5 Steps to a 5 AP Chemistry

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 equationis written by dropping out the spectator ions and showing only

those chemical species that are involved in the chemical reaction:

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 reactionsinvolve the formation of an insoluble compound, a precipitate,

from the mixing of two soluble compounds. Precipitation reactions normally occur in aque-

ous 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:

1. 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.


2. Be able to write the reactants and products in their ionic form, as in the ionic equation
   example above. Be sure, however, that you do not try to break apart molecular
   compounds such as most organic compounds, or insoluble species.


3. 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.


4. 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 rules:

Salts containing the following ions are normally soluble:

 All salts of Group IA (Li+, Na+, etc.) and the ammonium ion (NH 4 +) are soluble.

 All salts containing nitrate (NO 3 −), acetate (CH 3 COO−), and perchlorates (ClO 4 −) are

soluble.

 All chlorides (Cl−), bromides (Br−), and iodides (I−) are soluble,except those of Cu+, Ag+,

Pb^2 +, and Hg 22 +.

 All salts containing sulfate (SO 42 −) are soluble,except those of Pb^2 +, Ca^2 +, Sr^2 +, and Ba^2 +.

Pb aq^2 +−()+→SO^24 ()aq PbSO s 4 ()

70  STEP 4. Review the Knowledge You Need to Score High

STRATEGY

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STRATEGY