Question

Are solutions of the following salts acidic, basic, or neutral? For those that are not neutral, write balanced equations for the reactions causing the solution to be acidic or basic. The relevant \(K_{\mathrm{a}}\) and \(K_{\mathrm{b}}\) values are found in Tables 14.2 and \(14.3 .\) \(\begin{array}{ll}{\text { a. } \operatorname{Sr}\left(\mathrm{NO}_{3}\right)_{2}} & {\text { d. } \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3} \mathrm{ClO}_{2}} \\ {\text { b. } \mathrm{NH}_{4} \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}} & {\text { e. } \mathrm{NH}_{4} \mathrm{F}} \\ {\text { c. } \mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{O} \mathrm{l}} & {\text { f. } \mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{CN}}\end{array}\)

Short answer

The solutions are: a. Sr(NO₃)₂: neutral b. NH₄C₂H₃O₂: relatively neutral c. CH₃NH₃Ol: acidic (CH₃NH₂ + H₃Ol⁺) d. C₆H₅NH₃ClO₂: acidic e. NH₄F: basic (NH₃ + HF) f. CH₃NH₃CN: relatively neutral

Step-by-step solution

Identify the ions in the salts and classify as acidic, basic, or neutral

First, let's identify the ions present in each salt and classify them depending on their acidic, basic, or neutral properties. a. Sr(NO3)₂: Sr²⁺ (neutral), NO₃⁻ (neutral) b. NH₄C₂H₃O₂: NH₄⁺ (acidic), C₂H₃O₂⁻ (basic) c. CH₃NH₃Ol: CH₃NH₃⁺ (acidic), Ol⁻ (neutral) d. C₆H₅NH₃ClO₂: C₆H₅NH₃⁺ (acidic), ClO₂⁻ (neutral) e. NH₄F: NH₄⁺ (acidic), F⁻ (basic) f. CH₃NH₃CN: CH₃NH₃⁺ (acidic), CN⁻ (basic)

Determine the nature of the solutions using \(K_a\) and \(K_b\) values

We'll now determine the nature of the solutions based on the ions' conjugate acids and bases, and their \(K_a\) and \(K_b\) values. If the ion is derived from a weak acid or base, its conjugate should be stronger, making the solution acidic or basic. a. Sr(NO₃)₂: Since both ions are neutral, the solution is neutral. b. NH₄C₂H₃O₂: Acidic and basic ions are both present, so we need to compare their strengths. NH₄⁺ (conjugate acid of NH₃, a weak base) and C₂H₃O₂⁻ (conjugate base of acetic acid, a weak acid). Since their strengths are comparable, the solution should be relatively neutral. c. CH₃NH₃Ol: Since CH₃NH₃⁺ is acidic and Ol⁻ is neutral, the solution is acidic. d. C₆H₅NH₃ClO₂: C₆H₅NH₃⁺ is acidic, and ClO₂⁻ is neutral, the solution is acidic. e. NH₄F: Both NH₄⁺ and F⁻ are present, so we need to compare their strengths. NH₄⁺ is acidic, and F⁻ is the conjugate base of HF, a weak acid, so the basic ion is stronger, and the solution is basic. f. CH₃NH₃CN: Both CH₃NH₃⁺ and CN⁻ are present; we need to compare their strengths. Since CN⁻ is the conjugate base of HCN, a weak acid, and CH₃NH₃⁺ is the conjugate acid of CH₃NH₂, a weak base, the solution should be relatively neutral.

Write balanced equations for the acidic or basic reactions

c. CH₃NH₃Ol → CH₃NH₂ + H₃Ol⁺ e. NH₄F → NH₃ + HF In summary, we have the following solutions: a. Sr(NO₃)₂: neutral b. NH₄C₂H₃O₂: relatively neutral c. CH₃NH₃Ol: acidic d. C₆H₅NH₃ClO₂: acidic e. NH₄F: basic f. CH₃NH₃CN: relatively neutral

Key concepts

Salts

Salts are ionic compounds made up of a cation (positive ion) and an anion (negative ion). They are commonly formed from the neutralization reaction between an acid and a base. When dissolved in water, salts can affect the pH of the solution depending on the nature of the ions they release.

In a neutralization reaction, the hydrogen ions (\(H^+\)) from the acid and the hydroxide ions (\(OH^-\)) from the base combine to form water. The remaining ions form the salt. For instance, NaCl is formed when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH). However, not all salts are neutral. Some salts can make solutions acidic or basic due to the presence of conjugate acids or bases that alter the pH.

Understanding the behavior of salts in water involves recognizing the ions they dissociate into and determining whether these ions undergo further reactions with water that can affect the pH. This reaction is key to identifying whether a solution of salt will be acidic, basic, or neutral.

• If a salt formed from a strong acid and a strong base, the resulting solution is neutral.
• If a salt comes from a strong base and a weak acid, the solution tends to be basic.
• If it originates from a weak base and a strong acid, the solution is often acidic.

Conjugate Acids and Bases

In the context of acid-base equilibria, conjugate acids and bases are fundamental to understanding how solutions can be acidic or basic. When an acid donates a proton (\(H^+\)), it forms its conjugate base. Conversely, when a base accepts a proton, it forms its conjugate acid.

For example, consider the dissociation of acetic acid (\(CH_3COOH\)): when it donates a proton, it forms acetate (\(CH_3COO^-\)), which is its conjugate base. Similarly, ammonia (\(NH_3\)), when accepting a proton, becomes ammonium (\(NH_4^+\)), its conjugate acid.

Why is the concept of conjugate pairs important? It's because they help determine the pH of salt solutions.

• If a conjugate base of a weak acid is present in a salt, it can react with water to form hydroxide ions (\(OH^-\)), making the solution basic.
• On the other hand, the conjugate acid of a weak base can react with water to form hydronium ions (\(H_3O^+\)), making the solution acidic.

Understanding which ion acts as a conjugate acid or base allows us to predict the behavior of the solution once the salt dissolves in water.

pH of Solutions

The pH of a solution is a measure of its acidity or basicity. It is calculated based on the concentration of hydrogen ions (\([H^+]\)) in the solution. The pH scale ranges from 0 to 14 with 7 being neutral. Values below 7 indicate an acidic solution, and values above 7 indicate a basic solution.

When a salt dissolves in water, the ions that compose the salt may interact with water to release or absorb hydrogen ions, thus altering the pH.

• A solution is acidic if it increases the \([H^+]\) concentration, lowering the pH.
• A solution is basic if it decreases the \([H^+]\) concentration, raising the pH.

For example, ammonium fluoride (NH₄F) dissolves to provide NH₄⁺ and F⁻ ions. NH₄⁺ can release an \(H^+\) ion, while F⁻ tends to keep the solution basic due to its origin from hydrofluoric acid (HF), a weak acid.

Calculating pH often involves the dissociation constants of the weak acid or base from which the conjugate emerges. Using the \(K_a\) (acid dissociation constant) and \(K_b\) (base dissociation constant) values helps in estimating whether the added ions will push the solution towards acidity or basicity. By comparing these values, you can predict whether the salt solution will behave more like an acid or a base.

The key takeaway? Knowing the nature of the ions in a salt and their dissociation constants helps us estimate the resulting pH once the salt is in a solution.