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Binary Ionic Compound
Solid formed between a metal and a nonmetal
The oppositely charged ions together have lower energy
Lattice Energy
The change in energy that takes place when separated gaseous ions are packed together to form an ionic solid
Lattice Energy
The energy released when an ionic solid is formed
M+(g) + X-
(g) --> MX(s)
Sign will be negative b/c process is exothermic
Energy Changes
Look at the formation of an ionic solid from its elements
Keep in mind that energy is a STATE FUNCTION!!
Li(s) + ½ F2(g) --> LiF(s)
Sublimation of solid LiLi(s) --> Li(g)
Enthalpy for sublimation is 161 kJ/mol
Li(s) + ½ F2(g) --> LiF(s)
Ionization of Li atoms to form Li+
Li(g) --> Li+(g) + e-
Ionization is 520 kJ/mol
Li(s) + ½ F2(g) --> LiF(s)
Dissociation of F2 molecules to F atoms
½ F2(g) --> F(g)
154 kJ/mol Divide by two = 77 kJ/mol
Li(s) + ½ F2(g) --> LiF(s)
Formation of F- ionsElectron affinityF(g) + e- --> F-
(g)
Electron affinity = - 328 kJ/mol
Li(s) + ½ F2(g) --> LiF(s)
Formation of LiF(s)
Lattice energyLi+(g) + F-
(g) --> LiF(s)
Lattice energy = -1047 kJ/mol
Li(s) + ½ F2(g) --> LiF(s)
Sum of these 5 processes yields the desired overall reaction
-617 kJ (per mole of LiF)
K(s) + ½ Cl2(g) --> KCl(s)
Sublimation of K = +64 kJ Ionization of K = +419 kJ Bond energy of Cl2 = +240 kJ
e- affinity of Cl = -349 kJ Lattice energy = -690 kJ
K(s) + ½ Cl2(g) --> KCl(s)
Net energy of formation equals the sum of the energy changes
Hfo = -436 kJ
Lattice Energy Calculations
Lattice energy is important in contributing to the stability of the compounds
Lattice Energy Calculations
Modified from Coulomb’s Law
Lattice energy = k(Q1Q2/r)k = constant that depends
on structure of solid
Lattice EnergyThis value counteracts the
higher endothermic ionization energies, thus resulting in a more stable energetically stable crystal
Li(s) + ½ Br2(g) --> LiBr(s)
Ionization of Li = +520 kJ/mol e- affinity for Br = -324 kJ/mol sublimation of Li = +161 kJ/mol lattice energy = -787 kJ/mol bond energy Br2 = +193 kJ/mol