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2/20/2015

Chapter 11: Properties of - Their and Colligative Properties

Chapter Outline

. 11.1 Changes when Substances Dissolve . 11.2 Vapor Pressure . 11.3 of Volatile Substances . 11.4 Colligative Properties of Solutions . 11.5 Osmosis and Osmotic Pressure . 11.6 Using Colligative Properties to Determine Molar Mass

1 2/20/2015

Enthalpy of

. Dissolution of Ionic Solids:

• of solution (∆Hsoln) depends on: » holding solute ions in crystal lattice » Attractive force holding 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

2 2/20/2015

Calculating Lattice Energies Using the Born-Haber Cycle

Lattice energy (U) is the energy required to completely separate one mole of a solid into gaseous ions. It is always endothermic.

2+ - e.g. MgF2(s)  Mg (g) + 2F (g)

푄 푥푄 Lattice energy (E) increases as Q 퐸 = 2.31 푥 10−19퐽 ∙ 푛푚 1 2 푒푙 푑 increases and/or as r decreases.

Lattice Q1 is the charge on the cation Compound Energy (U) Q2 is the charge on the anion

d is the distance between the ions MgF2 2957 Q= +2,-1

MgO 3938 Q= +2,-2

LiF 1036 radius F < LiCl 853 radius Cl

3 2/20/2015

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) + X(g) M+(g) + X(g)

EA1 + EA2 etc IE1 + IE2 etc M+(g) + X-(g)

M(g) + X(g)

½ ΔHBE M(g) + ½ X2(g) ΔHlast = -Ulattice

ΔHsub M(s) + ½ X2(g)

ΔHf MX(s)

4 2/20/2015

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) + Cl(g) Na+(g) + Cl(g)

EA1 IE1 Na+(g) + Cl-(g)

Na(g) + Cl(g)

½ ΔHBE Na(g) + ½ Cl2(g) ΔHlast = -Ulattice

ΔHsub Na(s) + ½ Cl2(g)

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

7 2/20/2015

Lattice Energies and

푄 푥푄 퐸 = 2.31 푥 10−19퐽 ∙ 푛푚 1 2 푒푙 푑

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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