Problem 48 - Chemistry Matter and Change

Question

Challenge Write the equation for the reaction that occurs in a titration of ammonium hydroxide \(\left(\mathrm{NH}_{4} \mathrm{OH}\right]\) with hydrogen bromide \((\mathrm{HBr}) .\) Will the \(\mathrm{pH}\) at the equivalence point be greater or less than 7 ?

Step-by-Step Solution

Answer

The pH at the equivalence point will be less than 7.

Step 1: Identify Reactants and Products

The reaction involves titrating ammonium hydroxide \( (\mathrm{NH}_4\mathrm{OH}) \) with hydrogen bromide \((\mathrm{HBr})\). NH\(_{4}\)OH is a weak base, and HBr is a strong acid. The products of this acid-base reaction are ammonium bromide \((\mathrm{NH}_4\mathrm{Br})\) and water. Write the chemical equation: \[ \mathrm{NH}_4\mathrm{OH} + \mathrm{HBr} \rightarrow \mathrm{NH}_4\mathrm{Br} + \mathrm{H_2O} \]

Step 2: Determine the Nature of the Salt

In the reaction \(\mathrm{NH}_4\mathrm{OH} + \mathrm{HBr} \rightarrow \mathrm{NH}_4\mathrm{Br} + \mathrm{H_2O}\), ammonium bromide \((\mathrm{NH}_4\mathrm{Br})\) is formed. \(\mathrm{NH}_4\mathrm{Br}\) dissociates into \(\mathrm{NH}_4^+\) and \(\mathrm{Br}^-\) ions in solution. \(\mathrm{NH}_4^+\) is the conjugate acid of the weak base \(\mathrm{NH}_4\mathrm{OH}\), which can donate a proton (H+) to water, thereby making the solution slightly acidic.

Step 3: Evaluate the pH at the Equivalence Point

At the equivalence point, the acid and base have neutralized each other. The presence of \(\mathrm{NH}_4^+\), which works as a weak acid, will lower the pH. Since \(\mathrm{NH}_4^+\) releases H+ into the solution, the pH will be less than 7. Therefore, the pH at the equivalence point will be acidic due to the formation of \(\mathrm{NH}_4^+\).

Key Concepts

Equivalence Point

In the context of an acid-base titration, the equivalence point is a crucial concept. It represents the stage in the titration where the amount of acid equals the amount of base. At this point, the acid and base have completely reacted with each other. It is important to understand that although this might suggest a neutral solution with a pH of 7, that's not always the case.

When titrating a weak base, like ammonium hydroxide, with a strong acid, such as hydrogen bromide, the resulting solution at the equivalence point will not be neutral. Instead, it can be slightly acidic due to the nature of the products formed. Therefore, it's essential to evaluate the products of the reaction to understand the pH behavior at the equivalence point.

Weak Base

A weak base is a substance that does not completely dissociate in water. Unlike strong bases that completely ionize, weak bases only do so partially. Ammonium hydroxide (\(\mathrm{NH}_4\mathrm{OH}\)) is a classic example of a weak base. In solution, it only partially disassociates into hydroxide ions and ammonium ions.

This incomplete dissociation leads to the establishment of an equilibrium between the reactants and the products. As a result, when \(\mathrm{NH}_4\mathrm{OH}\) reacts in a titration process, the solution's pH is influenced by the products formed. Understanding the properties of weak bases can help predict the behavior of the solution after the acid-base reaction is completed.

Ammonium Bromide

Ammonium bromide \((\mathrm{NH}_4\mathrm{Br})\) is a compound formed in the reaction between ammonium hydroxide and hydrogen bromide. In an acid-base titration, the formation of the salt ammonium bromide introduces unique properties to the resulting solution.

Once dissolved, \(\mathrm{NH}_4\mathrm{Br}\) dissociates into ammonium ions \(\mathrm{NH}_4^+\) and bromide ions \(\mathrm{Br}^-\). The ammonium ion acts as a weak acid in water, having the ability to donate a proton \((\mathrm{H}^+)\) to the water, slightly increasing the solution's acidity. This behavior can have a significant impact on determining the pH value at the equivalence point of the titration.

Chemical Equation

Understanding chemical equations is vital in connecting reactants to products in a chemical reaction. In the acid-base titration of ammonium hydroxide with hydrogen bromide, the chemical equation is as follows:

• \(\mathrm{NH}_4\mathrm{OH} + \mathrm{HBr} \rightarrow \mathrm{NH}_4\mathrm{Br} + \mathrm{H_2O}\)

This equation matches the stoichiometry of the reaction, showing the conversion of reactants into the salt ammonium bromide and water.

The role of a balanced chemical equation is to ensure that mass and charge are conserved, illustrating the principle of conservation in chemical reactions. The interpretation of this equation also facilitates the prediction of changes in the solution, such as pH shifts during the titration process.