CHEM20018
Tutorial Week 5 Reactions and Synthesis: Thermodynamics of Inorganic Reactions
1. Provide a brief description of the structures of NaCl, zinc blende, wurtzite, CsCl and CaF2 in terms of the close packing of ions and the occupation of interstitial sites. What factors affect the type of packing that is found?
2. The calculated enthalpy change associated with the reaction:
Ca(s) + F2(g) —> Ca2+(g) + 2F-(g)
is +1415 kJmol-1
Given that the reaction to form Ca2+ ions and F- ions is highly endothermic, why does calcium react with fluorine to form. the ionic compound CaF2(s)?
3. The Madelung constant is used in both the Born-Landé and Born-Mayer equations.
a) What factor determines the value of the Madelung constant used in these equations?
b) Does a high Madelung constant lead to a greater or lower lattice enthalpy? Why?
4. The Born-Landé and Born-Mayer equations take into account the contribution of Born forces to the lattice enthalpy in addition to the Coulombic interactions. What is the origin of the Born forces?
5. The Kapustinskii equation allows the estimation of the lattice enthalpy but unlike the Born-Landé and Born-Mayer equations it does not include the Madelung constant.
a) Explain why the Kapustinskii equation does not include the Madelung constant.
b) Would you expect the Born-Mayer equation or the Kapustinskii equation to give a more accurate estimation of the lattice enthalpy?
6. BaCl2 crystallises in two crystalline forms one of which has the fluorite (CaF2) structure.
a) Using the Born-Haber cycle calculate the lattice enthalpy.
b) Using the Born-Landé equation estimate the lattice enthalpy.
c) Using the Born-Mayer equation estimate the lattice enthalpy.
d) Using the Kapustinskii equation estimate the lattice enthalpy.
Data and equations for Tutorial 1
ΔatomHo(Ba) = +180 kJ mol-1; ΔdisHo (Cl2) = +244 kJ mol-1; Δion1stHo (Ba) = 509 kJ mol-1; Δion2ndHo (Ba) = 971 kJ mol-1; ΔegHo (Cl) = -355 kJ mol-1; ΔfHo(BaCl2) = -859 kJ mol-1.
NA = 6.022 x 1023 ; e = 1.602 x 10-19 C; εo = 8.854 x 10-12 J-1C2m-1; d* = 34.5 pm; A (CaF2) = 2.519, Born exponent values, n: Ba2+ 12, Cl- 9; ionic radius of Ba2+ : 135 (6-cordinate), 142 (8-coordinate) pm;
radius Cl-: 181 (6-coordinate), 175 (4-coordinate) pm; k = 1.21 x 105 kJ pm mol-1.
Born-Landé equation: ΔLHo = NAA(½zAzB ½e2/4πεod)(1 – 1/n)
Born-Mayer equation: ΔLHo = NAA(½zAzB ½e2/4πεod)(1 – d*/d)
Kapustinskii equation: ΔLHo = Nion(½zAzB ½/d)(1 – d*/d)k
Radius of Mg2+ 72 pm (6–coordinate), Radius of Ba2+ 135 pm (6–coordinate)
Thermochemical radius of CO32- 178 pm.
Supplementary Questions
S1. Crystalline ionic solids represent examples of highly ordered materials.
i) What is the driving force for the formation of an ionic solid?
ii) What factors impact upon the stability of an ionic solid?
S2. The lattice enthalpies of MgCO3 and BaCO3 may be estimated using the Kapustinskii equation. The thermochemical radius of the carbonate anion is estimated as 178 pm. The 6-coordinate radii of Mg2+ and Ba2+ are 72 and 135 pm respectively.
a) What is a thermochemical radius?
b) Using the Kapustinskii equation estimate the lattice enthalpy of MgCO3 and BaCO3.
c) Would you expect MgCO3 or BaCO3 to be more stable with respect to decomposition? Why?
S3. For the following compounds indicate whether they would be considered as a basic oxide, an acidic oxide or an amphoteric oxide.
SO2, BaO, Li2O, Ga2O3
For the acidic oxides and basic oxides give equations that illustrate their acidic or basic behaviour. For the amphoteric oxides give equations that illustrate both acidic and basic behaviour.