Precipitation Reactions Spectator Ions

FIGURE 1.5 (a) Silver chloride precipitates immediately when sodium chloride solution is added to a solution of silver nitrate, (b) If we imagine the removal of the spectator ions from the complete ionic reaction (top), we can focus on the essential process, the net ionic reaction (bottom). 

Table 1.1 summarizes the solubility patterns of common ionic compounds in water. Notice that all nitrates and all common compounds of the Group 1 metals are soluble so they make useful starting solutions for precipitation reactions. Any spectator ions can be used, provided that they remain in solution and do not otherwise react. For example, Table 1.1 shows that mercury(I) iodide, Hg2I2, is insoluble. It is formed as a precipitate when solutions containing Hg22+ ions and I ions are mixed ... 

Each of the following five procedures results in the formation of a precipitate. For each reaction, write the chemical equations describing the formation of the precipitate the overall equation, the complete ionic equation, and the net ionic equation. Identify the spectator ions. 

A net ionic equation contains only those species that participate in a chemical reaction. Notice that neither K nor Cr appears in the equation for the precipitation of Fe (OH). Similarly, neither K nor NO3 appears in the equation for the precipitation of Pbl2. Although these other ions are present in the solution, they undergo no change during the precipitation reaction. Ions that are not involved in the chemical change are referred to as spectator ions. Spectator ions are omitted from net ionic equations. 

Mixing the two solutions will produce 2.50 X 10 mol of Fe (0H)3 precipitate, which is 2.67 g. The mixed solution contains Na cations and Cl anions, too, but we can ignore these spectator ions in our calculations. Notice that this precipitation reaction is treated just like other limiting reactant problems. Examples and further illustrate the application of general stoichiometric principles to precipitation reactions. 

C04-0086. Write the baianced net ionic equation for each of these precipitation reactions. Also, identify the spectator ions. 

C04-0107. A white precipitate forms when aqueous calcium nitrate is mixed with aqueous ammonium sulfate, (a) Identify the precipitate and write the net ionic equation for the reaction, (b) What are the spectator ions ... 

When equal volumes of 0.100 M solutions of sodium bromide and silver nitrate are mixed, a white solid precipitates from the solution. Identify the precipitate, write the net ionic reaction for the solubility equilibrium, and identify any spectator ions. 

The wastewater contains Cd +, so an anion must also be present in the solution to balance the charge of the cadmium ions. Other species may exist as well. The problem asks only about the cadmium in the wastewater, so assume that any other ions are spectators. The sodium hydroxide solution contains Na and OH, so the major species in the treated wastewater include B.2 O, Cd ", OH", and Na. The equilibrium constant for the precipitation reaction is the inverse of for Cd (OH)2 ... 

There are three important classes of aqueous reactions. Precipitation reactions occur when solutions of two ionic substances are mixed and a precipitate falls from solution. To predict whether a precipitate will form, you must know the solubility of each potential product. Acid-base neutralization reactions occur when an acid is mixed with a base, yielding water and a salt. The neutralization of a strong acid with a strong base can be written as a net ionic equation, in which nonparticipating, spectator ions are not specified ... 

Choose one of the reactions in the Precipitation, Redox, and Neutralization Reactions activity in eChapter 4.8, and draw the pictures used to represent it. Eliminate the spectator ions by crossing them out of the picture, and then write the net ionic equation. Draw similar pictures for the reaction NaCl(a ) + N NC fa ) — NaN03(fl(j) + Nt CKa ), and eliminate the spectator ions. What does this indicate about the reaction ... 

When a precipitate was formed in Activity 9.1, the reaction that occurred was a double displacement reaction. Each reactant contained two ions. When two reactants combined, a precipitate formed if two of the ions present were more stable and if they bonded to form a low solubility solid. An ionic equation for such a reaction is Ag+ (aq) + N03-(aq) + K+ (aq) + CL (aq) -> K+ (aq) + N03-(aq) + AgCl(s). The net ionic equation is Ag+ (aq) + Or (aq) — AgCl(s). Potassium and chloride ions are called spectator ions. They only watch the reaction. 

Q C99 State the name and formula of the precipitate that forms when aqueous solutions of copper(II) sulfate and sodium carbonate are mixed. Write the net ionic equation for the reaction. Identify the spectator ions. 

The ions present before any reaction occurs are Ba2+, N03, Na+, and S042. The Na+ and N03 ions are spectator ions, since NaN03 is very soluble in water and will not precipitate under these conditions. The net ionic equation of the reaction is therefore... 

The net ionic equation is the same as the total ionic equation. This is a precipitation and a neutralization reaction. There are no spectator ions because all the ions are used to form the two products. 

Section 4.1 polar molecule (109) solvated (110) electrolyte (110) nonelectrolyte (112) Section 4.2 molecular equation (113) total ionic equation (114) spectator ion (114) net ionic equation (114) Section 4.3 precipitation reaction (115) precipitate (115) metathesis reaction (116) Section 4.4 acid-base reaction (117) neutralization reaction (117) acid (117) base (118) salt (119) titration (11 9) equivalence point (120) end point (120) Section 4.5 oxidation-reduction (redox) reaction (123) oxidation (124) reduction (124) oxidizing agent (124) reducing agent (124) oxidation number (O.N.) (or oxidation state) (124) Section 4.6 activity series of the metals (130)... 

