The Chemistry of Solutes and Solutions

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Chapter 15

• The Chemistry of Solutes and Solutions

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Overview

• Solubility and Intermolecular Forces
• Enthalpy, Entropy, and Dissolving Solutes
• Solubility and Equilibrium
• Temperature and Solubility
• Pressure and Dissolving Gases in Liquids Henrys
  Law
• Solution Concentration Keeping Track of Units
• Vapor Pressure, Boiling Points, and Freezing
  Points of Solutions

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Solubility and Intermolecular Forces

• Solutions are comprised of
• Solute component in the least amount
• Solvent component in the greatest amount
• Solutions are typically thought of as solids
  dissolved in liquid or two liquids mixed together
• Other possibilities exist
• Gas in gas
• Gas in liquid
• Gas in solid
• Solid in solid

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Solubility and Intermolecular Forces

• Solute-Solvent Interactions
• Like Dissolves Like
• Substances with similar intermolecular forces
  will dissolve in each other miscible
• Substance with different intermolecular forces do
  not dissolve in each other and are said to be
  immiscible

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The major types of intermolecular forces in
solutions
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Solubility and Intermolecular Forces

• Concept of intermolecular force must be taken one
  step further
• It is the primary force that matters

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Problems
(a) Sodium chloride in methanol (CH3OH) or in
propanol (CH3CH2CH2OH)
(b) Ethylene glycol (HOCH2CH2OH) in hexane
(CH3CH2CH2CH2CH2CH3) or in water.
(c) Diethyl ether (CH3CH2OCH2CH3) in water or in
ethanol (CH3CH2OH)
SOLUTION
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Enthalpy, Entropy, and Dissolving Solutes
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Enthalpy, Entropy and Dissolving Solutes

• ?Hsoln ?Hsolute ?Hsolvent ?Hmix
• If ?Hsolute ?Hsolvent gt ?Hmix
• Dissolving is not favored by the enthalpy effect

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Enthalpy, Entropy and Dissolving Solutes

• Entropy A measure of the dispersal of energy in
  a system
• A more chaotic system is preferred to a more
  organized system
• ?S gt 0 indicates a process where energy is spread
  out more products tend to be favored

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Solubility and Equilibrium

• Solubility the maximum amount of solute that
  dissolves in a given concentration of solvent at
  a particular temperature
• Saturated solution solute concentration
  solubility dynamic equilibrium
• Unsaturated solution solute concentration lt
  solubility QltK hence more solute can dissolved
• Supersaturated solution more dissolved solute
  is present than is allowed by equilibrium Qgtk
  hence the reactants are favored

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Solubility and Equilibrium

• Process of dissolving ionic compounds in water -
  hydration

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Heats of Hydration
solvation of ions by water - always exothermic
DHhydr is related to the charge density of the
ion, that is, coulombic charge and size matter.
Lattice energy is the DH involved in the
formation of an ionic solid from its gaseous ions.
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Dissolving ionic compounds in water
NH4NO3
NaCl
NaOH
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Solubility and Equilibrium

• Entropy
• Dissolving of ions into solvent is a entropy
  favored process ?S gt 0
• A portion of the entropy change obtained by
  hydration is counteracted with the water
  molecules becoming more ordered ?S lt 0
• The difference between these two factors help
  explain why some ionic compounds dissolve while
  others do not
• Overall entropy change gt0 hydration favored (1,
  -1) compounds
• Overall entropy change lt0 only a small amount of
  solid will be hydrated (2, 3)

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Temperature and Solubility

• Gas solute increase in temperature, decrease in
  solubility
• Solid increase in temperature, a typical
  increase in solubility to determine must look
  at ?H and use Le Chateliers principle

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Pressure and Dissolving Gases in Liquids Henrys
Law

• Solubility of a gas increases with increased
  pressure

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Pressure and Dissolving Gases in Liquids Henrys
Law

