 – 19, page 483
Problems 20 – 23, page 484
Problems 71 – 74, page 508
Problems 5, 6, 8, 9, page 985
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Don’t Know Volume of
H2O Required
1.0 L water initially available to make 1.0 L of solution
25.0 g (0.100 mol) CuSO45H2O
Water left over
1.0 L of 0.100 M CuSO4
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For 1.0 M aqueous solution, cannot add 1.0 mol of compound to 1.0 L of H2O
Final volume of solution not guaranteed to be 1.0 L using this method
Need to use volumetric flask• Weigh out solute• Add to volumetric• Fill volumetric to mark
Preparing Molar Solutions
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Preparing Molar Solutions Using Volumetric Flask
Weigh Place in flask Fill to mark
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Preparing Solutions of Known Concentration
(Adds Solute in Dry Form to Flask)
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Prepare less concentrated solution from more concentrated one by adding solvent
Diluting Solutions
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Often want to use concentrated (stock) solution to prepare a precisely known concentration of a dilute solution
When diluting, add only solvent – moles of solute remains constant
mol solute = M1 V1 = M2 V2
1 = concentrated (stock) solution
2 = dilute solution
Diluting Solutions
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Making Solution by Dilution
M1 = 12.0 M V1 = ? LM2 = 1.50 M V2 = 5.00 L
M1 V1 = M2 V2
V1 = M2 V2 / M1
= 1.50 M 5.00 L / 12.0 MV1 = 0.625 L
Dilute 0.625 L of 12.0 M solution to 5.00 L
Make 5.00 L of 1.50 M KCl from 12.0 M Stock
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Example problem 14.3, page 486
Make 0.50 L of 0.300M CaCl2 from 2.00M stock CaCl2. mL stock needed?
M1 V1 = M2 V2
V1 = V2 M2
M1
V1 = 0.50L 0.300M 2.00M
V1 = 0.075 L = 75 mL
Diluting Solutions
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Practice (dilutions)
Problems 24 – 26, page 486
Problems 75 – 79, page 508
Problems 10 – 11, page 985
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Molality = m = moles of solute kg solventNote: only concentration unit that has solvent rather than solution in denominator; also in kg, not g
Prob. 14.4 4.5 g NaCl + 100.0 g H2OMolality?4.5 g NaCl 0.077 mol NaClm = 0.077 mol NaCl /0.1000 kg = 0.77m
Molality
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Practice (molality)
Problems 27 – 28, page 487
Problems 80 – 82, page 508
Problems 12 – 14, page 986
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Mole fraction = XA = moles of cpd A total moles in solution
= nA/(nA + nB)
A = solvent or solute cpd = compound
Mole Fraction
XA & XB represent mole fractions of
substances A and B (must add up to 1)
nB
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Practice (mole fraction)
Problems 29 – 30, page 488
Problems 83 – 85, pages 508-9
Problems 15 – 16, page 986
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Not discussed in book but widely used for concentrations in water, especially with regard to environmental issues
ppm = g solute x 106
g solution
ppm, ppb, etc
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Parts per million (ppm): often used to express concentration of very dilute aqueous solutions. “Technical" definition is:
ppm = g solute x 106
g solution
Since amount of solute relative to amount of solvent is typically very small, density of solution is ~ same as density of solvent (water). For this reason, parts per million also expressed as:
ppm = mg solute = mg solute kg solution L solution
ppm, ppb, etc
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Chapter 14 – Mixtures and Solutions
14.1 Types of Mixtures14.2 Solution Concentration14.3 Factors Affecting Solvation14.4 Colligative Properties of Solutions
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Section 14.3 Factors Affecting Solvation
• Describe how intermolecular forces affect solvation.
• Define solubility and the terms unsaturated, saturated, supersaturated, dilute and concentrated.
• List the factors affect solubility and be able to predict if a certain material will readily dissolve in another.
Factors such as temperature, pressure, and polarity affect the formation of solutions.
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Section 14.3 Factors Affecting Solvation
• Use Henry’s law to calculate gas solubility.
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Section 14.3 Factors Affecting Solvation
Key Concepts
• The process of solvation involves solute particles surrounded by solvent particles. Strong forces will occur between solute and solvent when both are of the same “type” (polar or non-polar).
