CHM 1046
General Chemistry II
Dr. Michael Blaber


Additional Aspects of Aqueous Equilibria

Qualitative Analysis of Metal Ions


Metathesis Reactions

In many aqueous reactions it seems that the reaction involves the ionic compounds swapping their ionic partners. For example, in the reaction involving the ionic compounds silver nitrate and potassium chloride we have:

The silver cation exchanges its nitrate anion partner for the chloride anion. Likewise, the potassium cation exchanges chloride anion for the nitrate anion:

This swapping of ions in aqueous reactions can be symbolically represented as follows:

This type of reaction is known as a Metathesis reaction

Note: metathesis is not pronounced "meta-thesis", but rather "meh-TATH-eh-sis" (apparently the Greeks prefer to pronounce it that way)

 

There is something subtle in the above example that is important to note.

The driving force for metathesis reactions is the removal of ions from solution

What are the ways in which ions can be removed from solution and thus drive a metathesis reaction?

1. Certain ions can associate to form an insoluble precipitate (as with the formation of AgCl(s))

2. Certain ions can chemically combine to form a neutral molecular compound (resulting in either a non-electrolyte, or a weak electrolyte).

3. Certain ions can chemically combine to form a gas, and the gas physically escapes from the solution

Precipitation Reactions

Metathesis reactions that result in an insoluble precipitate are called precipitation reactions

AX(s) Û A+(aq) + X-(aq)

Ksp = [A+(aq)][X-(aq)]

If solubility is 0.01M, then [A+(aq)] = 0.01M and [X-(aq)] = 0.01M, thus:

Ksp = (0.01)(0.01) = 1.0 x 10-4

Generally speaking, if Ksp < 1.0 x 10-4, the ionic compound is considered to be insoluble

The reaction of KI and Pb(NO3)2:

Can we predict whether an ionic compound will be soluble or not?

Ions that form soluble compounds

Exceptions

NO3-
C2H3O2-
Group 1A metal ions (Li+, Na+, K+, Rb+, Cs+)
NH4+

None (compounds involving any of these ions will be soluble)

Cl-
Br-
I-

Ionic compounds with these ions are soluble, except for compounds involving Ag+, Hg22+ and Pb2+

SO42-

Ionic compounds with these ions are soluble, except for compounds involving Ca2+, Sr2+, Hg22+ and Pb2+

 


Qualitative Analysis

Qualitative analysis is a type of analysis that informs you of the presence of a substance, but does not tell you exactly how much of the substance there is. It is like being able to get a "yes" or "no" answer to the question "is a particular metal ion present in this sample?"

Although automated instrumentation has superceded many qualitative methods, a general method for qualitative analysis of metal ions in solution is still in use. The general outline of the method is as follows:

This method will divide a wide collection of soluble metal ions into 5 general groups. These are (in order of the experimental steps needed to separate them):

  1. Insouble chlorides
  2. Acid-insoluble sulfides
  3. Base-insoluble hydroxides and sulfides
  4. Insoluble phosphates
  5. Alkali metal ions and ammonia

Additional analytical methods are required to identify the individual components in each group


© 2000 Dr. Michael Blaber