Problem 37 - Chemistry The Central Science

Question

Predict whether the equivalence point of each of the following titrations is below, above, or at \(\mathrm{pH}\) 7: (a) \(\mathrm{NaHCO}_{3}\) titrated with \(\mathrm{NaOH},\) (b) \(\mathrm{NH}_{3}\) titrated with \(\mathrm{HCl}\) (c) KOH titrated with HBr.

Step-by-Step Solution

Answer

For the titrations given: (a) NaHCO3 with NaOH will have an equivalence point with a pH above 7 due to a weak acid being titrated with a strong base. (b) NH3 with HCl will have an equivalence point with a pH below 7, as a weak base is being titrated with a strong acid. (c) KOH titrated with HBr will have an equivalence point with a pH at exactly 7, as both are strong species that balance each other out.

Step 1: Titration (a) NaHCO3 with NaOH

We have a bicarbonate (\(\mathrm{NaHCO}_{3}\)) being titrated with a strong base (\(\mathrm{NaOH}\)). Since bicarbonates are weak acids and \(\mathrm{NaOH}\) is a strong base, the equivalence point will occur when the pH is above 7 because the weak acid will be neutralized by the strong base, producing a basic solution at the equivalence point.

Step 2: Titration (b) NH3 with HCl

We have a weak base (\(\mathrm{NH}_{3}\)) being titrated with a strong acid (\(\mathrm{HCl}\)). When the reaction is complete, the equivalence point will be reached. In this case, since we have a strong acid reacting with a weak base, the equivalence point will occur when the pH is below 7 because the strong acid will cause the solution to become acidic.

Step 3: Titration (c) KOH with HBr

In this case, we have a strong base (\(\mathrm{KOH}\)) being titrated with a strong acid (\(\mathrm{HBr}\)). When the reaction is complete and the equivalence point is reached, these two strong species will balance each other, resulting in a neutral solution. Therefore, the pH at the equivalence point will be exactly 7.

Key Concepts

Equivalence Point

The equivalence point of a titration is the moment when the amount of titrant added is just enough to completely neutralize the analyte solution. This state is critical in titrations as it indicates that the ratio of reactants has reached a specific point based on their stoichiometric coefficients.

• In a strong acid-strong base titration, like (c) KOH with HBr, the equivalence point occurs at pH 7 because the strong acid and the strong base perfectly neutralize each other.
• In a weak acid-strong base titration, such as (a) NaHCO3 with NaOH, the equivalence point will be above pH 7. This happens because the conjugate base of the weak acid increases the pH.
• Conversely, in a weak base-strong acid titration, for instance (b) NH3 with HCl, the pH at the equivalence point will be below 7 because the conjugate acid of the weak base lowers the pH.

Knowing whether a solution is strong or weak helps predict your equivalence point’s pH, shedding light on the acidity or basicity of your final solution.

Acid-Base Reactions

Acid-base reactions in titrations are the backbone of determining the equivalence point. These reactions involve the transfer of protons (H+) from acids to bases.

• A strong acid, such as HBr, completely dissociates in solution, contributing many protons and leading to a significant pH change upon the addition of a base.
• Strong bases like KOH also dissociate completely, providing hydroxide ions (OH-) that counteract the protons from acids, helping to neutralize the solution.
• Weak acids and bases, like NaHCO3 and NH3 respectively, only partially dissociate. This partial dissociation means that strong acids or bases will alter the pH more dramatically near the equivalence point.

Titrations leverage these reactions to determine the precise point where the number of moles of acid equals the number of moles of base, leading us to the sought-after equivalence point.

pH

pH is a measurement of how acidic or basic a solution is, determined by the concentration of hydrogen ions (H+) in the solution. Titrations keenly observe the pH changes as a titrant is added.
Let's break down its importance:

• A pH of 7 is neutral, as seen during the titration of strong acids and strong bases, such as KOH with HBr.
• Values below 7 indicate acidity, often resulting from the titration of weak bases with strong acids, like NH3 with HCl, where extra H+ ions dominate post-reaction.
• Values above 7 indicate basicity, observed when weak acids react with strong bases, such as NaHCO3 with NaOH.

By carefully plotting the pH against the volume of titrant added, a titration curve can be derived. The steepness of this curve highlights the pH changes and pinpoints the equivalence point efficiently.