Question

Acid-Base Properties of Ions and Salts Determine if each salt will form a solution that is acidic, basic, or pH-neutral. \begin{array}{l}{\text { a. } \mathrm{FeCl}_{3}} \\ {\text { b. } \mathrm{NaF}} \\ {\text { c. CaBr }_{2}} \\ {\text { d. } \mathrm{NH}_{4} \mathrm{Br}} \\ {\text { e. } \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3} \mathrm{NO}_{2}}\end{array}

Short answer

\(\mathrm{FeCl}_{3}\) is acidic, \(\mathrm{NaF}\) is basic, \(\mathrm{CaBr}_{2}\) is neutral, \(\mathrm{NH}_{4}\mathrm{Br}\) is acidic, and \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{3}\mathrm{NO}_{2}\) is acidic.

Step-by-step solution

Understanding the Components

Examine each part of the salt to determine if it comes from a strong or weak acid or base. A strong acid or base completely dissociates in water, while a weak acid or base only partially dissociates. Cations of strong bases will not affect the pH of the solution, whereas anions of strong acids also do not affect the pH. However, cations from weak bases and anions from weak acids will hydrolyze, affecting the pH of the solution.

Analyzing each Salt

\(\mathrm{FeCl}_{3}\) comes from \(\mathrm{Fe}^{3+}\) (a weak base, ferric hydroxide) and \(\mathrm{Cl}^{-}\) (from hydrochloric acid, a strong acid). The cation \(\mathrm{Fe}^{3+}\) will hydrolyze to form an acidic solution. \(\mathrm{NaF}\) comes from \(\mathrm{Na}^{+}\) (from sodium hydroxide, a strong base) and \(\mathrm{F}^{-}\) (from hydrofluoric acid, a weak acid). The anion \(\mathrm{F}^{-}\) will hydrolyze to form a basic solution. \(\mathrm{CaBr}_{2}\) comes from \(\mathrm{Ca}^{2+}\) (from calcium hydroxide, a strong base) and \(\mathrm{Br}^{-}\) (from hydrobromic acid, a strong acid), having no effect on the pH of the solution, resulting in a neutral solution. \(\mathrm{NH}_{4}\mathrm{Br}\) comes from \(\mathrm{NH}_{4}^{+}\) (from the weak base, ammonia) and \(\mathrm{Br}^{-}\) (from hydrobromic acid, a strong acid). The cation \(\mathrm{NH}_{4}^{+}\) will hydrolyze to form an acidic solution. \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{3}\mathrm{NO}_{2}\) comes from \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{3}^{+}\) (from a weak base, aniline) and \(\mathrm{NO}_{2}^{-}\) (from nitric acid, a strong acid). The cation \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{3}^{+}\) will hydrolyze to form an acidic solution.

Concluding the pH nature of each solution

Based on the previous analysis, the nature of each solution can be summarized as follows: \(\mathrm{FeCl}_{3}\) forms an acidic solution, \(\mathrm{NaF}\) forms a basic solution, \(\mathrm{CaBr}_{2}\) forms a neutral solution, \(\mathrm{NH}_{4}\mathrm{Br}\) forms an acidic solution, and \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{3}\mathrm{NO}_{2}\) forms an acidic solution.

Key concepts

pH determination of salts

The pH of a salt solution is a measure of its acidity or basicity. When salts dissolve in water, they separate into cations and anions, which can then interact with water in a process called hydrolysis. This reaction can impact the pH of the solution. For instance, a salt formed from a strong acid and a strong base, like \textbf{CaBr}\(_2\), does not hydrolyze in water, resulting in a neutral solution. In contrast, salts like \textbf{FeCl}\(_3\), which originates from a weak base (ferric hydroxide) and a strong acid (hydrochloric acid), will produce an acidic solution due to the hydrolysis of the \textbf{Fe}\(^{3+}\) cation. To determine the pH of a salt solution, consider the strength of the original acid and base from which the cations and anions are derived.

Strong base cations or strong acid anions typically do not alter the pH significantly when they hydrolyze. However, if either ion comes from a weak counterpart (\textbf{NH}\(_4\)\textbf{Br} is a prime example), hydrolysis will shift the pH away from neutrality, making the solution acidic in the case of weak base-derived cations (\textbf{NH}\(_4$$^+\)) or basic for weak acid-derived anions (\textbf{F}\(^-\) from sodium fluoride).

The pH is not always easy to predict; for mixed systems, involving both weak acids and bases, it's necessary to carefully consider the extent to which each ion might hydrolyze. The pH level of such solutions requires more in-depth analysis, often involving equilibrium calculations and the use of constants like the acid dissociation constant (Ka) or base dissociation constant (Kb).

Hydrolysis of ions

Hydrolysis of ions is a chemical reaction that occurs when ions interact with water after dissolving. Ions that come from weak acids or bases can participate in this process, resulting in the formation of hydroxide ions (OH\(^-\)) in the case of a basic solution, or hydrogen ions (H\(_3\)O\(^+\)) in the case of an acidic solution. This ion-water interaction is the driving factor behind the pH of the solution.

For example, fluoride ions (\textbf{F}\(^-\)) from \textbf{NaF} represent the anion from hydrofluoric acid, a weak acid. In water, \textbf{F}\(^-\) ions attract protons to form HF and leave behind OH\(^-\) ions, thus increasing the solution's basicity. Conversely, when a cation like \textbf{NH}\(_4$$^+\), which comes from the weak base ammonia, is in the solution, it donates a proton to water, forming NH\(_3\) and H\(_3\)O\(^+\), making the solution more acidic.

The extent of the hydrolysis and its effect on the pH is dependent on the ion's strength, the concentration of ions in the solution, and temperature. Understanding the propensity of an ion to undergo hydrolysis helps predict the resulting pH of a solution. Stronger the acid or base from which the ion comes, the less likely it is to hydrolyze, thus having a minimal effect on pH.

Strong and weak acids and bases

Understanding the distinction between strong and weak acids and bases is crucial for predicting the behavior of salts in solution. Strong acids and bases, like hydrochloric acid (\textbf{HCl}) and sodium hydroxide (\textbf{NaOH}), dissociate entirely in water, releasing all of their hydrogen and hydroxide ions respectively. These strong electrolytes don't tend to undergo hydrolysis when they form salts, because their ions are already fully dissociated.

Weak acids and bases, in contrast, do not fully dissociate; they only partially release their ions into the water. An example of a weak base is aniline (\textbf{C}\(_6\)\textbf{H}\(_5\)\textbf{NH}\(_2\)), which does not completely release OH\(^-\) ions. Similarly, a weak acid like hydrofluoric acid (\textbf{HF}) only partially releases H\(^+\) ions. When salts are formed from weak acids or bases, such as \textbf{NH}\(_4\)\textbf{Br} (ammonium bromide), they can significantly affect the pH of their solutions because the ions can react with water molecules to a considerable extent.

It's the weak acids and bases that lead to the most interesting scenarios in pH determination due to their incomplete dissociation. The degree of dissociation is described by their dissociation constants (Ka for acids and Kb for bases). These constants are essential for calculating the exact pH of solutions and provide insight into the acid or base strength. The smaller the constant, the weaker the acid or base, and the more significant its potential effect on the pH of a solution when forming salts.