Question

14 (a) Give a definition of lattice energy. Does your definition mean that the associated enthalpy of reaction will be positive or negative? (b) Use the Born-Landé equation to calculate a value for the lattice energy of KBr, for which ro = 328 pm. KB adopts an NaCl structure; other data may be found in Tables 6.3 and 6.4.

Answer 1

Lattice energy is a measure of the strength of the ionic bonds in an ionic compound.

Lattice energy can be defined as the energy required to convert one mole of ionic solid into gaseous ionic constituents .

Alternately, it can be defined as the energy that must be supplied to one mole of an ionic crystal in order to separate it into gaseous ions in a vacuum via an endothermic process. Therefore, this quantity always holds a positive value. If the lattice energy is exothermic, i.e., the value of ΔHlattice is negative

some sources define lattice energy in the opposite manner, i.e. the amount of energy released when an ionic solid is formed from its gaseous ionic constituents via an exothermic process. According to this definition, lattice energy must always hold a negative value

calculating crystalline ionie Born Laude Equation is used to the lattice energy of a cry compound OU E NAM /**//2-le B untor NA = Avagadro constant = 6:022 X 1023 M = Madelung constant e = charge of an electron = 1.6022 x 10 19 C zt = charge of cation 2 = carge of anion to = permitivity of free space = 8.854 X 10th Em n = Born Exponent for K Br n for K+ = 9 n for Br = 10 n= 9+10= 19 9.5 KBM has Nad like structure Madelung constant (M) = 1 1476 Mo = 328pm = 328x10-12m

До 5 (- 6:622 123 х 1.4 1с х(1)11){1-4022 xorn) 14px 84 85 xio 12 х (328 x 10-12m) 20 г. – 7-109 110° xo'ss?“ (95-) 9:s 5 - 1-40 4 x 105 ( ) - - 1:409 x jos xo-8947 2 - 39ч. Ч7 ET