Volume 2

Aqueous Solutions of Various Salts Can Be Neutral, Basic, or Acidic. How Cations and Anions Produced on Dissociation of Salts Interact with Water • Exercises
2.7.1. Aqueous Solutions of Various Salts Can Be Neutral, Basic, or Acidic. How Cations and Anions Produced on Dissociation of Salts Interact with Water. On addition of an acid such as HCl or H2SO4 to water, the original pH value of 7 goes down because the concentration of the H+ goes up. On addition of a base such as NaOH or Ca(OH)2 to water the pH goes up because the concentration of the H+ decreases due to an increase in the concentration of the OH-. Will the neutral pH of 7 of pure water change on addition of a soluble salt? It might and it might not.

Some salts such as NaCl, Ba(NO3)2, and K2SO4 have no effect on the pH of water. However, if we dissolve baking soda (NaHCO3) in water, the resultant solution will be slightly basic (pH ≈ 8). On dissolution of Na2CO3 in water, the pH goes to 11 — no kidding! — thereby indicating that a solution of Na2CO3 is quite alkaline. On the contrary, aqueous solutions of ZnCl2 are acidic, the pH being as low as 1 at a high concentration of zinc chloride. Why? Because some ions produced on dissociation of a salt can and do interact with water molecules in an interesting way.

Let us consider an aqueous solution of ZnCl2 (Figure 2-87). Being a salt, ZnCl2 readily dissociates in water into the ions Zn2+ and Cl-. Water dissociates too, although to a much smaller degree, to give rise to H+ and OH-. The blue and red ellipses in Figure 2-87 show interactions of the oppositely charged ions derived from ZnCl2 and water.
Figure 2-87. Interactions between ZnCl2 and water.

Circled in red in Figure 2-87, are the Cl- (from ZnCl2) and H+ (from H2O). When they collide, a molecule of HCl is formed, which is a strong electrolyte that immediately dissociates back into the Cl- and H+. Consequently, the Cl- anion cannot and does not influence the concentrations of H+ and OH- in water.

Circled in blue in Figure 2-87, are the Zn2+ (from ZnCl2) and OH- (from H2O). When they come together, ZnOH+ is produced. What is ZnOH+ and will this cation dissociate back into the Zn2+ and OH-? The ion ZnOH+ could be formed on dissociation of one OH- from Zn(OH)2. As Zn(OH)2 is a weak electrolyte, ZnOH+ is also a weak electrolyte, not readily dissociating to Zn2+ and OH-. Consequently, the Zn2+ cation in our solution serves as a trap for the OH- ions. As the concentration of OH- goes down, the solution becomes less alkaline = more acidic, precisely as can be observed experimentally by a pH measurement.

Next, we consider Na2CO3, sodium carbonate, also known as soda ash and washing soda. To understand why solutions of Na2CO3 are alkaline, we will use the same type of analysis as in the previous case of ZnCl2. On dissolution in water, Na2CO3 dissociates to Na+ and CO32- (Figure 2-88). Water dissociates (to a much lesser extent) to give small quantities of H+ and OH-. A collision of Na+ and OH- produces NaOH that, being a strong electrolyte, dissociates back right away (Figure 2-88, blue ellipse). In contrast, when the H+ and CO32- meet, they produce hydrocarbonate, HCO3-, a weak electrolyte that does not dissociate readily. We know that HCO3- dissociates poorly from the fact that H2CO3 is a very weak electrolyte and, consequently, the product of its first dissociation, HCO3- is even a weaker acid. Therefore, the CO32- serves as a trap for the H+, thereby making the solution less acidic = more basic.
Figure 2-88. Interactions between Na2CO3 and water.

Finally, if we apply the same analysis to an aqueous solution of NaCl (Figure 2-89), we will see that neither Na+ can trap OH-, nor Cl- can sequester H+ because both NaOH and HCl are strong electrolytes. Consequently, the pH of an aqueous solution of NaCl is 7, the same as of pure water.
Figure 2-89. The pH of aqueous NaCl is neutral (7) because neither Na+ sequesters OH-, nor Cl- can trap H+.

We say that NaCl is not hydrolyzed in solution (Figure 2-89), whereas Na2CO3 (Figure 2-88) and ZnCl2 (Figure 2-87) are. Hydrolysis is a chemical reaction where water is involved as a reagent. Hydrolysis reactions can be reversible, like those of Na2CO3 or ZnCl2 considered above, but can also be irreversible. Irreversible hydrolysis means that the reaction with water goes to completion, as in the following example.

