Ionic Crystals
CHEMISTRY 362—Worksheet
April 1, 2016
1. Consider an ionic compound MX2 where M is a metal that forms a cation of +2 charge, and X is a nonmetal
that forms an anion of –1 charge. A Born-Haber cycle for MX2 is given below. Each step in this cycle has been
assigned a number (1 – 7).
a) Identify which steps (1 - 7) that you expect to be endothermic as
written. ______________
b) Which step (1 - 7) corresponds to ΔH°sub? _____________
c) Which step (1 - 7) corresponds to ΔH°form? _____________
d) Use the following energy values to calculate the lattice energy (in
kJ/mol) for MX2. ΔH°sub = 296 kJ/mol; ΔH°form = -421 kJ/mol; 1st
ionization energy = 378 kJ/mol; 2nd ionization energy = 555 kJ/mol; bond
dissociation enthalpy = 310 kJ/mol; electron affinity = -427 kJ/mol.
2. Given the following information for magnesium, oxygen, and magnesium oxide calculate the second electron
affinity enthalpy for oxygen {i.e. for the reaction: O-(g) + e- 
for Mg (s), Δ Hsub = +148 kJ/mol
st
O2- (g)}.
bond dissociation energy for O2
st
= +499 kJ/mol
1 ionization energy for Mg = +738 kJ/mol;
1
2nd ionization energy for Mg+ = +1450 kJ/mol;
for MgO (s), lattice energy = –3890 kJ/mol
for MgO (s), enthalpy of formation = –602 kJ/mol
electron affinity enthalpy for O = –141 kJ/mol
CHEM 362 – worksheet
3. The Pauling ionic radii of the alkali metals are Li+ = 0.60
Å, Na+ = 0.95 Å; K+ = 1.33 Å; and Rb+ = 1.48 Å. The ionic
radius of the chloride anion, Cl- , is 1.67Å. a) From radius
ratios, what is the expected crystal form (or structure type)
of the alkali metal chlorides? b) Actually all of the MCl
salts crystallize in the rock salt structure. Based on your
radius ratios, how might you assign the following melting points: 1160, 1261, 1347, and 1316 °C.
Rationalize your choices.
4.
Consider the graphic at right.
a) In terms of the parameters on the x-axis, why do CsF and LiI have the most
negative ΔHsol values (in water) while CsI and LiF have positive values?
b) Solutions of which salts will feel cool as the salt
is dissolved in water?
pg. 2