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Chapter 14. Solutions. “how things dissolve”. – homogeneous mixtures of 2 or more substances – does the dissolving (H 2 O is the universal solvent) – what gets dissolved 3 things happen when dissolving occurs: 1. solvent molecules split up 2. solute units split up

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Chapter 14

Chapter 14

Solutions


How things dissolve
“how things dissolve”

  • – homogeneous mixtures of 2 or more substances

    • – does the dissolving (H2O is the universal solvent)

    • – what gets dissolved

      3 things happen when dissolving occurs:

      1. solvent molecules split up

      2. solute units split up

      3. Form solvent/solute attractions


Rules for solutions likes dissolve like
Rules for solutions “likes dissolve like”

  • ~ water is highly polar, dissolves polar solutes and ionic solutes fairly well

  • – solvent/solute attraction

    H2O

    NaCl(s)  Na+(aq) + Cl-(aq)

  • electrolytes dissociate into ions


  • __________– molecules split up b/c of the weak attraction into smallest molecule unit

    H2O

    C12H22O11 C12H22O11

    Non-electrolytes undergo __________ __________

    Ex. oil/water vs. oil/benzene


~ liquid in liquid – often volumes are not additive, called __________.

Why? intermolecular attractions

 50 mL H2O + 50 mL methanol ≈ 97 mL total

gas in liquid – same attraction

  • gases tend to be less soluble at higher temps.

  • The __________the pressure, the __________soluble, b/c the gas is forced into the solution more. If pressure __________the gas is ________soluble



  • __________ crystals______– a sol’n has its maximum amount of solute dissolved in it. You always see un-dissolved solute. (ex. 40.0g NaCl in room temp. water) When a sol’n is saturated, the solute is still dissolving – it’s just that the solute is also re-crystallizing out of the sol’n at the same rate, so there is no apparent change.


  • __________ crystals______– by heating a saturated sol’n and carefully cooling it, you can get more solute dissolved then theoretically possible. It is an unstable condition that is the basis for rock candy.


Spontaneity of the dissolution process
Spontaneity of the Dissolution Process crystals

  • Assume solvent is a liquid

  • Major factors that affect dissolution of solutes (dissolving of solutes)

    • change of energy content, DHsolution

      • exothermic favors dissolution

      • endothermic does not favor dissolution

    • change in disorder, or randomness, DSmixing

      • increase in disorder favors dissolution

      • increase in order does not favor dissolution

  • Best conditions for dissolution

    • ____________________ __________


  • Molarity crystals: M= n/V

    n = moles of solute V = Liters of solution

  • Molality: m = n/kg

    n = same kg = mass of solvent

  • Mole fraction (x) number of moles of one part per moles of all parts of the solution

    xa = naxb = nb etc…

    ntotal ntotal


Ex. 1) What is the crystalsmolality and mole fraction of 50.0 g potassium chloride in 425 g water?


Colligative properties
Colligative crystals properties

  • __________ __________ of solutions that depend only on the number of particles dissolved and not the kinds of particles dissolved.

    __________

    • Vapor Pressure

    • Boiling Point Elevation

    • Freezing Point Depression

    • Osmotic Pressure


1 vapor pressure
1. crystals Vapor pressure

-- Pvap goes down with increase in concentration.

Water vs. water + sugar *more sugar

higher Pvap vs. lower Pvap

In solns, fewer molecules have the chance to turn into vapor

__________ – the vapor pressure of an ideal solution is directly proportional to the mole fraction of the solvent in the solution

Pvap = (Pvapo)(xsolvent)

~ ideal gases obey PV = nRT very well

~ Raoult’s Law is close to ideal when the solute and solvent have very similar I.F.


Effect of pressure on solubility
Effect of Pressure on Solubility crystals

  • Pressure changes have ________or __________ on solubility of liquids and solids in liquids

  • Pressure changes have __________effects on the solubility of gases in liquids

    • why carbonated drinks fizz when opened

    • cause of several scuba diving related problems including the “bends”


2 3 boiling point elevation freezing point depression
2. & 3. crystals Boiling Point Elevation & Freezing Point Depression

  • __________exist as liquids over a wider temp. range

  • solutions move the __________ ______of a substance further down the curve.

      pg. 563 & next slide show a phase diagram comparison between pure solvent and a solution

  • Why?