• Henrys law is the relationship between pressure
  and solubility

Sgas kH X Pgas
The solubility of a gas (Sgas) is directly
proportional to the partial pressure of the gas
(Pgas) above the solution.
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Probem
PLAN
Knowing kH and Pgas, we can substitute into the
Henrys law equation.
SOLUTION
0.1 mol/L
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Concentration Definitions
Concentration Term
Ratio
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Problem
PLAN
We have to convert the grams of CaCl2 to moles
and the grams of water to kg. Then substitute
into the equation for molality.
SOLUTION
0.288 mole CaCl2
1.06 m CaCl2
molality
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Problem
PROBLEM
(a) Find the concentration of calcium (in ppm)
in a 3.50-g pill that contains 40.5 mg of Ca.
(b) The label on a 0.750-L bottle of Italian
chianti indicates 11.5 alcohol by volume. How
many liters of alcohol does the wine contain?
(c) A sample of rubbing alcohol contains 142 g
of isopropyl alcohol (C3H7OH) and 58.0 g of
water. What are the mole fractions of alcohol
and water?
PLAN
(a) Convert mg to g of Ca, find the ratio of g
Ca to g pill and multiply by 103.
(b) Knowing the alcohol and total volume, we
can find volume of alcohol.
(c) Convert g of solute and solvent to moles
find the ratios of parts to the total.
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SOLUTION
(a)
1.16x104 ppm Ca
3.5 g
(b)
0.0862 L alcohol
(c)
moles ethylene glycol
142 g
2.36 mol C2H6O2
moles water
38.0g
3.22 mol H2O
2.39 mol C2H8O2
3.22 mol H2O
2.39 mol C2H8O2 3.22 mol H2O
2.39 mol C2H8O2 3.22 mol H2O
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(a) Molality
(b) Mole fraction of H2O2
(c) Molarity
PLAN
(a) To find the mass of solvent we assume the
is per 100 g of solution. Take the difference in
the mass of the solute and solution for the mass
of peroxide.
(b) Convert g of solute and solvent to moles
before finding c.
(c) Use the density to find the volume of the
solution.
70.0 g H2O
SOLUTION
(a)
g of H2O 100. g solution - 30.0 g H2O2
30.0 g H2O2
mol H2O2
34.02 g H2O2
molality
12.6 m H2O2
kg H2O
70.0 g H2O
103 g
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(b)
70.0 g H2O
3.88 mol H2O
0.882 mol H2O2
0.185 c of H2O2
0.882 mol H2O2 3.88 mol H2O
(c)
90.1 mL solution
100.0 g solution
0.882 mol H2O2
9.79 M H2O2
90.1 mL solution
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Vapor Pressure, Boiling points, and Freezing
Points of Solutions

• Vapor pressure of a pure solvent is lowered when
  a solute is added.

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Vapor Pressure, Boiling points, and Freezing
Points of Solutions

• The amount that a dissolved solute will lower the
  vapor pressure can be determined by Raoults law

Raoults Law (vapor pressure of a solvent above a
solution, Psolvent)
Psolvent csolvent P0solvent where
P0solvent is the vapor pressure of the pure
solvent and Psolvent is the vapor pressure of
the solvent over the solution and Xsolvent is
the mole fraction of solvent in the solution
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SOLUTION
10.0 mL C3H8O3
0.137 mol C3H8O3
x
27.4 mol H2O
500.0 mL H2O
x
0.137 mol C3H8O3
DP
92.5 torr
x
0.461 torr
0.137 mol C3H8O3 27.4 mol H2O
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Vapor Pressure, Boiling points, and Freezing
Points of Solutions

• Boiling point increases with a decrease in vapor
  pressure

DTb Kbmsolute ?Tb boiling point of solution
boiling point of pure solvent Kb the molal
boiling point elevation constant msolute
molality of the solute
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Vapor Pressure, Boiling points, and Freezing
Points of Solutions

• Freezing occurs due to a few molecules clustering
  together to form a solid to which other molecules
  will adhere
• Solute is going to lower the freezing point of a
  pure solvent

?Tf Kfmsolute
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SOLUTION
1.00x103 g C2H6O2
16.1 mol C2H6O2
16.1 mol C2H6O2
3.62 m C2H6O2
4.450 kg H2O
DTbp
0.512 0C/m
3.62m
x
1.850C
DTfp
1.86 0C/m
3.62m
x
BP 101.85 0C
FP -6.73 0C