Precipitation Reactions

Precipitation Reactions

Some chemical reactions produce an insoluble product -- called a precipitate. These types of reactions are usually carried out in a water solution where the reactants dissociate in water to give the appropriate cations and anions. Then if a cation and an anion combine to form a compound which is insoluble in water, then a precipitation reaction occurs.

An example is the reaction

AgNO3(aq) + KCl(aq) arrow AgCl(s) + KNO3(aq)

where we have introduced another nomenclature using the symbol "aq" to mean that the compound is water soluble.

If we write this in terms of the ions formed in solution we see that there is an intermediate step in the reaction:

AgNO3(aq) + KCl(aq) arrow Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)

Then the silver ion and the nitrate ion can combine to give the following:

Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) arrow AgCl(s) + K+(aq) + NO3-(aq)

So how do I know that AgCl is not water soluble and that KNO3 is water soluble so that the potassium and nitrate ions remain in solution? You probable were able to guess the answer to that question: Memorize a solubility table. I like the following one: -----------------------------------------------------------------------------------------

Soluble Compounds                             Exceptions

Almost all salts of Na+, ____________________________ K+, and NH4+ ----------- All salts of Cl-, Halides of Ag+, Pb2+, Br-, and I- Hg22+ ----------- Compounds containing F- Fluorides of II A except Be++ ----------- Salts of nitrate, chlorate, ____________________________ perchlorate, acetate ----------- Salts of sulfate Sulfates of Sr2+, Ba2+, Pb2+

Insoluble Compounds Exceptions

All salts of carbonate, phosphate, Salts of NH4+ and oxalate, chromate, sulfide and most the metal cations metal hydroxides and oxides


Net Ionic Equations

Let's consider again the reaction

AgNO3(aq) + KCl(aq) arrow AgCl(s) + KNO3(aq)

If we write this as we did above which is called the complete ionic equation:

Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) arrow AgCl(s) + K+(aq) + NO3-(aq)

Writing it this way we see that there are ions of the same type on both sides of the equation:

NO3- and K+(s). These ions do not directly participate in the reaction and are called spectator ions. If we remove the spectator ions we are left with what is called the net ionic reaction:

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

Of course the spectator ions are important -- without them we never would have had the silver and chlorine ions in solution. However the anion to go with silver initially could have been any ion as long as it forms water soluble products with silver and potassium. Similarly the cation initially with chlorine could have been any cation as long as it forms water soluble products with potassium and silver.

Practice, practice, practice.

The stoichiometric procedure is the same as for any reaction. We will do some examples.

Now take a practice quiz to help you understand if you understand the basic concepts.
You must use your real name when it asks for a name.
The test will only submit when you have answers all of the questions correctly.
If you are not taking this course for credit please do not answer all the questions correctly for I don't want to be flooded with email answers to the tests.


Web Author: Dr. Leon L. Combs
Copyright 1999 by Dr. Leon L. Combs - ALL RIGHTS RESERVED