Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Reactions in Aqueous Solutions
Describing Reactions in Aqueous Solutions

Objective

· To learn to describe reactions in solutions by writing molecular, complete ionic, and net ionic equations.

Much important chemistry, including virtually all of the reactions that make life possible, occurs in aqueous solutions. We will now consider the types of equations used to represent reactions that occur in water. For example, as we saw earlier, when we mix aqueous potassium chromate with aqueous barium nitrate, a reaction occurs to form solid barium chromate and dissolved potassium nitrate. One way to represent this reaction is by the equation

This is called the molecular equation for the reaction; it shows the complete formulas of all reactants and products. However, although this equation shows the reactants and products of the reaction, it does not give a very clear picture of what actually occurs in solution. As we have seen, aqueous solutions of potassium chromate, barium nitrate, and potassium nitrate contain the individual ions, not molecules as is implied by the molecular equation. Thus the complete ionic equation ,

better represents the actual forms of the reactants and products in solution. In a complete ionic equation, all substances that are strong electrolytes are represented as ions. Notice that is not written as the separate ions because it is present as a solid; it is not dissolved.

The complete ionic equation reveals that only some of the ions participate in the reaction. Notice that the and ions are present in solution both before and after the reaction. Ions such as these, which do not participate directly in a reaction in solution, are called spectator ions . The ions that participate in this reaction are the and ions, which combine to form solid :

This equation, called the net ionic equation , includes only those components that are directly involved in the reaction. Chemists usually write the net ionic equation for a reaction in solution, because it gives the actual forms of the reactants and products and includes only the species that undergo a change.

Types of Equations for Reactions in Aqueous Solutions

Three types of equations are used to describe reactions in solutions.

1. The molecular equation shows the overall reaction but not necessarily the actual forms of the reactants and products in solution.

2. The complete ionic equation represents all reactants and products that are strong electrolytes as ions. All reactants and products are included.

3. The net ionic equation includes only those components that undergo a change. Spectator ions are not included.

To make sure these ideas are clear, we will do another example. In Example 7.2 we considered the reaction between aqueous solutions of lead nitrate and sodium sulfate. The molecular equation for this reaction is

Because any ionic compound that is dissolved in water is present as the separated ions, we can write the complete ionic equation as follows:

The is not written as separate ions because it is present as a solid. The ions that take part in the chemical change are the and the ions, which combine to form solid . Thus the net ionic equation is

The and ions do not undergo any chemical change; they are spectator ions.

Interactive Example 7.3. Writing Equations for Reactions

For each of the following reactions, write the molecular equation, the complete ionic equation, and the net ionic equation.

a. Aqueous sodium chloride is added to aqueous silver nitrate to form solid silver chloride plus aqueous sodium nitrate.

b. Aqueous potassium hydroxide is mixed with aqueous iron(III) nitrate to form solid iron(III) hydroxide and aqueous potassium nitrate.

Solution

a. Molecular equation:

Complete ionic equation:

Net ionic equation:

b. Molecular equation:

Complete ionic equation:

Net ionic equation:

Self-Check: Exercise 7.2

· For each of the following reactions, write the molecular equation, the complete ionic equation, and the net ionic equation.

a. Aqueous sodium sulfide is mixed with aqueous copper(II) nitrate to produce solid copper(II) sulfide and aqueous sodium nitrate.

b. Aqueous ammonium chloride and aqueous lead(II) nitrate react to form solid lead(II) chloride and aqueous ammonium nitrate.

See Problems 7.25, 7.26, 7.27, 7.28, 7.29, and 7.30.