WRITING NET IONIC EQUATIONS

When a substance made up of ions is dissolved in water, the dissolved ions behave independently. That is, they undergo their own characteristic reactions regardless of what other ions may be present. For example, silver ions in solution, Ag+, always react with chloride ions in solution, Cl−, to form an insoluble ionic compound, AgCl(s), no matter what other ions are present in the solution. If a solution of sodium chloride, NaCl, and a solution of silver nitrate, AgNO3, are mixed, a white solid, silver chloride, is produced. The solid can be separated from the solution by filtration, and the resulting solution contains sodium nitrate, just as it would if solid NaNO3

were added to water. In other words, when the two solutions are mixed, the following reaction occurs:

AgNO3+NaCl−→AgCl(s)+NaNO3 or Ag++NO3−+Na++Cl−−→AgCl(s)+Na++NO3−

Written in the latter manner, the equation shows that, in effect, the sodium ions and the nitrate ions have not changed. They began as ions in solution and wound up as those same ions in solution. They are calledspectator ions.Since they have not reacted, it is really not necessary to include them in the equation. If they are left out, a net ionic equationresults:

Ag++Cl−−→AgCl(s) 134

This equation may be interpreted to mean thatanysoluble silver salt will react withanysoluble ionic chloride to produce (insoluble) silver chloride.

EXAMPLE 9.1. Write three equations that the preceding net ionic equation can represent.

Ans. The following equations represent three of many possible equations:

AgNO3+NaCl−→NaNO3+AgCl(s) AgClO3+KCl−→KClO3+AgCl(s) AgC2H3O2+NH4Cl−→NH4C2H3O2+AgCl(s)

Obviously, it is easier to remember the net ionic equation than the many possible overall equations that it represents.

(See Problem 9.12.)

Net ionic equations may be written whenever reactions occur in solution in which some of the ions originally present are removed from solution or when ions not originally present are formed. Usually, ions are removed from solution by one or more of the following processes:

1. Formation of an insoluble ionic compound (Table 8-2) 2. Formation of molecules containing only covalent bonds 3. Formation of new ionic species

4. Formation of a gas (a corollary of 2) Examples of these processes include

1. AgClO3+NaCl−→AgCl(s)+NaClO3 Ag++Cl−−→AgCl(s)

2. HI+NaOH−→H2O+NaI H++OH−−→H2O

3. Cu+2 AgNO3−→2 Ag+Cu(NO3)2 Cu+2 Ag+−→Cu2++2 Ag 4. NH4CO3+2 HCl−→CO2+H2O+2 NH4Cl CO32−+2 H+−→CO2+H2O

The question arises how the student can tell whether a compound is ionic or covalent. The following generalizations will be of some help in deciding:

1. Binary compounds of two nonmetals are covalently bonded. However, strong acids (Table 8-3) in water form ions completely.

2. Binary compounds of a metal and nonmetal are usually ionic.

3. Ternary compounds are usually ionic, at least in part, except if they contain no metal atoms or ammonium ion.

EXAMPLE 9.2. Predict which of the following will contain ionic bonds: (a) NiCl2, (b) SO2, (c)Al2O3, (d) NH4NO3, (e) H2SO4, (f) HCl, and (g) NCl3.

Ans. (a) NiCl2, (c) Al2O3, and (d) NH4NO3contain ionic bonds. NH4NO3also has covalent bonds within each ion. (e) H2SO4and (f) HCl would form ions if allowed to react with water.

When do we write compounds as separate ions, and when do we write them as complete compounds? Ions can act independently in solution, and so we write ionic compounds as separate ions only when they are soluble.

We write compounds together when they are not ionic or when they are not in solution.

EXAMPLE 9.3. Write each of the following compounds as it should be written in an ionic equation. (a) KCl, (b) BaSO4, (c) SO2, and (d) Ca(HCO3)2.

Ans. (a) K++Cl− (b) BaSO4(insoluble) (c) SO2(not ionic) (d) Ca2++2 HCO3−

The insoluble compound is written as one compound even though it is ionic. The covalent compound is written together because it is not ionic.

EXAMPLE 9.4. Each of the following reactions produces 56 kilojoules (kJ) per mole of water produced. Is this just a coincidence? If not, explain why the same value is obtained each time.

