Problem 50 - Chemistry

Question

For which salt in each of the following groups will the solubility depend on \(\mathrm{pH}\) ? \(\begin{array}{ll}{\text { a. AgF, AgCl, AgBr }} & {\text { c. } \operatorname{Sr}\left(\mathrm{NO}_{3}\right)_{2}, \operatorname{Sr}\left(\mathrm{NO}_{2}\right)_{2}} \\ {\text { b. } \mathrm{Pb}(\mathrm{OH})_{2}, \mathrm{PbCl}_{2}} & {\text { d. } \mathrm{Ni}\left(\mathrm{NO}_{3}\right)_{2}, \mathrm{Ni}(\mathrm{CN})_{2}}\end{array}\)

Step-by-Step Solution

Answer

The salts with solubility that depends on pH are AgF (Group a), Pb(OH)₂ (Group b), Sr(NO₂)₂ (Group c), and Ni(CN)₂ (Group d).

Step 1: Understand the effect of pH on solubility

pH affects a salt's solubility if the salt's anion can act as a base and react with H+ ions in the solution, or if its cation can form insoluble hydroxides that dissolve upon the addition of OH- ions.

Step 2: Group a - Analyzing the Anions in AgF, AgCl, and AgBr

The anions in these salts are F-, Cl-, and Br-, respectively. F- is the weakest base among them and can potentially react with H+ ions. In contrast, Cl- and Br- do not significantly react with H+ ions.

Step 3: Group a - Identifying the salt with pH-dependent solubility

Among AgF, AgCl, and AgBr, only AgF has an anion that can potentially react with H+ ions. Therefore, the solubility of AgF depends on pH.

Step 4: Group b - Analyzing the Components of Pb(OH)2 and PbCl2

Pb(OH)2 comprises Pb2+ ions and OH- ions. Pb2+ can form insoluble hydroxides, while OH- ions that can react with H+ ions. PbCl2 is formed by the combination of Pb2+ and Cl- ions. The ions in PbCl2 do not significantly react with H+ or OH- ions.

Step 5: Group b - Identifying the salt with pH-dependent solubility

Among Pb(OH)2 and PbCl2, only Pb(OH)2 has components that can react with H+ or OH- ions. Therefore, the solubility of Pb(OH)2 depends on pH.

Step 6: Group c - Analyzing the Components of Sr(NO3)2 and Sr(NO2)2

Sr(NO3)2 has Sr2+ ions and NO3- ions. The anion NO3- does not react with H+ or OH- ions. Sr(NO2)2 contains Sr2+ ions and NO2- ions. The anion NO2- can react with H+ ions and act as a weak base.

Step 7: Group c - Identifying the salt with pH-dependent solubility

Among Sr(NO3)2 and Sr(NO2)2, only Sr(NO2)2 has an anion that can significantly react with H+ ions. Therefore, the solubility of Sr(NO2)2 depends on pH.

Step 8: Group d - Analyzing the Components of Ni(NO3)2 and Ni(CN)2

Ni(NO3)2 has Ni2+ ions and NO3- ions. The anion NO3- does not react with H+ or OH- ions. Ni(CN)2 contains Ni2+ ions and CN- ions. CN- ions can react with H+ ions as a weak base.

Step 9: Group d - Identifying the salt with pH-dependent solubility

Among Ni(NO3)2 and Ni(CN), only Ni(CN)2 has an anion that can significantly react with H+ ions. Therefore, the solubility of Ni(CN)2 depends on pH. To summarize, the salts with solubility that depends on pH are: - AgF (Group a) - Pb(OH)2 (Group b) - Sr(NO2)2 (Group c) - Ni(CN)2 (Group d)

Key Concepts

Salt Solubility

When discussing solubility, we're talking about how well a substance, like a salt, can dissolve in a solvent, often water. Solubility is influenced by interactions between the ions in the salt and the molecules of the solvent. The dissolving process involves breaking these ions apart from the salt structure and surrounding them with solvent molecules.

Some salts dissolve easily, forming high ion concentrations in solution. Others dissolve poorly, with fewer ions released into the solution.

• Salts like NaCl dissolve well, producing many ions.
• Salts with low solubility produce only a few ions.

Additionally, the solubility of salts can depend on temperature, pressure, and the presence of other chemicals, such as acids or bases.

Effect of pH on Solubility

pH measures the concentration of hydrogen ions ( H^+ ) in a solution and indicates whether a solution is acidic or basic. The pH can significantly affect the solubility of certain salts.

Salts containing anions that can react with H^+ will see a change in solubility with pH shifts. An acidic environment can increase solubility if the salt’s anion is a weak base because the H^+ will shift the equilibrium, pulling more ions into the solution.

• For example, the solubility of AgF increases in an acidic solution since F^- can react with H^+ to form HF , a weak acid that further dissociates minimally.
• Similarly, when the solution becomes basic, salts forming insoluble hydroxides, such as Pb(OH)_2 , may dissolve more, as OH^- ions engage in complex equilibria.

Recognizing the nature of the salt’s anion or cation is essential to predict how pH affects solubility.

Anion Basicity

Anions are negatively charged ions that can act as bases if they can accept H^+ ions. The basic property of anions, or anion basicity, helps us understand how certain salts react in solutions with different pH levels.

A basic anion can grab onto H^+ ions, reducing the concentration of hydrogen ions and creating potential changes in solubility. The stronger the base, the more effectively it can react with H^+ .

• For instance, NO_2^- in Sr(NO_2)_2 can react with H^+ , forming HNO_2 , which slightly ionizes, decreasing the amount of Sr(NO_2)_2 in solid form.
• Weakly basic anions, like CN^- , also engage in similar reactions, affecting the solubility of salts like Ni(CN)_2 by reducing the free hydrogen ions in the solution.

Understanding anion basicity is crucial for predicting the changes in solubility in response to varying pH levels.

Hydroxide Formation

Hydroxide formation in solutions is vital when considering salts containing cations capable of forming insoluble hydroxides. These cations will react in basic solutions to form compounds that often precipitate out of solution.

When the solution is basic, it has high levels of OH^- ions. If these ions find reactive cations, they will form hydroxides. However, this can also be reversed: adding enough OH^- can sometimes increase solubility by complexing with the cation and pulling it back into the solution.

• For example, Pb^2+ can form Pb(OH)_2 in basic conditions, thus reducing solubility.
• In contrast, additional OH^- may cause Pb(OH)_2 to dissolve by forming more soluble lead species.

Recognizing which cations are prone to form insoluble hydroxides helps to predict their behavior in different pH conditions.

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, causing the concentrations of reactants and products to remain constant over time. This balance is crucial in understanding solubility because some solubility processes reach equilibrium.

When assessing how salts dissolve, it's important to consider solubility equilibria, represented by equations like:\[K_{sp} = [ ext{Cation}^{n+}][ ext{Anion}^{m-}]\]This expression represents the solubility product constant (K_{sp}), which defines the situation where the solution is saturated. Smaller K_{sp} values indicate that the salt has low solubility.

• The equilibrium can shift in response to changes in H^+ or OH^- concentration, altering ion balance and thus solubility.
• By removing H^+ in basic solutions, equilibrium shifts to increase product formation, increasing solubility.

Understanding chemical equilibrium is essential for predicting and manipulating salt solubility under different conditions.