Reactions and Periodicity ‹ 73
Although not required for the AP exam, the following solubility rules are very useful:
● All salts containing 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 +.
Salts containing the following ions are normally insoluble:
● Most carbonates (CO 32 - ) and phosphates (PO 43 - ) are insoluble, except those of Group
IA and the ammonium ion.
● Most sulfides (S^2 - ) are insoluble, except those of Groups IA and IIA and the ammo-
nium 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(NO 32 )(aq)N+→aI(aq)Convert to the ionic form:
Pb(^2 +−aq)2++NO 3 (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:
PbI 23 +NaNOApply the solubility rules to the two possible products:
PbI 2 (s) Insoluble; therefore a precipitate will form.NaNO 3 (aq) Soluble; no precipitate will form.
Complete the chemical equation and balance it:Pb(NO 32 )(aq)2+→NaI(aq)PbI (s)2 23 + NaNO(aq)Pb(^2 +−aq)2+→I(aq)PbI (s) 2If 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 reac-
tion 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.Oxidation–Reduction Reactions
Oxidation–reduction reactions, commonly called redox reactions, are an extremely important
category of reaction. Redox reactions include combustion, corrosion, respiration, photo-
synthesis, and the reactions involved in electrochemical cells (batteries). The driving forceENRICHMENT