Al2S3 + 6 H2O = 2 Al(OH)3↓ + 3 H2S↑

There is a simple rule for quickly predicting if an aqueous solution of a given salt would be acidic or alkaline. This rule is illustrated in Figure 2-90.
Figure 2-90. A rule of thumb to predict if a solution of a salt would be acidic or alkaline.

First, identify the parent acid and base of a given salt. If the parent acid is stronger an electrolyte than the parent base, an aqueous solution of the salt will be acidic (acid "defeats" base). If the parent base is stronger an electrolyte than the parent acid, the solution will be alkaline (base "defeats" acid). If both the parent acid and the base are strong electrolytes, neither one can "defeat" the other, the salt will not be hydrolyzed and its aqueous solution will be neutral (pH = 7).

As an example, the parent acid and base for KCl are HCl and KOH. As both are strong electrolytes, KCl is not hydrolyzed and aqueous solutions of KCl are neutral. For CuSO4, the parent acid and base are H2SO4 and Cu(OH)2. Since the acid is strong and the base is weak, solutions of CuSO4 are acidic. Sodium silicate (Na2SiO3) is formed by NaOH, a strong base, and metasilicic acid, H2SiO3, a weak acid. Therefore, an aqueous solution of Na2SiO3 is alkaline.

Take warning of the often tricky outcome of hydrolysis of hydro salts. A hydro salt is produced if not all of the protons on the molecules of a dibasic or tribasic acid have been displaced by a metal ion(s). Two examples of hydro salts are presented in Figure 2-91.
Figure 2-91. Empirical and structural formulas of NaHSO4 and NaHCO3.

Some people make a big mistake thinking that all hydro salts are acidic because of the remaining H atom(s) on the acid remainder. While aqueous solutions of some hydro salts are indeed acidic, those of some other hydro salts are alkaline! Every case should be carefully analyzed to avoid making a mistake.

Aqueous solutions of NaHSO4 are acidic. This salt is formed by a strong acid and a strong base, so should a solution of NaHSO4 not be neutral? True, neither OH- nor H+ can be captured by Na+ and HSO4- produced on dissociation of NaHSO4 (Figure 1-92). However, the HSO4- can and does dissociate to produce H+ and SO42-. The dissociation of HSO4- is not as efficient as that of H2SO4 because polybasic acids always dissociate step-wise and each consecutive step is considerably less facile than the previous one. Nonetheless, HSO4- is a medium strength acid, making aqueous solutions of NaHSO4 quite acidic (Figure 2-92). Note that the acidity of NaHSO4 solutions does not originate from hydrolysis but rather from dissociation of the HSO4- anion into H+ and SO42-.
Figure 2-92. NaHSO4 is not hydrolyzed. The acidity of aqueous NaHSO4 is due to dissociation of HSO4- to give H+.

Do not be deceived by the similarity of formulas and structures of NaHSO4 and NaHCO3 (Figure 2-91). The case of NaHCO3 (baking soda) is very different from that of NaHSO4. Solutions of NaHCO3 are not acidic but mildly alkaline. (This explains the long-time use of baking soda as a simple remedy for heartburn.) Figure 2-93 accounts for the basicity of a NaHCO3 solution. Unlike NaHSO4, NaHCO3 does undergo hydrolysis because when HCO3- and H+ collide, H2CO3 is formed, which is a weak electrolyte. Serving as a trap for H+, the HCO3- is responsible for the alkalinity of baking soda solutions. Unlike HSO4-, HCO3- is an extremely poor electrolyte (even weaker than H2CO3) that cannot produce H+ in any significant quantities (Figure 2-93).
Figure 2-93. Solutions of NaHCO3 are mildly alkaline due to hydrolysis. Unlike HSO4- (Figure 2-92), HCO3- is an extremely weak electrolyte that produces H+ only in negligible quantities.

2.7.2. Exercises.

1. By looking at the formulas below determine if an aqueous solution of each salt is acidic, alkaline, or neutral: (a) KBr; (b) K2CO3; (c) BaI2; (d) CuCl2; (e) BaSO4; (f) Al2(SO4)3; (g) Ba(NO3)2; (h) AgCl; (i) AgNO3; (j) K2SiO3. Answer

2. Using the ion collision analysis (as shown in Figures 2-87, 2-88, and 2-89), support your answers to questions in Exercise 1 above.

3. Would you expect an aqueous solution of Ca(NO3)2 to be neutral, acidic, or alkaline? Provide arguments to support your view. Answer