Problem 35 - Chemistry: The Central Science

Question

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

Step-by-Step Solution

Answer

(a) The equivalence point for the titration of \(\mathrm{NaHCO}_{3}\) (a weak acid) with \(\mathrm{NaOH}\) (a strong base) will be above pH 7. (b) The equivalence point for the titration of \(\mathrm{NH}_{3}\) (a weak base) with \(\mathrm{HCl}\) (a strong acid) will be below pH 7. (c) The equivalence point for the titration of \(\mathrm{KOH}\) (a strong base) with \(\mathrm{HBr}\) (a strong acid) will be at pH 7.

Step 1: (a) NaHCO3 titrated with NaOH

\(\mathrm{NaHCO}_{3}\) is a weak acid, and \(\mathrm{NaOH}\) is a strong base. When a weak acid is titrated with a strong base, the equivalence point pH will be greater than 7. So, the equivalence point for this titration will be above pH 7.

Step 2: (b) NH3 titrated with HCl

In this case, \(\mathrm{NH}_{3}\) is a weak base, and \(\mathrm{HCl}\) is a strong acid. When a weak base is titrated with a strong acid, the equivalence point pH will be less than 7. Thus, the equivalence point for this titration will be below pH 7.

Step 3: (c) KOH titrated with HBr

Here, \(\mathrm{KOH}\) is a strong base, and \(\mathrm{HBr}\) is a strong acid. When a strong base is titrated with a strong acid, the equivalence point pH will be 7. Therefore, the equivalence point for this titration will be at pH 7.

Key Concepts

Acid-Base Titration

Acid-base titration is a technique used to determine the concentration of an acid or base in a solution. It involves gradually adding a solution of known concentration, called the titrant, to a solution of unknown concentration, until the chemical reaction reaches the equivalence point.
This is where the amount of titrant added is exactly enough to completely react with the unknown solution. During a titration, an indicator is often used to signal when the equivalence point has been reached. Indicators change color at a certain range of pH levels.
However, the selection of an appropriate indicator depends on the expected pH at the equivalence point.

• For a titration involving strong acids and strong bases, the equivalence point occurs at pH 7.
• In titrations of strong acids with weak bases, the equivalence point is typically below pH 7.
• Conversely, when a weak acid is titrated with a strong base, the equivalence point is above pH 7.

Weak Acids and Bases

Weak acids and bases do not completely dissociate in solution. This means only a small fraction of their molecules ionize to produce hydrogen ions (H⁺) or hydroxide ions (OH^-), impacting their behavior during titration.
For instance, a weak acid reacting with a strong base will result in an equivalence point that is basic due to the formation of its conjugate base, which can further react with water to produce OH^- ions.The behavior of weak acids and bases during titration requires understanding their dissociation constants (Ka for acids and Kb for bases).
This allows us to predict the pH changes as the titrant is added.

• When titrating a weak base like ammonia (\(\text{NH}_3\), a relatively small amount of its molecules react with the acid, resulting in an equivalence point with a low pH.
• For a weak acid such as \(\text{NaHCO}_3\), the reaction with a strong base results in an equivalence point higher than pH 7.

Strong Acids and Bases

Strong acids and bases completely dissociate in water, meaning they release a high concentration of ions. This characteristic makes their behavior predictable when they undergo titration.
For example, when a strong acid titrates a strong base, or vice versa, the resulting solution at the equivalence point is neutral, having a pH of 7.

• Strong acids, such as hydrochloric acid \( ext{(HCl)}\), can completely dissociate, reacting efficiently with bases.
• Strong bases, like potassium hydroxide \( ext{(KOH)}\), also fully break down into ions, allowing full interaction with acids.

Strong acids and bases are perfect for precise titrations, usually resulting in a sharp and distinct equivalence point.While using these substances, safety precautions are necessary due to their highly reactive nature.