1.1 What is Chemistry?

http://www.ck12.org Chapter 11. Chemical Reactions

that are present in solution but do not participate in the overall reaction are known asspectator ions. The ionic
equation can be simplified to thenet ionic equationby canceling out all the spectator ions.

A+(aq)+B−(aq)+C+(aq)+D−(aq)→A+(aq)+D−(aq)+CB(s)

B−(aq)+C+(aq)→CB(s)

Let’s look at these three types of equations again using a real example. If we were to mix aqueous solutions of
potassium iodide and lead(II) nitrate, lead(II) iodide would precipitate as a solid, and potassium nitrate would remain
dissolved. This can be represented by any of the three following equations:

Molecular Equation

2KI(aq)+Pb(NO 3 ) 2 (aq)→2KNO 3 (aq)+PbI 2 (s)

Ionic Equation

2K+(aq)+2I−(aq)+Pb^2 +(aq)+2NO− 3 (aq)→2K+(aq)+2NO− 3 (aq)+PbI 2 (s)

Net Ionic Equation

Pb^2 +(aq)+2I−(aq)→PbI 2 (s)

Predicting Solubility of Ionic Compounds

How do we determine which ions are likely to form an insoluble precipitate and which will remain dissolved in
water? By combining various ionic solutions, chemists have come up with some general guidelines for whether a
given cation-anion pairing is likely to be soluble or insoluble in water. It should be noted that such an approach is
an oversimplification. Each compound has its own solubility value, so two "soluble" compounds might have very
different abilities to dissolve in water. Additionally, even "insoluble" salts can dissolve in water to a very limited
extent. We will take a more quantitative approach to solubility in the chapter on solutions. However, qualitative
rules like the ones in theTable11.9 are useful for predicting whether a precipitate is likely to form when combining
moderate amounts of specific cations and anions.

TABLE11.9: Solubility Properties to Predict Products of Chemical Reactions

Type of Particle Soluble Insoluble

Common Cations Alkali metal cation (Li+, Na+, K+,

Rb+, or Cs+) or the NH 4 +cation

Common Anions ClO 4 −and NO 3 −compounds

Halides Most Cl−, Br−, and I−compounds Compounds that include the Ag+,

Pb^2 +, or Hg 22 +cations

Sulfates Most SO 42 −compounds PbSO 4 , Ag 2 SO 4 , Hg 2 SO 4 , CaSO 4 ,

SrSO 4 , and BaSO 4

Sulfides Compounds with NH 4 +or a metal

from group IA or IIA as cation

Most S^2 −compounds

Hydroxides Compounds with NH 4 +, Ba^2 +, or a

metal from group IA as cation

Most OH−compounds

Carbonates, phosphates, and sul-

fites

Compounds with NH 4 +or a metal

from group IA as cation

Most CO 32 − and PO 43 −, and

SO 32 −compounds