  • __________– more attractions need to be overcome before boiling can occur. _______________– ions get in the way of freezing.


Phase diagram
Phase Diagram crystals


Dissociation of electrolytes colligative properties
Dissociation of Electrolytes & Colligative Properties crystals

  • _____________have larger effects on boiling point elevation & freezing point depression than soluble ____________________

  • one mole of sugar dissolves in water to produce one mole of aqueous sugar molecules

  • one mole of NaCl dissolves in water to produce 1 mole of Na+ & 1 mole of Cl- ions

  • colligative properties depend on number of __________particles

    • expect twice the effect for NaCl than for sugar


Ex for f p b p
Ex. for F. P & B.P. crystals

  • Salt in water for pasta raises the B.P.

  • Which of the following raises the B.P. more?

    • 0.1 M HF vs. 0.1 M HCl

      • Weak electrolyte vs. strong electrolyte

        (s.e. dissociates thus higher B.P.)

    • 0.1 M CH3OH vs. 0.1 M CH3COOH

      • Nonelectrolyte vs. weak electrolyte

        (w.e. slightly soluble so higher B.P.)

  • Salt on the road ~ often CaCl2 (3 particles dissolved) is chosen over NaCl (2 particles dissolved) CaCl2 lowers F.P. more

  • Homemade ice-cream ~ Yummy!


B p f p equations
B.P & F.P. Equations crystals

  • Boiling point elevation ∆Tb = imkb

    ∆T = change in temp. m = molality k = constant

    i = van’thoff factor ~ measures the extent of ionization (number of dissolved particles)

  • Freezing point depression ∆Tf = imkf pg 564. table 14-2


Ex. 2) 15.0 g of ethanol is dissolved in 750. g of formic acid. The freezing point of the solution is 7.20oC. The freezing point of pure formic acid is 8.40oC. What is kf for formic acid?


4 osmotic pressure
4. acid. The freezing point of the solution is 7.20Osmotic pressure

  • the pressure produced on the surface of a semipermeable membrane by osmosis.

    • __________– net flow of solvent between two solns separated by a semipermeable membrane

    • Solvent passes from __________concentration soln. into __________concentration soln.

    • Ex. of semipermeable membranes ~ skin, cell membranes, cellophane, and saran wrap

  • π = MRT

    π = symbol for osmotic pressure

    M = molarity T = temp

    R = 0.082057 L atm/mol K


Ex. 3) What is the molar mass (grams/mole) of 30.0 acid. The freezing point of the solution is 7.20mL of an unknown substance if 0.300 g of that substance has an osmotic pressure of 0.400kPa at room temperature?


Colloids
Colloids acid. The freezing point of the solution is 7.20

  • Mixtures that have particle sizes between those of true solutions and suspensions (suspensions have particles that are very large, they will settle out unless the mixture is constantly stirred).

    • Fog, smoke, paint, milk

  • The _______________– colloids scatter light when its shined on them.


Hydrophilic hydrophobic colloids
Hydrophilic & Hydrophobic Colloids acid. The freezing point of the solution is 7.20

  • ________________– water loving colloids

    • Blood plasma, some biological proteins

  • ________________– water hating colloids, require emulsifying agents to stabilize in water (emulsion helps keep two things together that normally would not mix)

    • __________– emulsion of fat and proteins mixed in water, casein is the emulsifier

    • __________– oil and eggs with water, lecithin from egg yolk is the emulsifier

    • __________and __________ are excellent emulsifying agents ~ hard water reacts with soap anions and precipitates making bathroom scum


Hydrophilic hydrophobic colloids1
Hydrophilic & Hydrophobic Colloids acid. The freezing point of the solution is 7.20


  • Medicines that are injected into humans such as shots must be at the same concentration as the existing chemical in our blood. These solutions are called isotonic. For example, if the medicine contains potassium ions, they must be the same concentration as the potassium ions in our blood. Why do medicines have to be made that way?


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