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Chapter 11: Properties of Solutions Their Concentrations and Colligative Properties
Chapter Outline
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11.1 Energy Changes when Substances Dissolve
11.2 Vapor Pressure
11.3 Mixtures of Volatile Substances
11.4 Colligative Properties of Solutions
11.5 Osmosis and Osmotic Pressure
11.6 Using Colligative Properties to Determine Molar Mass
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Enthalpy of Solution
 Dissolution of Ionic Solids:
• Enthalpy of solution (∆Hsoln) depends on:
» Energies holding solute ions in crystal lattice
» Attractive force holding solvent molecules together
» Interactions between solute ions and solvent
molecules
• ∆Hsoln = ∆Hion-ion + ∆Hdipole-dipole + ∆H ion-dipole
• When solvent is water:
» ∆Hsoln = ∆Hion-ion + ∆Hhydration
Enthalpy of Solution for NaCl
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Calculating Lattice Energies Using the Born-Haber Cycle
Lattice energy (U) is the energy required to completely separate one mole of
a solid ionic compound into gaseous ions. It is always endothermic.
e.g. MgF2(s)  Mg2+(g) + 2F-(g)
𝐸𝑒𝑙 = 2.31 𝑥 10−19 𝐽 ∙ 𝑛𝑚
𝑄1 𝑥𝑄2
𝑑
Q1 is the charge on the cation
Lattice energy (E) increases as Q
increases and/or as r decreases.
Compound
Lattice
Energy (U)
MgF2
2957
Q= +2,-1
MgO
3938
Q= +2,-2
LiF
LiCl
1036
853
radius F <
radius Cl
Q2 is the charge on the anion
d is the distance between the ions
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Born-Haber Cycles – Calculating Ulattice
Hess’ Law is used to calculate Ulattice
The calculation is broken down into a series of steps (cycles)
Steps:
1. sublimation of 1 mole of the metal
= ΔHsub
2. if present, breaking bond of a diatomic
= ½ ΔHBE
3. ionization of the metal(g) atom
= IE1 + IE2 etc
4. electron affinity of the nonmetal atom
= EA1 + EA2 etc
5. formation of 1 mole of the salt from ions(g) = ΔHlast = -Ulattice
In General for M(s) + ½ X2(g)  MX(s)
M+(g) +
M+(g) +
X(g)
X(g)
EA1 + EA2 etc
IE1 + IE2 etc
M+(g) +
M(g) +
X(g)
M(g) +
½ X2(g)
M(s) +
½ X2(g)
X-(g)
½ ΔHBE
ΔHlast = -Ulattice
ΔHsub
ΔHf
MX(s)
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Calculating Ulattice
ΔHf° = ΔHsub + ½ ΔHBE +  IE +  EA + ΔHlast
Rearranging and solving for ΔHlast -
ΔHlast = ΔHf − ΔHsub − ½ ΔHBE −  IE −  EA
Ulattice = -ΔHlast
Calculating Ulattice for NaCl(s)
Na+(g) +
Na+(g) +
Cl(g)
Cl(g)
EA1
IE1
Na+(g) +
Na(g) +
Cl-(g)
Cl(g)
½ ΔHBE
Na(g) +
½ Cl2(g)
Na(s) +
½ Cl2(g)
ΔHlast = -Ulattice
ΔHsub
ΔHf
NaCl(s)
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Calculating Ulattice for NaCl(s)
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ΔHlast = ΔHf − ΔHsub − ½ ΔHBE − IE1 − EA1
Sample Exercise 11.1
Calculating Lattice Energy
Calcium fluoride occurs in nature as the mineral fluorite, which is the
principle source of the world’s supply of fluorine. Use the following data to
calculate the lattice energy of CaF2.
ΔHsub,Ca = 168 kJ/mol
BEF2 = 155 kJ/mol
EAF = -328 kJ/mol
IE1,Ca = 590 kJ/mol
IE2,Ca = 1145 kJ/mol
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Lattice Energies and Solubility
𝐸𝑒𝑙 = 2.31 𝑥 10−19 𝐽 ∙ 𝑛𝑚
𝑄1 𝑥𝑄2
𝑑
Q1 is the charge on the cation
Q2 is the charge on the anion
d is the distance between the ions
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