• Solutions can be unsaturated, saturated, or supersaturated.
• Henry’s law states that at a given temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid.
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Solution Terminology
Solvation – process of surrounding solute particles with solvent particles
• Attractive intermolecular forces between solute and solvent must exist for this to happen
• In water, solvation process called hydration
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Intermolecular Forces in Solution
For a solute to dissolve,
strength of solvent–solvent
forces …
… and solute–solute
forces …
… must be comparable to solute–solvent
forces
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Intermolecular InteractionsAll Can Be Involved in Forming Solutions
Ion DipoleHydrogen Bond Dipole-Dipole
Ion – Induced DipoleDipole – Induced
Dipole Dispersion
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Solubility Rule
“Like dissolves like” general rule for determining solubility
Polar substances (especially water) dissolve other polar substances and ionic compounds
Nonpolar substances (especially hydrocarbons) dissolve other nonpolar substances
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Typical Nonpolar Hydrocarbon (C18)
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Immiscible Oil and Water
Nonpolar oil (hydrocarbon)
layer
Polar water layer
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Like Dissolves Like
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Solubility of Alcohols in H2O & Hexane
0.12
0.12
0.030
0.0058
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Aqueous Solutions - Ionic Compounds
Water strongly polar
Water molecules in constant motion (KMT) and collide with surfaces
Ionic solids in water tend to dissolve due to strong ion – dipole interactions
• Must be stronger than attraction among ions on crystal surface for dissolution to occur
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Ion–Dipole Forces in Dissolution
Positive ends of dipoles attracted to
anions
Negative ends of dipoles attracted to
cations
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Not all ionic compounds dissolve in water
• CaCO3 (calcium carbonate)
• CaSO42H2O (gypsum)
These compound have stronger forces between ions than between ions and the polar water molecules
Aqueous Solutions - Ionic Compounds
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Water is a good solvent for molecules with polar groups like OH
(sucrose)
Aqueous SolutionsMolecular Compounds
Dissolving Sugar Cube in Water
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Solvation process energy requirements• Solute particles separate (endothermic)• Solvent particles move apart
(endothermic)• Solute-solvent particles interact
(exothermic)
Overall energy change: heat of solution
Can be either endo- or exo-thermic
Heat of Solution
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Energetics of Solution Formation
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Solvation only occurs at point of contact between solvent and solute
Can accelerate process by• Agitation (stirring)• Increasing surface area of solute• Raising temperature
CaCO3 less soluble in hot water than in cold water but probably still reaches its solubility limit faster than at lower T
Factors That Affect Solvation Rate
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Maximum amount of solute that can dissolve in a given amount of solvent at a specified temperature and pressure
One common unit is g solute per 100 g solvent
Refer to table 14.4, page 494 for solubilities of aqueous solutions of ionic compounds, molecular compounds and gases (later slide will show)
Solubility
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Solubility limit occurs due to dynamic equilibrium process (similar to vapor pressure)
Solvated solute particles collide with undissolved solute – some stick
Rates of dissolution (solvation) and crystallization become equal at saturation
Maximum Solubility (saturation)
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Solution Formation and Equilibrium
Saturated: dynamic
equilibrium
Unsaturated:
All solvation
Unsaturated:
Some crystallization
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Unsaturated solution - more solute can be dissolved
Saturated – at solubility limit
Supersaturated – temporarily have more solute dissolved than the known solubility at a given temperature
• Unstable state
Solubility
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Solubility –Dynamic Equilibrium
SolvationCrystallization
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Many (but not all) substances have increased solublility at higher temperatures
Effect of Temperature on Solubility
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Aqueous Solubilities – Table 14.4
Increase with T
Decrease with TDecrease with T - gasesLittle change with T
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Solubility Variation with Temperature
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Variation of Solubility of
Ionic Compounds
with Temperature(large y scale)
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Can form by cooling a saturated solution (if solute has increased solubility with temperature)
Seed crystal, shock, rough surface (scratch) can initiate rapid crystallization
AgI crystals used to trigger rain in air supersaturated with water vapor
Supersaturated Solutions
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Supersaturated Sodium Acetate
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Gas Solubilities In Water
Note that these generally decrease with increasing temperature
(opposite in organic solvents)
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Pressure and Solubility of Gases
… thus more frequent collisions of gas molecules with surface …
… giving higher concentration of dissolved gas
Higher partial pressure means more molecules of gas per unit volume …
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Gases more soluble at higher pressure
Henry’s Law
S1/P1 = S2/P2
Si = solubility of gas
Pi = partial pressure of gas above solution
S2 = S1 (P2/P1)
Pressure and Solubility of Gases
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Solubility of Gases with Pressure
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Pressure and Solubility of Gases
Lowering pressure by unscrewing bottle cap lowers CO2 solubility and excess escapes
Warming also lowers solubility
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Pressure and Solubility of Gases
Increased pressure underwater causes N2 to dissolve in higher concentration in blood – can cause bends when diver surfaces
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Example problem 14.5, page 497
0.85 g of gas dissolves at 4.0 atm in 1.0 L at 25 CHow much dissolves in 1.0 L at 1.0 atm at same T?
