Precipitation Reactions for AP Chemistry

Practice problems for these concepts can be found at:

• Reactions and Periodicity: Review Questions for AP Chemistry
• Reactions and Periodicity: Free-Response Questions for AP Chemistry

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:

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 (Li²⁺, Na⁺, etc.) and the ammonium ion (NH₄⁺) are soluble.
• All salts containing nitrate (NO₃^–), acetate (CH₃COO^–), and perchlorates (ClO₄^–) are soluble.
• All chlorides (Cl^–), bromides (Br^–), and iodides (I^–) are soluble, except those of Cu⁺, Ag⁺, Pb⁺, and Hg₂²⁺.
• All salts containing sulfate (SO₄^2–) are soluble, except those of Pb²⁺, Ca²⁺, Sr²⁺, and Ba²⁺.

Salts containing the following ions are normally insoluble:

• Most carbonates (CO₃^2–) and phosphates (PO₄^3–) are insoluble, except those of Group IA and the ammonium ion.
• Most sulfides (S^2–) are insoluble, except those of Group IA and IIA and the ammonium ion.
• Most hydroxides (OH^–) are insoluble, except those of Group IA, calcium, and barium.
• Most oxides (O^2–) are insoluble, except for those of Group IA, and Group IIA which react with water to form the corresponding soluble hydroxides.

Let's see how one might apply these rules. Suppose a solution of lead(II) nitrate is mixed with a solution of sodium iodide. Predict what will happen.

Write the formulas:

Pb(NO₃)²(aq) + NaI(aq) →

Convert to the ionic form:

Pb^2–(aq) + 2NO₃^–(aq) + Na⁺(aq) + I^–(aq) →

Predict the possible products by combining the cation of one reactant with the anion of the other and vice versa:

PbI₂ + NaNO₃

Apply the solubility rules to the two possible products:

PbI₂(s) Insoluble, therefore a precipitate will form.

NaNO₃(aq) Soluble, no precipitate will form.

Complete the chemical equation and balance it:

If both possible products are soluble, then the reaction would be listed as NR (No Reaction). In the reaction question part of the AP exam, there will be a possible reaction for every part of the question. If at least one insoluble product is formed, the reaction is sometimes classified as a double displacement (replacement) or metathesis reaction.

Practice problems for these concepts can be found at:

• Reactions and Periodicity: Review Questions for AP Chemistry
• Reactions and Periodicity: Free-Response Questions for AP Chemistry