Question

Acid-Base Properties of Ions and Salts Determine whether each anion is basic or neutral. For the anions that are basic, write an equation that shows how the anion acts as a base. $$\begin{array}{l}{\text { a. } \mathrm{C}_{7} \mathrm{H}_{5} \mathrm{O}_{2}^{-}} \\ {\text { b. } \mathrm{I}^{-}} \\ {\text { c. } \mathrm{NO}_{3}^{-}} \\ {\text { d. } \mathrm{F}^{-}}\end{array}$$

Short answer

a. Basic: \( \mathrm{C}_{7} \mathrm{H}_{5}\mathrm{O}_{2}^{-} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{C}_{7} \mathrm{H}_{6}\mathrm{O}_{2} + \mathrm{OH}^{-} \). b. Neutral. c. Neutral. d. Basic: \( \mathrm{F}^{-} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{HF} + \mathrm{OH}^{-} \).

Step-by-step solution

Review Acid-Base Theories

Remember that according to the Bronsted-Lowry theory, an acid is a proton donor and a base is a proton acceptor. An anion can act as a base if it has the ability to accept a proton. For anions to be neutral, they must be the conjugate bases of strong acids, which do not accept protons in water.

Identify the Nature of Each Anion

Determine whether each anion is the conjugate base of a weak acid (which would make it basic) or a strong acid (which would make it neutral).

Determine the Nature of Anion a. \( \mathrm{C}_{7} \mathrm{H}_{5}\mathrm{O}_{2}^{-} \)

\( \mathrm{C}_{7}\mathrm{H}_{5}\mathrm{O}_{2}^{-} \) is the conjugate base of benzoic acid, which is a weak acid. Thus, this anion is basic.

Write Equation for Anion a.

The anion acts as a base by accepting a proton: \[ \mathrm{C}_{7} \mathrm{H}_{5}\mathrm{O}_{2}^{-} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{C}_{7} \mathrm{H}_{6}\mathrm{O}_{2} + \mathrm{OH}^{-} \]

Determine the Nature of Anion b. \( \mathrm{I}^{-} \)

\( \mathrm{I}^{-} \) is the conjugate base of hydroiodic acid, which is a strong acid. Therefore, this anion is neutral.

Determine the Nature of Anion c. \( \mathrm{NO}_{3}^{-} \)

\( \mathrm{NO}_{3}^{-} \) is the conjugate base of nitric acid, a strong acid. Hence, the nitrate anion is neutral.

Determine the Nature of Anion d. \( \mathrm{F}^{-} \)

\( \mathrm{F}^{-} \) is the conjugate base of hydrofluoric acid, a weak acid. Therefore, this anion is basic.

Write Equation for Anion d.

The anion acts as a base through proton acceptance: \[ \mathrm{F}^{-} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{HF} + \mathrm{OH}^{-} \]

Key concepts

Bronsted-Lowry Theory

The Bronsted-Lowry theory provides a fundamental framework for understanding how acids and bases behave. According to this theory, an acid is a substance that can donate a proton (a hydrogen ion, \(H^+\)), and a base is a substance that can accept a proton. This concept revolutionized the way we think about acid-base reactions, as it introduced the idea that acids and bases do not need to be in aqueous solution to act as proton donors or acceptors.

For instance, when an acid like hydrochloric acid (\(HCl\)) is dissolved in water, it donates a proton to the water molecule, which is acting as a base. This interaction is represented by the following equation:
\[ HCl + H_2O \rightarrow Cl^- + H_3O^+ \]
The proton-transferring behavior underpins our understanding of numerous chemical reactions and guides us through complex acid-base concepts.

Conjugate Base

The concept of a conjugate base is essential in the study of acid-base reactions. When an acid loses a proton, the species that remains is its conjugate base. It is crucial because the conjugate base retains the ability to re-accept the proton, essentially reversing the reaction that took place. In essence, conjugate base forms are simply the acid less one \(H^+\) ion.

For example, when acetic acid (\(CH_3COOH\)) donates a proton, the acetate ion (\(CH_3COO^-\)) is its conjugate base:
\[ CH_3COOH \rightarrow CH_3COO^- + H^+ \]
This relationship allows us to understand the reversibility of acid-base reactions and predict the relative strengths of acids and bases.

Weak Acids and Strong Acids

We categorize acids based on their ability to dissociate in solution: strong acids dissociate completely, while weak acids only partially. For a strong acid like hydroiodic acid (\(HI\)), practically all of the acid molecules donate their protons when in water, resulting in a neutral conjugate base, like iodide (\(I^-\)).

Conversely, weak acids like acetic acid (\(CH_3COOH\)) only partially dissociate in solution. Since they don't completely release their protons, their conjugate bases (like acetate \(CH_3COO^-\)) have a significan ability to accept protons, making them basic in nature.

Understanding the distinction between weak and strong can help predict the behavior of substances in a chemical reaction and their corresponding pH levels in solution.

Proton Acceptance

Proton acceptance is the key action that defines a substance as a base within the Bronsted-Lowry framework. When an anion acts as a base, it accepts a proton from another substance, usually water in aqueous solutions. This interaction is crucial to the mechanism of acid-base reactions.

For instance, when the fluoride ion (\(F^-\)), a conjugate base of weak hydrofluoric acid, is in water, it can attract and hold onto a proton from a water molecule, forming hydrofluoric acid and a hydroxide ion (\(OH^-\)):
\[ F^- + H_2O \rightarrow HF + OH^- \]
The ability to accept protons makes the fluoride ion basic. The readiness with which a base accepts a proton can give us insight into its basicity and potential reactivity.

Neutral Anions

Neutral anions are the conjugates of strong acids and do not act as bases in aqueous solution; they do not accept protons from water. This is due to the corresponding strong acid's proficiency at proton donation, leading to a complete dissociation in water and, therefore, no tendency for the anion to reaccept a proton.

As seen with nitrate (\(NO_3^-\)), the conjugate base of strong nitric acid (\(HNO_3\)), or iodide (\(I^-\)) from hydroiodic acid (\(HI\)), these anions remain in solution without altering the acid-base balance, characterized by their inability to increase the concentration of \(OH^-\) ions:
\[ NO_3^- + H_2O ot\rightarrow HNO_3 + OH^- \]
Recognizing neutral anions aids in predicting the outcome of reactions involving strong acids and in determining the pH of solutions.