S1 = 0.85 g/L P1 = 4.0 atm P2 = 1.0 atm
S1/P1 = S2/P2 S2 = S1 (P2/P1)
S2 = 0.85 g/L (1.0 atm / 4.0 atm)
S2 = 0.21 g/L
Pressure and Solubility – Henry’s Law
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Practice (Henry’s Law)
Problems 36 – 38, page 497
Problems 89, 90, 92 – 94, page 509
Problems 17 – 18, page 986
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Chapter 14 – Mixtures and Solutions
14.1 Types of Mixtures14.2 Solution Concentration14.3 Factors Affecting Solvation14.4 Colligative Properties of Solutions
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Section 14.4 Colligative Properties of Solutions
• Describe colligative properties.
• Identify four colligative properties of solutions.
• Calculate the boiling point elevation and freezing point depression of a solution.
• Explain the meaning of the terms osmotic pressure, dialysis and reverse osmosis and be able to give an example.
Colligative properties depend on the number of solute particles in a solution.
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Key Concepts
• Nonvolatile solutes lower the vapor pressure of a solution.
• Boiling point elevation is directly related to the solution’s molality.
∆Tb = Kbm• A solution’s freezing point depression is always lower
than that of the pure solvent.
∆Tf = Kfm• Osmotic pressure depends on the number of solute
particles in a given volume.
Section 14.4 Colligative Properties of Solutions
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Key Concepts
• Dialysis is a purification process that employs the same type of semipermeable membrane that is involved in the phenomenon of osmotic pressure. Artificial kidney dialysis is one example of its use.
• Applying pressure to the side opposite the pure solvent side of a semipermeable membrane is a purification process called reverse osmosis. It has many uses, among them being the desalinization of seawater to generate fresh water for drinking.
Section 14.4 Colligative Properties of Solutions
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Physical properties of solutions that depend upon number of particles and not identity of dissolved solute particles
Colligative Properties
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Electrolytes completely ionize
NaCl(s) Na+(aq) + Cl-(aq)
1m solution yields 2m ions
Nonelectrolytes dissolve but don’t ionize
Sucrose (table sugar)
1m solution yields 1m dissolved particles
Can distinguish based on conductivity
Electrolytes & Nonelectrolytes
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Electrolytes & Nonelectrolytes
Nonelectrolyte Electrolyte
Solution Conductivity Tests
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Adding nonvolatile solute lowers VP of solvent
Mixture of solvent and solute occupies surface; fewer solvent particles enter vapor phase
The greater the number of solute particles, the lower the VP
Vapor Pressure (VP) Lowering
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VP lowering due to number of solute particles and is a colligative property
Vapor Pressure (VP) Lowering
H2OPure waterSucroseSugar & water
Water VP lowered
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1 mole of nonelectrolyte lowers VP by given amount1 mole of electrolyte lowers VP by n , where n = # of particles produced
H3PO4 3H+ + PO43-
4 particles, 4 effect relative to nonelectrolyte
Vapor Pressure (VP) Lowering
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When nonvolatile solute lowers VP of solvent, BP also affectedMore heat needed to supply additional kinetic energy to raise VP to atmospheric pressure
Boiling Point (BP) Elevation
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Vapor pressure effect directly related to BP
Tb = Kbm
Kb = BP elevation constant (units C/m)Difference in BP between pure solvent and 1m solution of nonelectrolyte
Except for number of particles, Tb does not depend on solute
Boiling Point (BP) Elevation
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Boiling Point Elevation
Solvent Molecule
Nonvolatile Solute Molecule
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Boiling Point Elevation
Solvent BP (C) Kb (C/m)
Water 100.0 0.512
Benzene 80.1 2.53
CCl4 76.7 5.03
