4.2 Precipitation Reactions


Figure 4.4 shows two clear solutions being mixed, one containing lead nitrate, and the other containing potassium iodide (KI). The reaction between these two solutes produces an insoluble yellow product. Reactions that result in the formation of an insoluble product are known as precipitation reactions. A precipitate is an insoluble solid formed by a reaction in solution. In Figure 4.4 the precipitate is lead iodide a compound that has a very low solubility in water:

[4.4]


Figure 4.4 The addition of a colorless solution of potassium iodide (KI) to a colorless solution of lead nitrate [Pb(NO3)2] produces a yellow precipitate of lead iodide (Pbl2) that slowly settles to the bottom of the beaker.


The other product of this reaction, potassium nitrate, remains in solution.

Precipitation reactions occur when certain pairs of oppositely charged ions attract each other so strongly that they form an insoluble ionic solid. To predict whether certain combinations of ions form insoluble compounds, we must consider some guidelines or rules concerning the solubilities of common ionic compounds.

Solubility Guidelines for Ionic Compounds

The solubility of a substance is the amount of that substance that can be dissolved in a given quantity of solvent. Only mol of dissolves in a liter of water at In our discussions any substance with a solubility less than 0.01 molL will be referred to as insoluble. In those cases the attraction between the oppositely charged ions in the solid is too great for the water molecules to separate them to any significant extent, and the substance remains largely undissolved.

Unfortunately, there are no rules based on simple physical properties such as ionic charge to guide us in predicting whether a particular ionic compound will be soluble or not. Experimental observations, however, have led to guidelines for predicting solubility for ionic compounds. For example, experiments show that all common ionic compounds that contain the nitrate anion, are soluble in water. Table 4.1 summarizes the solubility guidelines for common ionic compounds. The table is organized according to the anion in the compound, but it reveals many important facts about cations. Note that all common ionic compounds of the alkali metal ions (group 1A of the periodic table) and of the ammonium ion are soluble in water.



SAMPLE EXERCISE 4.2

Classify the following ionic compounds as soluble or insoluble: (a) sodium carbonate (b) lead sulfate

Solution

Analyze: We are given the names and formulas of two ionic compounds and asked to predict whether they are soluble or insoluble in water.

Plan: We can use Table 4.1 to answer the question. Thus, we need to focus on the anion in each compound because the table is organized by anions.

Solve: (a) According to Table 4.1, most carbonates are insoluble, but carbonates of the alkali metal cations (such as sodium ion) are an exception to this rule and are soluble. Thus, is soluble in water.

(b) Table 4.1 indicates that although most sulfates are water soluble, the sulfate of is an exception. Thus, is insoluble in water.

PRACTICE EXERCISE

Classify the following compounds as soluble or insoluble: (a) cobalt(II) hydroxide; (b) barium nitrate; (c) ammonium phosphate.

Answers:(a) insoluble; (b) soluble; (c) soluble


To predict whether a precipitate forms when we mix aqueous solutions of two strong electrolytes, we must (1) note the ions present in the reactants, (2) consider the possible combinations of the cations and anions, and (3) use Table 4.1 to determine if any of these combinations is insoluble. For example, will a precipitate form when solutions of and NaOH are mixed? Because and NaOH are both soluble ionic compounds, they are both strong electrolytes. Mixing and NaOH(aq) first produces a solution containing and ions. Will either of the cations interact with either of the anions to form an insoluble compound? In addition to the reactants, the other possible interactions are with and with From Table 4.1 we see that is insoluble and will thus form a precipitate. however, is soluble, so and will remain in solution. The balanced equation for the precipitation reaction is

[4.5]

Exchange (Metathesis) Reactions

Notice in Equation 4.5 that the cations in the two reactants exchange anions— ends up with and ends up with The chemical formulas of the products are based on the charges of the ions—two ions are needed to give a neutral compound with and one ion is needed to give a neutral compound with (Section 2.7) It is only after the chemical formulas of the products are determined that the equation can be balanced.

Reactions in which positive ions and negative ions appear to exchange partners conform to the following general equation:

Example:

[4.6]

Such reactions are known as exchange reactions, or metathesis reactions (meh-TATH-eh-sis, which is the Greek word for "to transpose"). Precipitation reactions conform to this pattern, as do many acid-base reactions, as we will see in Section 4.3.


SAMPLE EXERCISE 4.3

(a) Predict the identity of the precipitate that forms when solutions of and are mixed. (b) Write the balanced chemical equation for the reaction.

Solution

Analyze: We are given two ionic reactants and asked to predict the insoluble product that they form.

Plan: We need to write down the ions present in the reactants and to exchange the anions between the two cations. Once we have written the chemical formulas for these products, we can use Table 4.1 to determine which is insoluble in water. Knowing the products also allows us to write the equation for the reaction.