KOH+HCl−→KCl+H2O LiOH+HBr−→LiBr+H2O NaOH+HNO3 −→NaNO3+H2O RbOH+HI−→RbI+H2O

Ans. The same quantity of heat is generated per mole of water formed in each reaction because it is really the same reaction in each case:

OH−+H+−→H2O

It does not matter whether it is a K+ion in solution that undergoes no reaction or an Na+ion in solution that undergoes no reaction. As long as the spectator ions undergo no reaction, they do not contribute anything to the heat of the reaction.

EXAMPLE 9.5. Write a net ionic equation for the reaction of aqueous Ba(OH)2with aqueous HNO3. Ans. The overall equation is

Ba(OH)2(aq) + 2 HNO3−→Ba(NO3)2+2 H2O In ionic form:

Ba2++2 OH−+2 H++2 NO3−−→Ba2++2 NO3−+2 H2O Leaving out the spectator ions yields

2 OH−+2 H+−→2 H2O Dividing each side by 2 yields the net ionic equation

OH−+H+−→H2O The net ionic equation is the same as that in Example 9.4.

Writing net ionic equations does not imply that any solution can contain only positive ions or only negative ions. For example, the net ionic equation

Ba2++SO42−−→BaSO4(s)

does not imply that there is any solution containing Ba2+ions with no negative ions, or any solution containing SO42− ions with no positive ions. It merely implies that whatever negative ion is present with the barium ion and whatever positive ion is present with the sulfate ion, these unspecified ions do not make any difference to the reaction that will occur.

Net ionic equations must always have the same net charge on each side of the equation. (The same number of each type of spectator ion must be omitted from both sides of the equation.) For example, the equation

Cu+Ag+−→Cu2++Ag (unbalanced)

has the same number of each type of atom on its two sides, but it is still not balanced. (One cannot add just one nitrate ion to the left side of an equation and two to the right.) The net charge must also be balanced:

Cu+2 Ag+−→Cu2++2 Ag There is a net 2+charge on each side of the balanced equation.

Solved Problems

WRITING NET IONIC EQUATIONS

9.1. How many types of ions are generally found in any ionic compound?

Ans. Two—one type of cation and one type of anion. (Alums are an exception. They are composed of two different cations and sulfate ions.)

9.2. Which of the following compounds are ionic? Which are soluble? Which would be written as separate ions in an ionic equation as written for the first equation in this section? Write the species as they would appear in an ionic equation. (a) CuCl, (b) NH4C2H3O2, (c) Hg2Cl2, (d) CoCl2, (e) Na3PO4, and (f) CH3OH(aq).

Ans.

Written Formulas in Ionic Ionic Soluble Separately Equation

(a) CuCl Yes No No CuCl

(b) NH4C2H3O2 Yes Yes Yes NH4++C2H3O2−

(d) CoCl2 Yes Yes Yes Co2++2 Cl−

(e) Na3PO4 Yes Yes Yes 3 Na++PO43−

(f) CH3OH(aq) No Yes No CH3OH

9.3. Write the formulas for the ions present in each of the following compounds: (a) NaClO4, (b) BaCl2, (c) KClO, (d) Ba(NO3)2, (e) LiClO3, (f) (NH4)3PO4, (g) Co3(PO3)2, (h) AgCl(s), and (i) Mg(HCO3)2. Ans. (a) Na+and ClO4−

(b) Ba2+and Cl− (c) K+and ClO− (d) Ba2+and NO3−

(e) Li+and ClO3−

(f) NH4+and PO43−

(g) Co2+and PO33−

(h) Ag+and Cl−(even though it is a solid)

(i) Mg2+and HCO3−

9.4. Write a net ionic equation for the equation in each of the following parts:

(a) NH4I+AgClO3−→AgI(s)+NH4ClO3

(b) H2SO4+BaCl2−→BaSO4+2 HCl

Ans. (a) I−+Ag+−→AgI (b) SO42−+Ba2+−→BaSO4

9.5. Write a net ionic equation for the equation in each of the following parts:

(a) HNO3+NaHCO3−→NaNO3+CO2+H2O (b) NaOH+NH4ClO3 −→NH3+H2O+NaClO3

Ans. (a) H++HCO3−−→CO2+H2O (HNO3is strong) (b) OH−+NH4+−→NH3+H2O

9.6. Given that BaCO3is insoluble in water and that NH3and H2O are covalent compounds, write net ionic equations for the following processes:

(a) NH4Cl+NaOH−→NH3+H2O+NaCl (b) BaCl2+Li2CO3−→BaCO3+2 LiCl

Ans. (a) NH4++OH−−→NH3+H2O (b) Ba2++CO32−−→BaCO3

9.7. Write a net ionic equation for the following overall equation:

Cr+2 Cr(NO3)3−→3 Cr(NO3)2

Ans. Cr+3 Cr3+−→3 Cr2+

Chromium ions appear on both sides of this equation, but they are not spectator ions since they are not identical. One is a 3+ion and the other is a 2+ion. The neutral atom is different from both of these.

