(^72) › STEP 4. Review the Knowledge You Need to Score High
Showing the soluble reactants and products in the form of ions yields the ionic
equation (sometimes called the total ionic equation):
Pb^2 +−(aq)++2NO( 34 aq)2Na+−(aq)+→SO^2 (aq) PbSO 43 (s)2++Na+−(aq) 2NO(aq)
Writing the equation in the ionic form shows clearly which species are really reacting
and which are not. In the example above, Na+ and NO 3 - appear on both sides of the equa-
tion. They do not react, but are simply there in order to maintain electrical neutrality of
the solution. Ions like this, which are not actually involved in the chemical reaction taking
place, are called spectator ions.
The net ionic equation is written by dropping out the spectator ions and showing only
those chemical species that are involved in the chemical reaction:
Pb^2 +−(aq)+→SO 42 (aq) PbSO 4 (s)
This net ionic equation focuses only on the substances that are actually involved in
the reaction. It indicates that an aqueous solution containing Pb^2 + (any solution, not just
Pb(NO 3 ) 2 (aq)) will react with any solution containing the sulfate ion to form insoluble
lead(II) sulfate. If this equation form is used, the spectator ions involved will not be known,
but in most cases, this is not a particular problem, since the focus is really the general
reaction, and not the specific one. You will be expected to write the balanced net ionic
equation for many of the reactions on the test.
Precipitation Reactions
Precipitation reactions involve the formation of an insoluble compound, a precipitate,
from the mixing of two soluble compounds. Precipitation reactions normally occur in
aqueous solution. The example above that was used to illustrate molecular equations, ionic
equations, etc., was a precipitation reaction. A solid, lead(II) sulfate, was formed from the
mixing of the two aqueous solutions. In order to predict whether or not precipitation will
occur if two solutions are mixed, you must:
- Learn to write the correct chemical formulas from the names; on the AP exam, names
are frequently given instead of formulas in the reaction section. - Be able to write the reactants and products in their ionic form, as in the ionic equa-
tion example above. Be sure, however, that you do not try to break apart molecular
compounds such as most organic compounds, or insoluble species. - Know and be able to apply the following solubility rules by combining the cation of one
reactant with the anion of the other in the correct formula ratio, and determining the
solubility of the proposed product. Then do the same thing for the other anion/cation
combination. - On the AP exam, you will be expected to explain why a substance is soluble/insoluble.
Simply quoting the solubility rule is not sufficient.
Learn the following solubility rule:
All sodium, potassium, ammonium, and nitrate salts are soluble in water.
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