Dressen

Understand the dynamics of a dissolving substance.

Be able to construct a Ksp equlibrium expression (solubility product constant).

Be able to calculate a Ksp value.

Understand the common ion effect.

Salts are generally more soluble in HOT water(Gases are more soluble in COLD water)

Alkali Metal salts are very soluble in water. NaCl, KOH, Li3PO4, Na2SO4 etc...

Ammonium salts are very soluble in water.NH4Br, (NH4)2CO3 etc…

Salts containing the nitrate ion, NO3-, are very

soluble in water. Most salts of Cl-, Br- and I- are very soluble in

water - exceptions are salts containing Ag+ and Pb2+.soluble salts: FeCl2, AlBr3, MgI2 etc...

“insoluble” salts: AgCl, PbBr2 etc...

A salt is an ionic compound - usually a metal cation bonded to a non-metal anion.

The dissolving of a salt is an example of equilibrium.

The water molecules will start to pull out some of the ions from the salt crystal.

At first, the only process occurring is the dissolving of the salt - the dissociation of the salt into its ions.

However, soon the ions floating in the water begin to collide with the salt crystal and are “pulled back in” to the salt. (precipitation)

Eventually the rate of dissociation is equal to the rate of precipitation.

The solution is now “saturated”. It has reached equilibrium.

AgCl (s) Ag+(aq) + Cl-(aq)

The equilibrium expression can be written ……

Keq =Ag+ Cl-

AgCl

But if we recognize that the amount of undissolved solid does not change, we can consider it is a constant. Therefore ….

Keq x Ag+ Cl-AgCl = = Ksp

Salt dissolving

Molecular dynamics of salt dissolving

Sample Problem

What is the concentration of lead ions and chromate ions insaturated lead chromate solution at 25oC? (Ksp = 1.8 x 10-14)

Pb2+=Ksp CrO42-

PbCrO4 (s) Pb2+(aq) + CrO42-(aq)

Pb2+=1.8 x 10-14 CrO42-

Pb2+ = CrO42- = 1.8 x 10-14

= 1.3 x 10-7 M

Common Ion Effect

The lowering of solubility of an ionic compound as a resultof the addition of a common ion is called the common ion effect.

Common Ion Effect-Precipitation

18.3 Homework #37

Page 565# 21, 22, 23, 24, 27, 28Due Friday