Ethanol 78.5 1.22
Chloroform 61.7 3.63
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Solute disrupts organization of crystal
Tf = Kfm
Kf = FP constant (units C/m)
Difference in FP between pure solvent and 1m solution of nonelectrolyte
1.86 C/m for water
Use of road salt a common application
Freezing Point (FP) Depression
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Freezing Point Depression
Solvent FP (C) Kf (C/m)
Water 0.0 1.86
Benzene 5.5 5.12
CCl4 -23 29.8
Ethanol -114.1 1.99
Chloroform -63.5 4.68
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FP & BP Changes – Effective m
Must use effective molality or particle molality in equations
1m NaCl solution = 2m effective
For 1m CaCl2 solution in water
BP elevation?
Effective molality = 3m
BP elevation = 3m 0.512 C/m
= 1.54 C
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FP & BP Changes
Example problem 14.6, page 503
BP and FP of 0.029 m aqueous soln. NaCl?
m = 0.029 x 2 = 0.058 m (2 particles)
Kb = 0.512 C/m Kf = 1.86 C/m
Tb=Kb m= 0.512 C/m 0.058 m = 0.030C
Tf = Kf m = 1.86 C/m 0.058 m = 0.11C
BP = 100.000 C + 0.030C = 100.030 C FP = 0.00 C 0.11C = 0.11C
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Practice (FP & BP Changes)
Problems 45 – 47, page 503
Problems 99 – 103, page 509
Problems 19 – 21, page 986
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FP & BP Changes – Phase Diagram
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Osmosis – diffusion of solvent particles across a semipermeable membrane from region of low to region of high solute concentration
Membrane allows solvent and smaller size particles to pass, blocks larger ones
Membrane pore radius typically 0.1 to 100 nm (bond lengths 0.1 to 0.25 nm)
Osmosis
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Semipermeable membrane separating water and aqueous
solution of glucose
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Movement of solvent (water) from dilute to concentrated solution across membrane
Osmosis
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If membrane blocks solute from passing, pressure difference develops
Pressure = osmotic pressure
Also a colligative property - depends on # particles
Osmotic pressure MM = solution molarity of solution on one side of membrane (pure solvent on other side)
Osmotic Pressure
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Osmosis & Osmotic Pressure
Pure solvent
Pure solvent Solution
Solution
Semipermeable membrane
Osmotic pressure
Membrane allows solvent to pass but not solute
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Osmosis and Osmotic Pressure
Net flow of water from
outside (pure H2O) to solution
Solution increases in
volume until … … height of solution exerts
osmotic pressure (π) of solution
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Drinking Seawater will Cause Dehydration of Body Tissues
Water leaves cells through osmosis to dilute seawater in digestive tract
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Dialysis - process that relies on concentration-driven transport (osmosis) for separation/purification
Common application: artificial kidney dialysis
Osmosis / Dialysis
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Renal Dialysis – Artificial Kidney
Hemodialysis attempts to mimic the action of the nephron (kidney) -separates low MW solutes (e.g., urea, creatinine) from the blood of patients with chronic uremia
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Renal Dialysis – Artificial KidneyKidney dialysis filters and removes waste products and excess water from blood. Inventor (Kollf, 1943, Holland) originally used cellophane as the membrane.
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Principle of Dialysis
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If apply pressure opposite that of direction of osmotic pressure, can purify solution
Reverse Osmosis
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Reverse Osmosis
Pure solvent Solution
Semipermeable membrane
Pressure applied > osmotic pressure
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Reverse Osmosis Water Purification
>30
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END OF CHAPTER