How do we know when to expect an acid-base reaction One of the most difficult tasks for someone inexperienced in chemistry is to predict what reaction might occur when two solutions are mixed. With precipitation reactions, we found that the best way to deal with this problem is to focus on the species actually present in the mixed solution. This idea also applies to acid-base reactions. For example, when an aqueous solution of hydrogen chloride (HCI) is mixed with an aqueous solution of sodium hydroxide (NaOH), the combined solution contains the ions H, Cl, Na, and OH . The separated ions are present because HCI is a strong acid and NaOH is a strong base. How can we predict what reaction occurs, if any First, will NaCI precipitate From Table 4.1 we can see that NaCI is soluble in water and thus will not precipitate. Therefore, the Na and Cl ions are spectator ions. On the other hand, because water is a nonelectrolyte, large quantities of H and OH ions cannot coexist in solution. They react to form H2O molecules ... 

Both sodium ions and nitrate ions are spectator ions and therefore do not enter into the precipitation reaction... 

In general terms, what are the spectator ions in a precipitation reaction Why are the spectator ions not included in writing the net ionic equation for a precipitation reaction Does this mean that the spectator ions do not have to be present in the solution ... 

Spectator ions are ions that remain in solution during a precipitation/double-displacement reaction. For example, in the reaction BaCl2(a[Pg.671]

Three solutions are mixed together to form a single solution. One contains 0.2 mol Pb(CH3COO)2, the second contains 0.1 mol Na2S, and the third contains 0.1 mol CaCl2. (a) Write the net ionic equations for the precipitation reaction or reactions that occur, (b) What are the spectator ions in the solution ... 

These are called precipitation reactions because they take place in water ( aqueous or aq ) solution and one compound is precipitated out of the solution (and can then be filtered, washed, recrystallised etc.). As with displacement reactions, some of the species present do not actually take any active part in the process (and so are called spectator ions). So in the first example, sodium chloride and silver nitrate are both soluble, so what is actually being mixed is a solution containing hydrated sodium ions and hyclrated chloride ions, with one containing hydrated silver ions and hydrated nitrate ions. The compound silver chloride has very low solubihty (the bonding between the ions is not readily broken down to allow hydration of the ions), so is precipitated from the solution ... 

The preceding equation is an example of an ionic equation, which shows dissolved species as free ions. To see whether a precipitate might form from this solution, we first combine the cation and anion from different compounds that is, Pbl2 and KNO3. Referring to Table 4.2, we see that PW2 is an insoluble compound and KNO3 is soluble. Therefore, the dissolved KNO3 remains in solution as separate and NO3 ions, which are called spectator ions, or ions that are not involved in the overall reaction. Because spectator ions appear on both sides of an equation, they can be eliminated from the ionic equation... 

Use Table 4.1 to determine which of the following combinations leads to a precipitation reaction. How can you identify the spectator ions in the reaction ... 

The precipitate forms as a result of the very strong attractive forces between the Pb + cations and the 1 anions. The other product is the water-soluble salt potassium nitrate, KNO3. The potassium and nitrate ions do not take part in the reaction. They remain in solution as aqueous ions and therefore are often referred to as spectator ions. The guidelines that help identify which ions form a precipitate and which ions remain in solution are developed in a later chapter on ions in aqueous solutions. 

Write the possible double-displacement reaction between Zn(N03)2 and (NH )2S. Use Figure 1.3 to determine if any of the products are insoluble and will precipitate. Write a formula equation and an overall ionic equation, and then cancel the spectator ions to produce a net ionic equation. 

Using Figure 1.3, write the balanced chemical equation, write the overall ionic equation, identify the spectator ions and possible precipitates, and write the net ionic equation for each of the following reactions. (Hint See Sample Problem B.)... 

In the case of precipitation reactions, we can identify which ions did not react using the solubility rules. We call these ions spectator ions. In a football stadium, the action is taking place on the playing field. The spectators are sitting in the stands. The spectators are present in the stadium but do not participate in the action taking place on the field. Spectator ions are like that—they are present in the solution but do not participate in the reaction that is taking place. Thus, a net ionic equation is one that leaves out any spectator ions that might be present. 

The ions listed in solubility rule 1 are always spectator ions in all precipitation reactions. Therefore, we would never write a net ionic equation that contains Na+, K+, NHJ, ... 

CjHjOi > CIO3, CIO4 > or NO 3. The negative ions listed in rules 2-9 could be spectator ions in some precipitation reactions, but they are not spectator ions in all precipitation reactions. For example, using rule 2, in the reaction between AgNOj and KCl, we would write the following net ionic equation ... 

Any ion not listed in solubility rule 1 can be a spectator ion some of the time and part of a precipitate at other times. Also, it is possible for a precipitation reaction to have only one spectator ion. It is also possible that a reaction has no spectator ions if both products are insoluble. An example is the reaction between BaCOH) and MgSO. Since BaSO and MgCOH) are both insoluble, the net ionic equation is as follows ... 

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