11.2. Types of Chemical Reactions http://www.ck12.org
FIGURE 11.8
A double replacement reaction is used to
form lead(II) iodide. The reactants shown
here are colorless solutions of potassium
iodide and potassium nitrate. When com-
bined, these produce a yellow precipitate
of lead(II) iodide.http://www.crescent.edu.sg/crezlab/webpages/PptReaction_PbI2.htm
Representing Ionic Reactions as Chemical Equations
For reactions that involve ions dissolved in water, there are several different ways to express the overall process as a
chemical equation. For example, the overallmolecular equationshows all ionic components as neutral compounds,
but the ones that are dissolved in water are denoted with "(aq)." Note that the ionic substances do not exist as
molecules, but we write them out as though they were. In the following example, two water-soluble compounds
trade partners to produce one dissolved ionic compound and one solid precipitate:
AB(aq)+CD(aq)→AD(aq)+CB(s)
In reality, the aqueous substances do not exist as molecules or ionic crystal lattices. Instead, the individual ions are
dissolved and distributed throughout the solution. If the reaction above were written as anionic equation, it would
look something like the following:
A+(aq)+B−(aq)+C+(aq)+D−(aq)→A+(aq)+D−(aq)+CB(s)
In this example, the various reaction components are presented in a form that is closer to the way they actually exist
during the reaction. The aqueous components are separated into ions, and the precipitate is found as a combined
solid. We are assuming in this example that A and C form cations with a charge of 1+, while B and D form anions
with a charge of 1-. In real examples, we would look at which group each element is found in on the periodic table
to determine its likely charge.
Notice that in the ionic equation,A+andD−were unchanged over the course of the reaction; they exist as aqueous
ions on both the reactant and product sides. In other words, these species did not experience any net change. Ions