SOLUTIONS
A. A Tendency toward Randomness, One Driving Force behind the Formation of a Solution
1. The tendency of a system, left to itself, to become increasingly disordered
2. This process is spontaneous
B. Attractions betweeen Solute and Solvent - Another Driving Force behind the Formation of a Solution
1. IM forces need to be broken up in solutes and solvents
2. IM forces between solutes and solvents are made
C. Solutions of Liquids in Liquids
1. Miscible -- two liquids that do dissolve in each other
2. Immiscible - two liquids that do not dissolve in each other
3. "like dissolves like"
a. Polar and ionic solutes dissolve in polar solvents
b. Nonpolar solutes dissolve in nonpolar solvents
D. Solutions of Solids in Liquids
1. Ion-dipole attractions pull ions from the crystal
2. Hydration (Solvation) of ions -- ions are surrounded by polar solvent molecules
a. "like dissolves like" applies here also
II. Heats of Solution (DeltaHsoln)
A. Molar enthalpy of solution
1. Particles of solute and solvent spread apart (endothermic)
2. Particles of solute and solvent then move back together (exothermic)
3. DeltaHsoln = difference of the two steps
B. Lattice Energies and Solvation Energies
1. Step 1 -- increase PE, lattice energy needed to change solute into vapor
2. Step 2 -- gas particles enter solvent and are solvated, decrease PE, solvation (hydration) energy
3. DeltaHsoln = lattice energy - solvation energy
a. lattice energy > solvation energy; solution forms endothermic
b. lattice energy < solvation energy; solution forms exothermic
C. Solutions of Liquids in Liquids
1. Step 1 -- (endothermic) spread solute particles apart
2. Step 2 -- (endothermic) spread solvent particles apart
3. Step 3 -- (exothermic) solute and solvent particle intermingle (drive for randomness)
4. Ideal Solutions -- DeltaHsoln = 0
D. Solutions of Gases in Liquids
1. Usually exothermic when water is the solvent
III. Solubility and the Effect of Temperature
A. Solubility -- mass of solute that forms a saturated solution with a given mass of solvent at a specific temperature
1. grams of solute/100 g solvent
2. Effect of Temperature on the Solubility of Solids and Liquids in Other Liquids
a. Increase the temperature of the system, the equilibrium shifts towards dissolved solute
1. More soluble
b. Increase the temerature of the system, the equilibrium shifts towards undissolved solute
1. Less soluble
3. Effect of temperature on the Solubility of a Gas in a Liquid
a. Increase temperature, solubility of gases in water decreases
IV. Effects of Pressure on the Solubilities of Gases
A. Increase pressure, solubility of gases in water increases
1. Henry's Law -- concentration of a gas in a liquid at any given temperature id directly proportional to the partial pressure of the gas on the solution
C(g) = k(g)P(g) (T is constant)
C1/P1 = C2/P2
B. Solubilities of Gases That Are Strongly Hydrated
1. Polar gases attract water molecules
2. Polar gases actually react with water
3. As gases react, more can dissolve to take the place of the reacted molecules
a. Cause of acid rain problem
5. Activity continues (evaporation and condensation are always occurring)
V. Temperature-Independent Concentration Units
A. Mole Fractions (Mole percent)
B. Weight Fraction and Weight Percent (percent by weight)
w(comp) = mass of component/mass of solution
1. Percent by weight = # of grams of solute in 100 grams of solvent (w/w)
2. can also use parts per million (ppm) and parts per billion (ppb)
C. Molal Concentration -- # of moles of solute per kilogram of solvent
m = mol of solute/kg of solvent
1. not the same as Molarity
D. conversions among Concentration Units
Examples on pg. 538-540
VI. Effects of Solutes on Vapor Pressures of Solutions
A. Colligative Properties
1. properties that depend on the relative population of particles in mixtures
B. Solutions of a Nonvolatile Solute
1. Raoult's Law -- the vapor pressure of the solution equals the product of the mole fraction of the solvent and its vapor pressure when pure
a. Liquid solutions of nonvolatile solutes have lower vapor pressures than their pure solvents
b. P(soln) = X(solvent)P(solvent)
c. DeltaP = X(solute)P(solvent)
2. Concentration of vapor to reach equilibrium is less, lower vapor pressure over solution than over pure solvent, amount a vapor pressure lowering corresponds th Raoult's Law
