Problem 28 - General Chemistry

Question

A chemist prepares dilute solutions of equal molar concentrations of \(\mathrm{NH}_{3}, \mathrm{NH}_{4} \mathrm{Br}, \mathrm{NaF},\) and \(\mathrm{NaCl} .\) Rank these solutions from highest \(\mathrm{pH}\) to lowest \(\mathrm{pH}\).

Step-by-Step Solution

Answer

NaF > NH3 > NaCl > NH4Br.

Step 1: Identifying the Nature of Compounds

Identify the nature of each compound based on their chemical properties. \(\mathrm{NH}_{3}\) is a weak base, \(\mathrm{NH}_{4} \mathrm{Br}\) is a salt that forms from a weak base (\(\mathrm{NH}_{3}\)) and a strong acid (\(\mathrm{HBr}\)), and it acts as an acidic solution. \(\mathrm{NaF}\) is a salt that forms from a strong base (\(\mathrm{NaOH}\)) and a weak acid (\(\mathrm{HF}\)), making it a basic solution. \(\mathrm{NaCl}\) forms from a strong base (\(\mathrm{NaOH}\)) and a strong acid (\(\mathrm{HCl}\)), and it is neutral.

Step 2: Determining the Effect on pH

Determine each compound’s effect on pH. \(\mathrm{NH}_{3}\), being a weak base, increases pH slightly (more basic). \(\mathrm{NH}_{4} \mathrm{Br}\) hydrolyzes to produce \(\mathrm{H}^{+}\) ions, lowering the pH (acidic effect). \(\mathrm{NaF}\) hydrolyzes to produce \(\mathrm{OH}^{-}\) ions, increasing the pH (basic effect). \(\mathrm{NaCl}\) remains neutral and does not affect the pH substantially.

Step 3: Ranking Based on pH Influence

Rank the solutions according to their pH influence: \(\mathrm{NaF}\) (most basic), \(\mathrm{NH}_{3}\) (weakly basic), \(\mathrm{NaCl}\) (neutral), \(\mathrm{NH}_{4} \mathrm{Br}\) (acidic). The order from highest pH to lowest is: NaF > NH3 > NaCl > NH4Br.

Key Concepts

Acid-Base Properties

The acid-base properties of a solution are central to determining its pH level. When we talk about acids and bases, we are referring to substances that either donate or accept hydrogen ions (\(\mathrm{H}^+\)). Acids are hydrogen ion donors and result in the solution becoming more acidic, whereas bases accept hydrogen ions, thereby increasing the solution's basicity.

• Weak Bases: These do not completely ionize in solution and only partially increase pH levels.
• Neutral Compounds: Substances like \(\mathrm{NaCl}\) have no significant effect on pH.

These properties help us determine the pH rank of different solutions by understanding if the compound in question will donate \(\mathrm{H}^+\) ions and become acidic, or accept \(\mathrm{H}^+\) ions and become basic.

Hydrolysis

Hydrolysis is a chemical reaction involving water that breaks bonds within a compound. In the context of salts and their effects on pH, hydrolysis occurs when a salt dissolves and its ions interact with water, potentially forming \(\mathrm{H}^+\) or \(\mathrm{OH}^-\) ions.

• Acid Hydrolysis: Salts such as \(\mathrm{NH}_{4} \mathrm{Br}\) can undergo hydrolysis to release \(\mathrm{H}^+\) ions, making the solution more acidic.
• Base Hydrolysis:\(\mathrm{NaF}\) undergoes hydrolysis to form \(\mathrm{OH}^-\) ions, which increases the basicity and pH of the solution.

Understanding hydrolysis is essential for predicting how the pH of a solution will change upon dissolving salts derived from weak acids and bases.

Weak Bases

Weak bases like \(\mathrm{NH}_3\) do not fully dissociate in water, which means they do not release a large quantity of \(\mathrm{OH}^-\) ions. As a result, they only slightly increase the pH of the solution compared to strong bases.

• Characteristics of Weak Bases: - Partially ionize in water - Only increase pH slightly as \(\mathrm{NH}_3\) does
• Base Strength: Their strength is determined by their dissociation constant (\(K_b\)), indicating how well a base can pull H+ ions out of water, forming \(\mathrm{OH}^-\) ions.

In our exercise, \(\mathrm{NH}_3\) serves as a weak base, slightly raising the solution's pH because of its limited dissociation.

Salts from Weak Acids and Bases

Salts formed from weak acids and bases can significantly influence the pH of their aqueous solutions. When these salts dissolve, they often do not behave neutrally like salts from strong acids and bases.

• Salt of Weak Acid and Strong Base: Salts like \(\mathrm{NaF}\) arise from a strong base (\(\mathrm{NaOH}\)) and a weak acid (\(\mathrm{HF}\)). These salts generally form basic solutions.
• Salt of Weak Base and Strong Acid:\(\mathrm{NH}_{4} \mathrm{Br}\) is an example of this type, forming an acidic solution as it hydrolyzes to produce \(\mathrm{H}^+\) ions.

By examining these interactions, we can see why solutions like \(\mathrm{NaF}\) and \(\mathrm{NH}_{4} \mathrm{Br}\) would have differing effects on pH based on their constituent properties.