Problem 26 - General Chemistry

Question

You have the following solutions, all of the same molar concentration: \(\mathrm{KBr}, \mathrm{HBr}, \mathrm{CH}_{3} \mathrm{NH}_{2},\) and \(\mathrm{NH}_{4} \mathrm{Cl}\). Rank them from the lowest to the highest hydroxide-ion concentration.

Step-by-Step Solution

Answer

HBr < NH₄Cl < KBr < CH₃NH₂

Step 1: Identify Compound Types

Determine whether each compound is an acid, a base, or a neutral salt. - **KBr**: This is formed by KOH (strong base) and HBr (strong acid); it's a neutral salt. It does not affect hydroxide-ion concentration. - **HBr**: This is a strong acid, which will dissociate completely and produce hydrogen ions in solution. - **CH₃NH₂**: This is a weak base, which can partially dissociate into hydroxide ions. - **NH₄Cl**: This is an acidic salt formed by NH₄OH (weak base) and HCl (strong acid). It will increase hydrogen ions in solution.

Step 2: Effect on Hydroxide Ion Concentration

Evaluate how each compound affects hydroxide ion (OH⁻) concentration. - **KBr**: No change in OH⁻ concentration because it's a neutral salt. - **HBr**: Decreases OH⁻ concentration due to the generation of H⁺ ions. - **CH₃NH₂**: Increases OH⁻ concentration slightly because it is a weak base. - **NH₄Cl**: Reduces OH⁻ concentration as it increases H⁺ ions, similar to adding a weak acid to the solution.

Step 3: Rank the Concentrations

Rank the solutions based on increasing OH⁻ ion concentration. - **Lowest OH⁻ concentration**: **HBr**, as it is a strong acid. - **NH₄Cl**: as it slightly lowers OH⁻ due to acidic properties. - **KBr**: neutral, no effect on OH⁻ concentration. - **Highest OH⁻ concentration**: **CH₃NH₂**, as it acts as a weak base, adding OH⁻.

Key Concepts

Strong Acids

In chemistry, strong acids are substances that completely dissociate into their ions in a solution. This means they fully release hydrogen ions ( H^+ ) when dissolved in water. Hydrochloric acid ( HCl ) and hydrobromic acid ( HBr ) are common examples. Because they completely dissociate, strong acids significantly increase the H^+ concentration of the solution, which in turn decreases the hydroxide ion ( OH^- ) concentration due to the reaction between H^+ and OH^- to form water. For instance, when HBr is added to water, it quickly breaks apart into H^+ and Br^- ions, effectively lowering the OH^- concentration.

Weak Bases

Weak bases are substances that partially dissociate in solution, producing hydroxide ions ( OH^- ) but not to the full extent as strong bases do. An example of a weak base is methylamine ( CH_3NH_2 ). When methylamine is added to water, it reacts to form a small number of OH^- ions. Unlike strong bases, which fully ionize, weak bases only partially dissociate, meaning only a fraction of the base molecules will produce OH^- ions. This partial dissociation results in a lower increase in OH^- concentration compared to what would be expected with a strong base of the same molar concentration. Therefore, although weak bases do increase the OH^- concentration, the effect is modest compared to strong bases. Hence, in the context of ranking hydroxide ion concentrations, solutions like CH_3NH_2 would have a higher OH^- concentration than neutral salts or acidic solutions, but a lower one compared to strong bases.

Neutral Salts

Neutral salts comprise compounds formed by the neutralization of a strong acid and a strong base. A typical example is potassium bromide ( KBr ), which originates from the reaction between potassium hydroxide ( KOH ), a strong base, and hydrobromic acid ( HBr ), a strong acid. When dissolved, neutral salts separate into their respective ions but do not affect the concentration of hydrogen ions ( H^+ ) or hydroxide ions ( OH^- ) in the solution. This is because the ions do not undergo further reactions that would alter H^+ or OH^- levels, hence maintaining neutrality. Therefore, a solution of a neutral salt like KBr will not change the OH^- concentration, which is vital to understand when analyzing acid-base reactions and the neutrality of solutions.

Hydroxide Ion Concentration

Hydroxide ion concentration ( OH^- ) is a crucial factor in determining the acidity or basicity of a solution. OH^- is the ion associated with bases, and its concentration changes depending on the nature of the solute added to the water. For solutions with strong acids like HBr , the H^+ ions react with OH^- ions, decreasing the OH^- concentration due to the formation of water. In weak bases, such as CH_3NH_2 , partial dissociation leads to an increase in OH^- concentration, though limited. Neutral salts do not alter OH^- levels; solutions remain balanced unless an external addition changes their composition. Understanding these changes in OH^- concentration is essential for ranking solutions and predicting their chemical behavior. The lower the OH^- ion concentration, the more acidic the solution, while higher concentrations indicate basic conditions.