Solve: (a) The reactants contain and ions. If we exchange the anions, we will have and KCl. According to Table 4.1, most compounds of are soluble but those of are not. Thus, is insoluble and will precipitate from solution. KCl, on the other hand, is soluble.

(b) From part (a) we know the chemical formulas of the products, and KCl. The balanced equation with phase labels shown is

PRACTICE EXERCISE

(a) What compound precipitates when solutions of and LiOH are mixed? (b) Write a balanced equation for the reaction. (c) Will a precipitate form when solutions of and KOH are mixed?

Answers: (a) (b) (c) No (both possible products are water soluble)


Ionic Equations

In writing chemical equations for reactions in aqueous solution, it is often useful to indicate explicitly whether the dissolved substances are present predominantly as ions or as molecules. Let's reconsider the precipitation reaction between and 2KI, shown previously in Figure 4.4:

An equation written in this fashion, showing the complete chemical formulas of the reactants and products, is called a molecular equation, because it shows the chemical formulas of the reactants and products without indicating their ionic character. Because KI, and are all soluble ionic compounds and therefore strong electrolytes, we can write the chemical equation to indicate explicitly the ions that are in the solution:

[4.7]

An equation written in this form, with all soluble strong electrolytes shown as ions, is known as a complete ionic equation.

Notice that and appear on both sides of Equation 4.7. Ions that appear in identical forms among both the reactants and products of a complete ionic equation are called spectator ions.

They are present but play no direct role in the reaction. When spectator ions are omitted from the equation (they cancel out like algebraic quantities), we are left with the net ionic equation:

[4.8]

A net ionic equation includes only the ions and molecules directly involved in the reaction. Charge is conserved in reactions, so the sum of the charges of the ions must be the same on both sides of a balanced net ionic equation. In this case the charge of the cation and the two charges of the anions add to give zero, the charge of the electrically neutral product. If every ion in a complete ionic equation is a spectator, then no reaction occurs.

Net ionic equations are widely used to illustrate the similarities between large numbers of reactions involving electrolytes. For example, Equation 4.8 expresses the essential feature of the precipitation reaction between any strong electrolyte containing and any strong electrolyte containing The and ions combine to form a precipitate of

Thus, a net ionic equation demonstrates that more than one set of reactants can lead to the same net reaction. The complete equation, on the other hand, identifies the actual reactants that participate in a reaction.

Net ionic equations also point out that the chemical behavior of a strong electrolyte solution is due to the various kinds of ions it contains. Aqueous solutions of KI and for example, share many chemical similarities because both contain ions. Each kind of ion has its own chemical characteristics that differ very much from those of its parent atom.

The following steps summarize the procedure for writing net ionic equations:

  1. Write a balanced molecular equation for the reaction.

  2. Rewrite the equation to show the ions that form in solution when each soluble strong electrolyte dissociates or ionizes into its component ions. Only strong electrolytes dissolved in aqueous solution are written in ionic form.

  3. Identify and cancel spectator ions.


SAMPLE EXERCISE 4.4

Write the net ionic equation for the precipitation reaction that occurs when solutions of calcium chloride and sodium carbonate are mixed.

Solution

Analyze: Our task is to write a net ionic equation for a precipitation reaction, given the names of the reactants present in solution.

Plan: We first need to write the chemical formulas of the reactants and products and to determine which product is insoluble. Then we write and balance the molecular equation. Next, we write each soluble strong electrolyte as separated ions to obtain the complete ionic equation. Finally, we eliminate the spectator ions to obtain the net ionic equation.

Solve: Calcium chloride is composed of calcium ions, and chloride ions, hence an aqueous solution of the substance is Sodium carbonate is composed of ions and ions; hence an aqueous solution of the compound is

In the molecular equations for precipitation reactions, the anions and cations appear to exchange partners. Thus, we put and together to give and and together to give NaCl. According to the solubility guidelines in Table 4.1, is insoluble and NaCl is soluble. The balanced molecular equation is

In a complete ionic equation, only dissolved strong electrolytes (such as soluble ionic compounds) are written as separate ions. As the (aq) designations remind us, and NaCl are all dissolved in the solution. Furthermore, they are all strong electrolytes. is an ionic compound, but it is not soluble. We do not write the formula of any insoluble compound as its component ions. Thus, the complete ionic equation is

and are spectator ions. Canceling them gives the following net ionic equation:

Check: We can check our result by confirming that both the elements and the electric charge are balanced. Each side has 1 Ca, 1 C, and 3 O, and the net charge on each side equals 0.

Comment: If none of the ions in an ionic equation is removed from solution or changed in some way, then they all are spectator ions and a reaction does not occur.

PRACTICE EXERCISE

Write the net ionic equation for the precipitation reaction that occurs when aqueous solutions of silver nitrate and potassium phosphate are mixed.

Answer: