Lecture notes: CHEM103 – October 28, 2008

 

LECTURE OUTLINE:

 

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

          Some vocabulary

          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

 

 

 

EXAMPLES:

 

          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!

 

 

0902

 

 

            ionization energy

              electron affinity

    +   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)

 

 

 

siL48593_02_12

 

 

0310a

 

 

another example: KI vs. NaCl

 

siL48593_09_07

 

 

Examples: application of ionic radius AND charge number to ion pair energy

 

Li+ & Mg2+:   ~70 – 80 pm (nearly the same – why?)

 

          F– & O2–:   ~ 100 or 140 pm (which is which?)

 

 

          Ion pair energy:   LiF     -1050 kJ/mol

                                      MgO  -3920 kJ/mol

 

          note: approximately a factor of 4 difference in energy…  WHY?

 

 

MACROSCALE EFFECT OF THIS DIFFERENCE

 

siL48593_t09_01