In: Chemistry
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Lattice enthalpy can be estimated using Born-Mayer equation
given below.
HL =
[(NA |ZAZB| e2) /
(4od)]
{[1 - (do/d)]A}
HL is
lattice enthalpy;NA = 6.022 *1023
mol-1 is avogadro number;ZA and ZB
are charges on cation and anion respectively;
e = fundamental charge = 1.602 * 10-19C
o
= permittivity of free space = 8.854 * 10-12
J-1C2m-1
d = distance between centres of nearest cation and anion or sum of
radii of cation and anion
do is constant that depends on compressibility of
crystal; for alkali metal halide do =34.5 pm
=34.5*10-12m (1pm=10-12m)
A = Madelung constant; for NaCl, A = 1.748
For NaCl, |ZAZB| = |+1*-1| =1;
Given, d = 283pm = 283 * 10-12m; A = 1.748
(1 pm = 10-12m)
HL =
[(NA |ZAZB| e2) /
(4od)]
{[1 - (do/d)]A}
={[6.022*1023 *1*(1.602*10-19)2] /
(4 * 3.14* 8.854 * 10-12 * 283 *
10-12)}{[1-(34.5*10-12/283*10-12)]*1.748}
HL =
7.53 *
105Jmol-1
Hence, lattice enthalpy of NaCl is 7.53 *
105Jmol-1
Answer: (b) 7.53 * 105Jmol-1