Thermodynamics of Crystal Formation

Thermodynamics of Crystal Formation ! All stable ionic crystals have negative standard enthalpies of formation, ΔHof, and negative standard free energies of formation, ΔGof. Na(s) + ½ Cl2(g) ÷ NaCl(s) ΔHof = –410.9 kJ ΔGof = –384.0 kJ Cs(s) + ½ Cl2(g) ÷ CsCl(s) ΔHof = –442.8 kJ ΔGof = –414.4 kJ Mg(s) + ½O2(g) ÷ MgO(s) ΔHof = –385.2 kJ ΔGof = –362.9 kJ Ca(s) + C(s) + 3/2 O2(g) ÷ CaCO3(s) ΔHof = –1216.3 kJ ΔGof = –1137.6 kJ ! The exothermic and spontaneous formation of ionic solids can be understood in terms of a Hess's Law cycle, called the Born-Haber cycle. ! The lattice energy is the most important factor in making the formation of ionic crystals exothermic and spontaneous. ! Lattice energy, U, is defined as the enthalpy required to dissociate one mole of crystalline solid in its standard state into the gaseous ions of which it is composed; e.g., NaCl(s) ÷ Na+(g) + Cl–(g) U = +786.8 kJ U Defined in this way, lattice energy is a positive (endothermic) quantity. U Sometimes lattice energy is defined by the reverse reaction, in which case the values are negative (exothermic). Born-Haber Cycle for NaCl(s) Na+(g) + Cl-(g) I = +496 kJ A = -349 kJ -U = ? Na(g) + Cl(g) o Hsub = +107.7 kJ 2D = +121.7 kJ Na(s) + 2Cl2(g) o H f Na(s) 6 Na(g) Na(g) 6 Na+(g) + e– ½Cl2(g) 6 Cl(g) Cl(g) + e– 6 Cl–(g) Na+(g) + Cl–(g) 6 NaCl(s) Na(s) + ½Cl2(g) 6 NaCl(s) = -410.9 kJ NaCl(s) ΔHosub = 107.7 kJ I = 496 kJ ½D = 121.7 kJ A = –349 kJ –U = ? ΔHof = –410.9 kJ ΔHof = ΔHosub + I + ½D + A – U Y ˆ U = ΔHosub + I + ½D + A – ΔHof = 107.7 kJ + 496 kJ + 121.7 kJ + (–349 kJ) – (–410.9 kJ) = 787 kJ Factors Favoring a More Stable Crystal Lattice Large values of lattice energy, U, are favored by 1. Higher ionic charges 2. Smaller ions 3. Shorter distances between ions Selected Lattice Energies, Uo (kJ/mol) (Born-Haber Cycle Data) F– Cl– Br– I– O2– Li+ 1049.0 862.0 818.6 762.7 2830 Na+ 927.7 786.8 751.8 703 2650 K+ 825.9 716.8 688.6 646.9 2250 Rb+ 788.9 687.9 612 625 2170 Cs+ 758.5 668.2 635 602 2090 Mg2+ 2522 3795 Ca2+ 2253 3414 Sr2+ 2127 3217 Calculating Lattice Energy U In principle, the lattice energy for a crystal of known structure can be calculated by summing all the attractive and repulsive contributions to the potential energy. ! For a pair of gaseous ions where Z+, Z– = ionic charges ro = distance between ions e = electronic charge = 1.602 × 10–19 C 4πεo = vacuum permittivity = 1.11 × 10–10 C2@J–1@m–1 ! Potential energy is negative for the attraction of oppositely charged ions and positive for repulsion of like-charged ions. ! The potential energy arising from repulsions and attractions acting on one reference ion can be calculated. ! Scaled up to a mole of ion pairs (and with a change of sign) this should equal the lattice energy of the crystal. Calculating U for NaCl Consider the potential energy arising from attractions and repulsions acting on a central Na+ ion of NaCl. Neighbors Distanc e 6 Cl– ro 12 Na+ 8 Cl– 6 Na+ 24 Cl– @@@ L @@@ The series in parentheses converges at a value that defines the Madelung constant, M. Calculating U for NaCl ! For a mole of ion pairs (N), using the Madelung constant (M), the expression for the potential energy of an NaCl-type lattice due to Coulombic interactions is L For the NaCl-type lattice M = 1.74756, and for NaCl ro = 280 pm. Using these values, UNaCl = –867 kJ/mol, which is too negative (cf., –786.8 kJ/mol from Born-Haber data). • Discrepancy arises from assuming ions are point charges. • Electron clouds of adjacent ions repel each other as they approach one another. ! Born proposed that the repulsive (positive) contribution to the potential energy is given by where B is a constant specific to the ionic compound and n is a power in the range 6 - 12. ! Adding the Born repulsion correction to the Coloumbic term gives Born-Landé Equation U At r = ro the potential energy must be a minimum, so U Solving for B gives U Substituting for B in the equation for the Coulombic and Born contributions to potential energy gives the Born-Landé equation, ! The value of n can be calculated from measurements of compressibility or estimated from theory. L For NaCl, n = 9.1 from experiment, and the Born-Landé equation gives Uo = -771 kJ/mol. ! In the absence of experimental data, Pauling's approximate values of n can be used. Ion configuration He Ne n 5 7 Ar, Cu+ Kr, Ag+ Xe, Au+ 9 10 12 Born-Mayer Equation ! Born-Landé values are approximate. ! Mayer showed that e–r/ρ, where ρ is a constant dependant on the compressibility of the crystal, gives a better repulsion term than 1/rn. ! Using this improved repulsion term leads to the Born-Mayer equation: • ρ = 30 pm works well for all alkali metal halides and other simple cases when ro values are in pm. ! Further refinements involve terms for van der Waals (dispersion) energy and evaluation of the zero point energy. Kapustinskii's Equation ! In the absence of detailed structural data, Kapustinskii's equation1 can be used to estimate U: where r+ and r– are ionic radii (pm) and V is the number of ions per formula unit (e.g., 2 for NaCl, 3 for CaCl2). ! Kapustinskii's equation has been used with ionic compounds containing polyatomic ions as a means of calculating their thermochemical radii, in which the ions are treated as spheres.2 Ion r– (pm) Ion r– (pm) NO3– 189 SO42– 230 BrO3– 191 CrO42– 240 IO4– 249 BeF42– 245 BF4– 228 BO33– 191 CO32– 185 PO43– 238