Chemistry Reference
In-Depth Information
Let's suppose we were to carry out a chemical reaction between aque-
ous solutions of lead (II) chlorate and sodium iodide. We mix the two solu-
tions in a test tube, and we find that a solid precipitate forms and falls to
the bottom of the test tube. We would want to be able to identify what the
solid was. The first thing that we might want to do is make a word equation
for the reaction that we think is taking place. Treating this as a double
displacement reaction, we would get the word equation shown here:
lead (II) chlorate + sodium iodide
sodium chlorate + lead (II) iodide
We can assume that the precipitate that formed in our test tube must
be one (or both) of the possible products from our word equation. In order
to identify which of the products formed, we would need to find out which
of the products is insoluble in water and would therefore precipitate out.
Our most handy reference tables for this type of problem will contain the
solubility rules for ionic compounds, shown here, and/or a solubility table
(see Figure 6-3a).
General Solubility Rules for Ionic Compounds in Water
Compounds That Tend to be Soluble
Compounds that contain ions of alkali metals (e.g., K
+
or Na
+
).
Compounds that contain halogens (e.g., Cl
-
or Br
-
).
Compounds that contain the ammonium (NH
4
+
) ion.
Compounds that contain the sulfate (SO
4
2-
) ion.
Compounds That Tend to be Insoluble
Compounds that contain the carbonate (CO
3
2-
) ion.
Compounds that contain the hydroxide (OH
-
) ion.
Compounds that contain the phosphate (PO
4
2-
) ion.
Compounds that contain the sulfide (S
2-
) ion.
If we refer to the general solubility rules, in this case, we are not really
able to identify the precipitate, because there seem to be conflicting trends.
We notice that compounds that contain ions of alkali metals tend to be
soluble, which would seem to suggest that the sodium chlorate is probably
not the precipitate, but we also read that compounds that contain halogens
also tend to be soluble, which would seem to let the lead (II) iodide off the
hook! How is it possible for us to have a solid precipitate and then have no
suspect?
