Lecture notes: CHEM103 – October 30, 2008

Ionic compounds conduct electricity: NOT as solids, but EITHER in solutions (below) or as liquids (not shown).

The difference between liquid salts (high melting points, see above) and solutions is the presence of a solvent (H₂O).

(and the interations between the solvent and the solute…)

What liquid salts and aqueous ionic solutions have in common is MOBILE ions; the ability to MOVE charge!

SO – there is an analogy to be made between MELTING a salt to produce MOBILE ions, NOT in a lattice.

AND DISSOLVING a salt to produce MOBILE ions, but now SOLVATED, and also NOT in a lattice.

BOTH must overcome the lattice energy holding a solid ionic compound together!

Ionic Compounds in Solution

DEFINE: SOLUTE…

DEFINE: SOLVENT…

DEFINE: SOLUTION…

DEFINE: SOLVATION…

DEFINE: DISSOCIATION…

DEFINE: DISSOLUTION & PRECIPITATION…

DISSOLUTION

PRECIPITATION

WHY DO IONIC COMPOUNDS DISSOLVE AT ALL???

Back to lattice energy…

ENERGY OF SOLVATION VS LATTICE ENERGY… THIS IS A COMPETITION!!!

Ion pair energy: LiF -1050 kJ/mol

MgO -3920 kJ/mol

What does this suggest about the solubility of these two compounds???

(P.S. this is NOT the whole picture)

SO WHAT DO WE DO INSTEAD???

Examples:

barium sulfate; mercury (II) chloride; copper (I) acetate, zinc (II) perchlorate, sodium phosphate, calcium phosphate

(when writing precipitation & dissolution reactions – note: do NOT include water!)

NOTE: partially soluble salts – what do these look like in solution at the atomic scale???