Lattice Energy

ionic cmpds =   -- ∆ H_f^o

Ionization E.: always  endo + E

Na (g)  à   Na+ (g)   +   e-              496 kJ

e- affinity: generally exo –E

Cl (g)    +    e-   à   Cl-  (g)              -349 kJ

Attraction of ions pulls ions together, created lattice/array in crystal

Na (g)    à   Na+ (g)  +   e-                    +496 kJ

Cl (g)     +     e-    à   Cl-  (g)                -349 kJ

Na (g)  + Cl (g)  à Na+ (g) + Cl- (g)        +147 kJ

Transfer e- not only factor in forming ionic bond

Lattice Energy: energy required to separate a mole of solid ionic compd into gaseous ions

(measure of stabilization resulting from arranging opp. charged ions in ionic solid)

NaCl (s)à  Na+ (g)   +  Cl- (g)         ∆ H_latt= +788 kJ

Na+ (g)   +  Cl- (g) à  NaCl (s)       ∆ H_latt= -788 kJ

Release E when form cmpd, require E to sep. ions

Born Haber Cycle:

Na(s)     à   Na (g)           + 107.7 kJ

½ Cl₂ (g)       à   Cl (g)                          +121.7 kJ

Na (g)    à   Na+ (g)  +   e-                    +496 kJ

Cl (g)     +     e-    à   Cl-  (g)                -349 kJ

Na+ (g)   +  Cl- (g) à  NaCl (s) -788 kJ

Na (s)    + ½ Cl (g) à NaCl (s)               -411.6 kJ

Overall formation of ionic cmpds- exothermic

E = k q¹ q²                     q = charge

         d                         d = dist

                                    k = 8.99 x 10⁹ J x m

                                                                     C²

E ↑ as q ↑                E ↑  as d ↓