9.8. Write a net ionic equation for each of the following overall equations:

(a) HNO3+NaOH−→NaNO3+H2O (b) HCl+KOH−→KCl+H2O (c) HClO4+LiOH−→LiClO4+H2O

(d) HNO3+RbOH−→RbNO3+H2O (e) HClO3+KOH−→KClO3+H2O Ans. In each case, the net ionic equation is

H++OH−−→H2O 9.9. Write a net ionic equation for each of the following overall equations:

(a) HClO3+LiOH−→LiClO3+H2O (b) HClO3+RbOH−→RbClO3+H2O

Ans. In each case, the net ionic equation is

H++OH−−→H2O 9.10. Write a net ionic equation for each of the following overall equations:

(a) H3PO4+3 NaOH−→Na3PO4+3 H2O (b) MgCO3(s)+CO2+H2O−→Mg(HCO3)2

(b) MgCO3+CO2+H2O−→Mg2++2 HCO3−

9.11. Write a net ionic equation for each of the following overall equations:

(a) 2 AgNO3+H2S−→Ag2S(s)+2 HNO3

(b) Zn+2 HCl−→ZnCl2+H2

(d) 2 Al+3 HgCl2−→2 AlCl3+3 Hg (e) Zn+FeCl2−→ZnCl2+Fe Ans. (a) 2 Ag++H2S−→Ag2S+2 H+

(b) Zn+2 H+−→Zn2++H2

(d) 2 Al+3 Hg2+−→2 Al3++3 Hg (Note the overall charge balance.) (e) Zn+Fe2+−→Fe+Zn2+

9.12. Write six more equations that can be represented by the net ionic equation of Example 9.1. Use the same reactants that are used in the equations in Example 9.1

Ans. AgNO3+KCl−→KNO3+AgCl(s)

AgClO3+NH4Cl−→NH4ClO3+AgCl(s) AgC2H3O2+NaCl−→NaC2H3O2+AgCl(s)

AgNO3+NH4Cl−→NH4NO3+AgCl(s) AgClO3+NaCl−→NaClO3+AgCl(s) AgC2H3O2+KCl−→KC2H3O2+AgCl(s)

9.13. A student used H3PO4to prepare a test solution that was supposed to contain phosphate ions. Criticize this choice.

Ans. Phosphoric acid is weak, and relatively few ions are present. The student should have used an ionic phosphate—sodium phosphate or ammonium phosphate, for example.

9.14. Write one or more complete equations for each of the following net ionic equations:

(a) NH3+H+ −→NH4+

(b) Cu2++S2−−→CuS

Ans. (a) NH3+HNO3−→NH4NO3 or NH3+HCl−→NH4Cl or ammonia plus any other strong acid to yield the ammonium salt.

(b) CuSO4+K2S−→CuS+K2SO4 or Cu(NO3)2+BaS−→CuS+Ba(NO3)2

or CuSO4+(NH4)2S−→CuS+(NH4)2SO4 or any soluble copper(II) salt with any soluble sulfide.

or CO2plus any other soluble hydroxide to give a soluble carbonate. Note that the following equation cannot be used, because BaCO3is insoluble: CO2+Ba(OH)2−→BaCO3+H2O

9.15. What chemical would you use to prepare a solution to be used for a test requiring the presence of Ca2+ ions?

Ans. Ca(NO3)2, CaCl2, or any other soluble calcium salt.

9.16. What chemical would you use to prepare a solution to be used for a test requiring the presence of Br− ions?

Ans. NaBr, BaBr2, HBr(aq), or any other soluble ionic bromide.

9.17. What chemical would you use to prepare a solution to be used for a test requiring the presence of SO42−

ions?

Ans. Na2SO4, FeSO4, or any other soluble sulfate.