C. Solutions That Contain Two or More Volatile Components
1. P(total) = X(a)P(a) + X(b)P(b) [Dalton's Law!]
D. Ideal Solutions and Raoult's Law
1. Only ideal solutions abey Raoult's Law exactly
2. Ideal solution -- DeltaH(soln) is zero (all IM forces are identical
3. Deviations from Raoult's Law
a. Negative deviations -- when solutions form exothermically
1. Stronger IM forces between solute and solvent particles than in the pure form
b. Positive deviations -- when solutions form endothermically
1. Weaker IM forces between solute and solvent particles than in the pure form
VII. Effects of Solutes on Freezing and Boiling Points of Solutions
A. Effect of a Solute on the Phase Diagram of Water
1. Ice/vapor curve unaffected -- contain no solute particles
2. Solute in water lowers vapor pressure -- liquid/vapor curce is lower
3. Shifts triple point down and to the left -- solid/liquid curve is also lower
B. Freezing Point Deptression and Boiling Point Elevation
1. Freezing Point Depression -- lowering the freezing point of a solvent due to the addition of a nonvolatile solute (colligative property)
a. DeltaT(f) = K(f)m
2. Boiling Point Elevation -- raising the boiling point of a solvent due to the addition of a nonvolatile solute (colligative property)
a. DeltaT(b) = K(b)m
C. Determination of Molecular Masses
1. Remember that Colligative properties depend on the relative number of particles
Example:
VIII. Dialysis and Osmosis. Osmotic Pressure
A. Dialysis -- when a membrane is able to let both water and small solute particles through
B. Osmosis -- when a semipermeable membrane will only let solvent molcules get through
1. Direction of osmosis is from the more dilute solution to the more concentrated solution
a. Escaping tendency of more dilute solution is greater since there are less solute particles
C. Osmotic Pressure -- back pressure needed to prevent any osmotic flow when one liquid is a pure solvent
1. Can reverse osmosis by exceeding the back pressure
2. Concentration increases, the amount of water transfered to it increases
3. van'tHoff equation for osmotic pressure
IIV = nRT
4. Molecular Mass Determination using Osmotic Pressure
Example:
IX. Colligative Properties of Solutions of electrolytes
A. Expected Effect of the Dissociation of a Solute
1. To roughly estimate a colligative property of a solution of an electrolyte, we recalculate the solution's molality using an assumption about the way the solute dissociates or ionizes.
a. depends on number of particles
1. NaCl -- freezing point drops 3.37 degrees instead of 1.86 degrees (NaCl has two ions(particles) not one)
B. Effects of Interionic attractions
1. Problem with assumption -- electrolytes do not completely separate into ions
a. Some ions stick together in ion pairs, that act as one particle
2. Compare degree of dissociation of electrolytes at different dilutions by the van't Hoff factor (i)
a. i = (DeltaTf)measured/(DeltaTf)calc as a nonelectrolyte
b. greater dilution, the van't Hoff factor gets closer to the hypothetical answer
4. Estimation of Percent Dissociation from Freezing Point Depression Data
a. Estimate percent ionization
b. Apparent molality = DeltaT(f)/K(f)
c. Percent Ionization = mol of acid ionized/mol of acid available X 100%
C. Association of Solute Molecules and Colligative Properties
1. Association of solute particles -- some particles associate (stick together by hydrogen bonding)
2. Larger effective molecular mass reduces the molality and the freezing point depression by half
X. Colloidal Dispersions
A. Importance of Particle Size: Solutions, Collodial Dispersions, and Suspensions
1. Suspension -- a mixture made out of large particles that can be filtered
a. Particles settle to bottom when left alone
b. Particles are larger than 1000 nm
c. Heterogeneous mixtures
d. Tyndall Effect -- Scattering of light by solute particles
B. Properties and Types of Collodial Dispersions
1. Colloid -- a mixture made out mid-sized particles that can not be filtered
a. Particles range from 1 - 1000 nm
b. Heterogeneous mixtures
c. display the Tyndal Effect
2. Brownian Movement -- random, erratic motion of collodially dispersed particles in a fluid
3. Possible Colloidal combinations -- Gas in Liquid, Gas in Solid, Liquid in Gas, Liquid in Liquid, Liquid in Solid, Solid in Gas, Solid in Liquid, Solid in Solid
a. Emulsions -- liquids in liquids
C. How Soaps and Detergents Work
1. Structure -- long nonpolar hydrocarbon tail and a very polar or ionic head
2. The tail likes to dissolve oils and grease (like dissolve like)
a. Both are nonpolar (Hydrophobic -- water fearing)
3. The ionic head likes to attract to polar water molecules
a. Hydrophilic (water loving)
4. Soap anions gather together to form collodial groups called micelles taht are washed away in the water
Outline based upon:
Brady, J. E., Holum, J. R., Russell, J. W. (2000). Chemistry: The Study of Matter and Its Changes. (3rd ed.). New York: John Wiley & Sons, Inc. pp. 521-564.
