Introduction to Inorganic Chemistry/Ionic and Covalent Solids - Energetics
- 1 Chapter 9: Ionic and Covalent Solids - Energetics
- 2 9.1 Ionic radii and radius ratios
- 3 9.2 Structure maps
- 4 9.3 Energetics of crystalline solids: the ionic model
- 5 9.4 Born-Haber cycles for NaCl and silver halides
- 6 9.5 Kapustinskii equation
- 7 9.6 Discovery of noble gas compounds
- 8 9.7 Stabilization of high and low oxidation states
- 9 9.8 Alkalides and electrides
- 10 9.9 Resonance energy of metals
- 11 9.10 Lattice energies and solubility
- 12 9.11 Discussion questions
- 13 9.12 Problems
- 14 9.13 References
Chapter 9: Ionic and Covalent Solids - Energetics
9.1 Ionic radii and radius ratios
9.2 Structure maps
9.3 Energetics of crystalline solids: the ionic model
9.4 Born-Haber cycles for NaCl and silver halides
9.5 Kapustinskii equation
9.6 Discovery of noble gas compounds
9.7 Stabilization of high and low oxidation states
9.8 Alkalides and electrides
9.9 Resonance energy of metals
9.10 Lattice energies and solubility
9.11 Discussion questions
- Explain why lattice energy calculations are very accurate for NaCl and CaCl2, but less accurate (by about 10%) for AgCl and PbCl2. Does the Born-Mayer equation under- or overestimate the latter values?
- Fluorine is more electronegative than oxygen. However, for many transition metals, we can make higher oxidation states in oxides than we can in fluorides. For example, Mn(IV) is stable in an oxide (MnO2), but MnF4 is unstable relative to MnF3 and fluorine. Can you explain this in terms of lattice energies?
1. Use lattice energies to explain why MgSO4 decomposes to magnesium oxide and SO3 at a much lower temperature than does BaSO4.
2. Solid MgO might be formulated as Mg+O- or Mg2+O2-. Use the thermochemical data below (some of which are irrelevant) and Kapustinskii's formula to determine which is more stable. The lattice constant for MgO (NaCl structure) is 4.213 Å. While the idea of an O- ion might seem strange, note that the second electron affinity of O and the second ionization potential of Mg (in the table below) are both quite endothermic.
|Mg(s) = Mg(g)||35.3|
|Mg(g) = Mg+(g) + e-||176.6|
|Mg(g) = Mg+(g) + e-||347.0|
|O2(g) = 2 O(g)||119.0|
|O(g) + e- = O-(g)||-33.7|
|O-(g) + e- = O2-(g)||188.9|
3. From the heat of formation of solid NH4Cl (-75.2 kcal/mol) and gaseous NH3 (-11.0), the bond dissociation energies of H2 (104.2) and Cl2 (58.2), the ionization potential of atomic hydrogen (313.4), and the electron affinity of atomic chlorine (-83.4), calculate the gas-phase proton affinity of NH3. The lattice energy of NH4Cl may be estimated from Kapustinskii's formula using rN-Cl = 3.50 Å.
4. Bottles of aqueous ammonia are often labeled “ammonium hydroxide.” We will test this idea by using a lattice energy calculation to determine whether the salt NH4+OH- can exist.
The heats of formation of gaseous OH- and H2O are respectively -33.7 and -57.8 kcal/mol. Assuming that NH4+ is about the same size as Rb+, and OH- about the same size as F-, using Kapustinskii's formula, ionic radii, and the NH3 proton affinity calculated in problem 3, determine whether NH4+OH- should be a stable salt relative to NH3 and H2O. At what temperature should NH4+Cl- be unstable relative to NH3 and HCl, if ΔHfo for HCl is -22.0 kcal/mol and ΔSo (NH4Cl --> NH3 + HCl) = 67 cal/mol K?
5. (a) Do you expect BaSO4 or MgSO4 to be more soluble in water? (b) Is LiF more soluble than LiClO4? Explain.
6. Which polymorph of ZnS (zincblende or wurzite) would you expect to be more stable on the basis of electrostatic energy?