Supplementary Problems

9.18. Write a net equation for the following overall equation:

4 Au+16 KCN+6 H2O+3 O2−→4 KAu(CN)4(aq)+12 KOH Ans. 4 Au+16 CN−+6 H2O+3 O2−→4 Au(CN)4−+12 OH− 9.19. Write a net ionic equation for each of the following equations:

(a) CdS(s)+I2−→CdI2(aq)+S

(b) 4 KOH+4 KMnO4(aq)−→4 K2MnO4(aq)+O2+2 H2O (c) 2 HI +2 HNO2−→2 NO+2 H2O+ I2

(d) AlCl3+4 NaOH−→NaAl(OH)4(aq) +3 NaCl Ans. (a) CdS(s)+I2−→ Cd2++2 I−+S

(b) 4 OH−+4 MnO4−−→4 MnO42−+ O2+2 H2O

9.20. Balance the following net ionic equations:

(a) Ag++Fe−→Fe2++Ag (b) Fe3++I−−→Fe2++I2

(d) Cd+Cr3+−→Cr2++Cd2+ Ans. In each part, the net charge as well as the number of each type of atom must balance.

(a) 2 Ag++Fe−→Fe2++2 Ag (b) 2 Fe3++2 I−−→2 Fe2++I2

(d) Cd+2 Cr3+−→2 Cr2++Cd2+

9.21. Try to write a complete equation corresponding to the unbalanced and the balanced net ionic equations of the prior problem. What do you find?

Ans. You cannot write a complete equation for an unbalanced net ionic equation. [In part (a), for example, you might have one acetate ion on the left and two on the right.] One complete equation for the balanced net ionic equation might be

2 AgC2H3O2+ Fe−→Fe(C2H3O2)2+2 Ag

9.22. Would the following reaction yield 56 kJ of heat per mole of water formed, as the reactions in Example 9.4 do?

Explain.

HC2H3O2+ NaOH−→NaC2H3O2+H2O

Ans. No. Since HC2H3O2 is a weak acid, there is a different net ionic equation and thus a different amount of heat:

HC2H3O2+OH−−→C2H3O2−+H2O 9.23. Per mole of water formed, how much heat is generated by the reaction of Example 9.5?

Ans. 56 kJ. It is the same reaction as that of Example 9.4.

9.24. Would 56 kJ per mole of water formed by generated by the following reaction? Compare your answer with that of the prior problem.

Ba(OH)2(s)+2 HCl−→BaCl2+2 H2O

Ans. No. It is not represented by the same net ionic equation. Some heat is involved in dissolving the solid Ba(OH)2.

9.25. Write a net ionic equation for each of the following overall equations:

(a) Ca(OH)2(s)+2 HClO3−→Ca(ClO3)2+2 H2O (b) CuSO4(aq)+H2S−→CuS(s)+H2SO4

Ans. (a) Ca(OH)2(s)+2 H+−→Ca2++2 H2O (b) Cu2++ H2S−→CuS+2 H+

9.26. Write 12 more equations represented by the net ionic equation given in Example 9.4, using only the reactants used in that example.

Ans. NaOH+HCl−→NaCl+H2O NaOH+HBr−→NaBr+H2O RbOH+NHO3 −→RbNO3+H2O KOH+HI−→KI+H2O

RbOH+HBr−→RbBr+H2O LiOH+HCl−→LiCl+H2O NaOH+HI−→NaI+H2O KOH+HNO3 −→KNO3+H2O RbOH+HCl−→RbCl+H2O KOH+HBr−→KBr+H2O LiOH+HNO3 −→LiNO3+H2O LiOH+HI−→LiI+H2O

9.27. Indicate how you could choose different compounds to enable you to write 100 additional equations in response to Problem 9.12.

Ans. Choose 34 or more soluble, ionic chlorides—the other alkali metal chlorides (4), the alkaline earth chlorides (6), the first transition metal chlorides, most of the metals with two different charges (14), many second and third transition metal chlorides (about 10), aluminum and tin(II) chloride (2). Combine each of these with each of the three silver salts that you know are soluble (given in the example), and you have over 100 overall equations.

Stoichiometry

ELECTRONIC STRUCTURE AND THE PERIODIC TABLE

DETAILED ELECTRONIC CONFIGURATIONS OF IONS (OPTIONAL)