Question

Acid-Base Properties of Ions and Salts Arrange the solutions in order of increasing basicity. $$\mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{Br}, \mathrm{KOH}, \mathrm{KBr}, \mathrm{KCN}, \mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NHNO}_{2}$$

Short answer

Increasing basicity: \(\mathrm{CH}_{3}\mathrm{NH}_{3}\mathrm{Br}\) < \(\mathrm{C}_{5}\mathrm{H}_{5}\mathrm{NHNO}_{2}\) < \(\mathrm{KBr}\) < \(\mathrm{KCN}\) < \(\mathrm{KOH}\).

Step-by-step solution

Identify Acids, Bases, and Salts

Recognize each substance as an acid, base, or salt. Acids donate protons, bases accept protons, and salts are ionic compounds that can dissociate into cations and anions.

Determine the Strength of Acids and Bases

Identify strong and weak acids and bases. Strong acids and bases dissociate completely in water, while weak ones do not.

Assessing the Effects of Ions on pH

Understand that cations derived from weak bases and anions derived from weak acids can affect the pH of a solution by respectively acting as weak acids or weak bases.

Rank the Solutions by Basicity

Arrange the substances in order of increasing basicity. Strong bases are more basic than weak bases, and salts resulted from strong bases and weak acids are more basic than salts resulting from weak bases and strong acids.

Arrange the Given Solutions

Rank from least basic to most basic: 1. \(\mathrm{CH}_{3}\mathrm{NH}_{3} \mathrm{Br}\) - a salt from a weak base and a strong acid, mildly acidic. 2. \(\mathrm{C}_{5}\mathrm{H}_{5}\mathrm{NHNO}_{2}\) - a salt from a weak base and a weak acid, neutral to slightly basic. 3. \(\mathrm{KBr}\) - a neutral salt, as both K+ and Br- are from strong acid and strong base. 4. \(\mathrm{KCN}\) - a salt from a weak acid and a strong base, basic. 5. \(\mathrm{KOH}\) - a strong base, most basic.

Key concepts

Acid-Base Reactions

Understanding acid-base reactions is essential when studying chemistry, as it deals with the transfer of protons between reactants. A proton donor is an acid, and a proton acceptor is a base. When they react, they can form water and a salt in a neutralization reaction. For example, in the given exercise, when KOH (a base) reacts with an acid like HBr, it can produce KBr (a salt) and H₂O (water).

Acid-base reactions also involve conjugate acid-base pairs, where acids lose a proton becoming conjugate bases, and bases gain a proton becoming conjugate acids. This concept is pivotal in predicting the outcomes of reactions and comprehending the acidity or basicity of the resultant solution.

pH Scale

The pH scale measures the acidity or basicity of a solution and ranges from 0 to 14. A pH of 7 is considered neutral, which is the pH of pure water. Solutions with a pH less than 7 are acidic, and those with a pH greater than 7 are basic (or alkaline). In our exercise, the presence of KOH, a strong base, implies a solution with a high pH, generally above 11, indicating strong basicity. On the other hand, the pH resulting from CH₃NH₃Br would be less than 7, since it is mildly acidic due to the proton-releasing tendency of the CH₃NH₃⁺ ion.

Strength of Acids and Bases

The strength of acids and bases refers to their ability to dissociate in water. Strong acids and bases dissociate completely, yielding a high concentration of H⁺ (hydrogen ions) or OH^- (hydroxide ions) in solution. In contrast, weak acids and bases only partially dissociate, leading to an equilibrium state with undissociated molecules and ions.

In an educational context, it's critical for students to grasp that a strong base like KOH will result in a solution with a high pH, indicating strong basicity, while a compound like CH₃NH₃Br, which is derived from a weak base (CH₃NH₂) and a strong acid (HBr), will exhibit weak acidic properties, because the weak base cannot effectively neutralize the strong acid.

Salts and Basicity

Salts from Strong Bases and Weak Acids

Solutions of salts formed from strong bases and weak acids, like KCN, tend to be basic. This is because the conjugate base (CN^-) from the weak acid (HCN) has a stronger tendency to accept protons, generating OH^- ions that increase the pH of the solution.

Salts from Weak Bases and Strong Acids

On the other hand, salts like CH₃NH₃Br, which come from weak bases and strong acids, will often result in a slightly acidic solution since the conjugate acid (CH₃NH₃⁺) from the weak base tends to donate protons, slightly lowering the pH. By understanding how the origin of a salt influences its effect on pH, students can effectively predict the basicity or acidity of a solution. The concept of ionic strength and the dissociation constants (K_a and K_b) for the acids and bases that form the salts also play a significant role in determining their pH behavior in aqueous solutions.