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 \(14.3 .\) a. \(\mathrm{KCl}\) c. \(\mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{Cl}\) e. \(\mathrm{NH}_{4} \mathrm{~F}\) b. \(\mathrm{NH}_{4} \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) d. \(\mathrm{KF}\) f. \(\mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{CN}\)

Short answer

The solutions of the given salts are as follows: a. KCl: Neutral c. CH₃NH₃Cl: Acidic (CH₃NH₃⁺(aq) + H₂O(l) ⇌ CH₃NH₂(aq) + H₃O⁺(aq)) e. NH₄F: Slightly basic (F⁻(aq) + H₂O(l) ⇌ HF(aq) + OH⁻(aq)) b. NH₄C₂H₃O₂: Neutral d. KF: Basic (F⁻(aq) + H₂O(l) ⇌ HF(aq) + OH⁻(aq)) f. CH₃NH₃CN: Slightly basic (CN⁻(aq) + H₂O(l) ⇌ HCN(aq) + OH⁻(aq))

Step-by-step solution

Identify the ions in the salts

We must first identify the cations and anions in each salt. These ions may act as acids or bases when dissolved in water. a. KCl: K⁺ (cation) and Cl⁻ (anion) c. CH₃NH₃Cl: CH₃NH₃⁺ (cation) and Cl⁻ (anion) e. NH₄F: NH₄⁺ (cation) and F⁻ (anion) b. NH₄C₂H₃O₂: NH₄⁺ (cation) and C₂H₃O₂⁻ (anion) d. KF: K⁺ (cation) and F⁻ (anion) f. CH₃NH₃CN: CH₃NH₃⁺ (cation) and CN⁻ (anion)

Determine the acidic or basic properties of the ions

Now we need to examine the acidic or basic properties of the ions by checking their K_a and K_b values. Keep in mind that neither K⁺ nor Cl⁻ react with water, meaning their corresponding K_a and K_b values are negligible. a. KCl: K⁺ (no reaction) and Cl⁻ (no reaction) - Neutral c. CH₃NH₃Cl: CH₃NH₃⁺ (K_a = 2.3 x 10⁻¹¹) and Cl⁻ (no reaction) e. NH₄F: NH₄⁺ (K_a = 5.6 x 10⁻¹⁰) and F⁻ (K_b = 1.4 x 10⁻¹¹) b. NH₄C₂H₃O₂: NH₄⁺ (K_a = 5.6 x 10⁻¹⁰) and C₂H₃O₂⁻ (K_b = 5.3 x 10⁻¹⁰) d. KF: K⁺ (no reaction) and F⁻ (K_b = 1.4 x 10⁻¹¹) f. CH₃NH₃CN: CH₃NH₃⁺ (K_a = 2.3 x 10⁻¹¹) and CN⁻ (K_b = 2.0 x 10⁻⁵)

Determine if the solution is acidic, basic, or neutral and write balanced equations where needed

a. KCl: Neutral - No reaction with water c. CH₃NH₃Cl: Acidic Reaction: CH₃NH₃⁺(aq) + H₂O(l) ⇌ CH₃NH₂(aq) + H₃O⁺(aq) e. NH₄F: Slightly basic due to the higher K_b value of F⁻ compared to the K_a value of NH₄⁺ Reaction: F⁻(aq) + H₂O(l) ⇌ HF(aq) + OH⁻(aq) b. NH₄C₂H₃O₂: Neutral - NH₄⁺ and C₂H₃O₂⁻ have similar acidity and basicity, so their effects cancel each other out. d. KF: Basic Reaction: F⁻(aq) + H₂O(l) ⇌ HF(aq) + OH⁻(aq) f. CH₃NH₃CN: Slightly basic due to the higher K_b value of CN⁻ compared to the K_a value of CH₃NH₃⁺ Reaction: CN⁻(aq) + H₂O(l) ⇌ HCN(aq) + OH⁻(aq)

Key concepts

pH of Salts

When we talk about salts in chemistry, we often refer to them as either acidic, basic, or neutral. These properties are determined by the ions that make up the salt. Once dissolved in water, salts can affect the pH of the solution.

A salt like KCl is neutral because both ions, K⁺ and Cl⁻, do not react significantly with water. They do not affect the pH because the acid or base derived from them are very weak or negligible.

However, in some cases, salts result in acidic or basic solutions. This is often due to one of the ions being a conjugate acid or base formed during the ionization process. For instance, CH₃NH₃Cl yields an acidic solution since CH₃NH₃⁺, its cation, is a conjugate acid of the weak base CH₃NH₂ and it raises the concentration of hydrogen ions ( H₃O⁺ ) when dissolved in water.

Understanding the resulting pH of a salt solution requires identifying the ions and knowing their reactions in water.

Equilibrium Constants

Equilibrium constants, denoted as Ka for acids and Kb for bases, help us understand the strength of acids and bases when they dissolve in water. The larger the value, the stronger the acid or base will be. This is critical when determining whether a solution will be acidic, basic, or neutral.

• For acidic solutions, where cation such as NH₄⁺ combines with water to produce H₃O⁺, we refer to Ka .
• For basic solutions, such as when anion F⁻ combines with water to produce OH⁻, we look at Kb values.

If both an acid and a base are formed from a salt in a balanced way, the solution may be neutral if the respective Ka and Kb values are similar. In a solution, the higher value indicates which ion is dominant and determines whether the solution shifts towards being more acidic or basic. For example, for NH₄F, it becomes basic as the Kb value of the fluoride ion (F⁻) outstrips the Ka of NH₄⁺.

Chemical Ionization

Chemical ionization is a process where molecules split into ions, typically when dissolved in water. This process is crucial for understanding how salts behave in solution.

• For salts like NH₄⁺, they may donate a proton to water, creating H₃O⁺ and making the solution acidic.
• Conversely, anions like CN⁻ can accept a proton from water, creating OH⁻ and rendering the solution basic.

The ability of an ion to produce one of these changes depends on its tendency to ionize or hydrolyze when interacting with water. By observing the reactions, you can predict whether a solution is acidic, basic, or neutral. This is determined by evaluating what ions are formed through chemical ionization. For instance, if CN⁻ is more predominant in a solution such as CH₃NH₃CN, it will lean towards basicity.

Balanced Chemical Equations

Chemical equations balance to represent the conservation of mass, ensuring that the number of atoms for each element remains constant on both sides of the equation. For acid-base reactions of salts, balanced chemical equations help demonstrate the changes occurring as ions interact with water.

Consider a basic solution like KF. When dissolved in water, the fluoride ion ( F⁻ ) reacts as follows: F⁻(aq) + H₂O(l) ⇌ HF(aq) + OH⁻(aq) This balanced equation shows the fluoride ion pulling a hydrogen from water, creating hydroxide ions ( OH⁻ ) and thus raising the pH.

Creating balanced chemical equations involves ensuring that all charge and mass are equal on both sides. This allows us to accurately predict the resulting pH of the solution based on the ions' interactions with water. Balanced equations are essential to fully understanding the behavior of salts in solution, particularly in predicting the resultant acidic or basic nature of the solution.