Lecture notes: CHEM103 – October 28, 2008
0) REVIEW – BRINGING IT ALL TOGETHER…
Formation of ionic compounds & their properties
1) Ion pair energy (lattice energy, kinda) as driving force of ionic bond formation
Relative strengths of ion pair energies
2) Ionic compounds in solution
Energy of the solvation process – driving force of dissolution
(competition with ion pair energy)
3) SOLUBILITY RULES AS AN ALTERNATIVE
Writing dissolution & precipitation reactions
Strong and weak electrolytes and non-electrolytes
lithium fluoride; magnesium oxide; potassium oxide; calcium bromide; aluminum oxide
This combination results in lower energy products than reactants!
(How does ionic bond formation result in lower energy?)
Energy balance accounting:
Na à Na+ + e-- large +DE (atom required energy)
+ Br + e-- à Br-- small –DE (atom gave off energy)
sign of SUM is: +DE (we need more energy to complete this)
Question: Where does it come from? Answer: Lattice energy!
+ LATTICE ENERGY!
à net negative CHANGE in energy ß
Definition of lattice energy: amount of energy involved (either pos. or neg.) in the creation of a lattice.
(NOTE: this is related to – but not exactly the same as
energy of formation of an ion pair found on page 378 in the textbook.)
What is the sign of the lattice energy? How can this be?
What is the direction of energy change (sign)?
(in other words, did this produce or require energy?)
NOW, what about the reverse process?
(formation of an ionic bond)
Lattice (or ion pair) energy
(need qualitative understanding of this equation only…)
where n = number of charges (e.g. for Ca2+, (n+) = 2; for Cl--, (n–) = 1)
and d = distance between ions (related to ionic radius)
Note: “e” = 1.602 x 10-19 C (amount of charge on an electron)
another example: KI vs. NaCl