Question

Acid-Base Properties of Ions and Salts Determine whether each cation is acidic or pH-neutral. For the cations that are acidic, write an equation that shows how the cation acts as an acid. \(\begin{array}{l}{\text { a. } \mathrm{Sr}^{2+}} \\ {\text { b. } \mathrm{Mn}^{3+}} \\ {\text { c. } \mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}} \\ {\text { d. } \mathrm{Li}^{+}}\end{array} \)

Short answer

a. Sr2+ is pH-neutral. b. Mn3+ is considered pH-neutral in Brønsted-Lowry terms. c. C5H5NH+ is acidic, with the reaction \( C5H5NH^+ + H2O \rightarrow C5H5N + H3O^+ \). d. Li+ is pH-neutral.

Step-by-step solution

Understanding the concept of acidity in cations

To determine whether a cation is acidic or pH-neutral, consider if it is the conjugate acid of a weak base. Strong acid cations tend to be neutral, but cations derived from weak bases can donate protons and are therefore acidic.

Analyzing Strontium Cation (Sr2+)

Strontium comes from strontium hydroxide, Sr(OH)2, which is a strong base. Its cation, Sr2+, does not act as an acid in water, since it cannot donate a proton. Therefore, it is pH-neutral.

Examining Manganese Cation (Mn3+)

Mn3+ is a cation that can act as a Lewis acid by accepting an electron pair. However, since it doesn't donate protons, it's not typically considered an acid in the Brønsted-Lowry sense. For simplicity, we assume it is not acidic under that definition.

Assessing the Pyridinium Cation (C5H5NH+)

C5H5NH+ is the conjugate acid of pyridine, a weak base. As such, it can donate a proton (H+) and is considered acidic. The equation for its dissociation is: \( \mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+} + H_{2}O \rightarrow \mathrm{C}_{5} \mathrm{H}_{5} N + H_{3}O^{+} \).

Considering Lithium Cation (Li+)

Lithium cation comes from lithium hydroxide, which is a strong base. Thus, the Li+ cation is pH-neutral because it does not have the ability to donate a proton to water.

Key concepts

pH-neutral Cations

When we talk about pH-neutral cations, we're referring to positively charged ions that do not change the acidity or basicity of a solution. These cations are typically derived from strong bases. A quintessential example is the strontium cation (Sr²⁺), which, due to its origin from strontium hydroxide (a strong base), does not have any protons to donate and thus, does not affect the pH when dissolved in water.

Similarly, the lithium cation (Li⁺) is another example of a pH-neutral cation. It stems from lithium hydroxide, also a strong base, and consequently, it has no acidic properties. These cations do not participate in proton transfer reactions when in aqueous solutions; thus, they maintain a neutral pH environment.

Conjugate Acids and Bases

Understanding the relationship between conjugate acids and bases is essential for grasping the acid-base properties of ions. This relationship is crucial when evaluating whether a cation is acidic. If a cation is the conjugate acid of a weak base, it has the potential to donate a proton according to the Brønsted-Lowry definition of an acid. The pyridinium cation (C₅H₅NH⁺) illustrates this concept well as it can donate a proton and revert to its conjugate base, pyridine, hence exhibiting acidic behavior.

The reaction can be represented as follows:\[ \text{C}_{5}\text{H}_{5}\text{NH}^{+} + \text{H}_{2}\text{O} \rightarrow \text{C}_{5}\text{H}_{5}\text{N} + \text{H}_{3}\text{O}^{+} \].Such reactions highlight how the cation, initially the conjugate acid, releases a proton, which is then accepted by a water molecule, forming hydronium ions and indicating the presence of an acid in the solution.

Brønsted-Lowry Acid

The concept of a Brønsted-Lowry acid is defined by its ability to donate a proton to another substance, which is the core of the Brønsted-Lowry acid-base theory. Typically, when we consider a cation's acidity, we examine its capacity to behave as a Brønsted-Lowry acid in water. Cations that can donate a proton to water molecules are deemed acidic; those that cannot are characterized as pH-neutral.

For instance, the manganese cation (Mn³⁺) was discussed in the solution with the recognition that it can act as a Lewis acid by accepting an electron pair. However, it does not meet the criteria for an acid according to the Brønsted-Lowry theory because it does not donate protons. This distinction is critical as it shapes our understanding of acid-base reactions in different contexts, emphasizing the importance of proton transfer.