Problem 38 - Chemistry: The Central Science in SI Units, Global Edition

Question

Predict whether the equivalence point of each of the following titrations is below, above, or at pH 7: (a) benzoic acid titrated with KOH, (b) ammonia titrated with iodic acid, (c) hydroxylamine with hydrochloric acid.

Step-by-Step Solution

Answer

In summary, for the given titrations: (a) Benzoic acid (a weak acid) with KOH (a strong base) results in an equivalence point above pH 7. (b) Ammonia (a weak base) with iodic acid (a strong acid) results in an equivalence point below pH 7. (c) Hydroxylamine (a weak base) with hydrochloric acid (a strong acid) results in an equivalence point below pH 7.

Step 1: Titration (a) Benzoic acid with KOH

In this case, we have a weak acid (benzoic acid) titrated with a strong base (KOH). According to case 2 mentioned in the analysis, the equivalence point will be above pH 7.

Step 2: Titration (b) Ammonia with iodic acid

Here, we have a weak base (ammonia) titrated with a strong acid (iodic acid). This corresponds to case 3 in the analysis, meaning the equivalence point will be below pH 7.

Step 3: Titration (c) Hydroxylamine with hydrochloric acid

In this titration, we have a weak base (hydroxylamine) being titrated with a strong acid (hydrochloric acid). This is again in line with case 3 mentioned in the analysis, so the equivalence point will be below pH 7. In conclusion: - The equivalence point of titration (a) is above pH 7. - The equivalence point of titration (b) is below pH 7. - The equivalence point of titration (c) is below pH 7.

Key Concepts

Titration

Titration is a laboratory technique utilized to determine the unknown concentration of a solution. It involves gradually adding a titrant, a solution of known concentration, to a known volume of a sample until the chemical reaction reaches its endpoint.
This carefully controlled addition helps identify the point where the reactants have reacted in stoichiometrically equivalent amounts, known as the equivalence point.

There are several indicators used to detect the endpoint, often involving a color change.

• Equivalence Point: This is the stage in the titration where the amount of titrant added is just enough to completely react with the analyte.
• Endpoint: The visible sign (such as a color change) signifying the point where the reaction appears complete.

Understanding these concepts is vital for accurately identifying the completion of titration, especially when predicting the pH at the equivalence point.
The nature of the acid and base involved in titration directly influences the pH at the equivalence point, which can be acidic, basic, or neutral.

Weak Acid-Strong Base

When a weak acid is titrated with a strong base, the solution at the equivalence point tends to be basic. This is because the strong base completely reacts with the weak acid forming its conjugate base, which can hydrolyze with water.
Let's dive deeper into this concept using benzoic acid and potassium hydroxide (KOH) as an example:

• Benzoic Acid (C₆H₅COOH): A weak acid that doesn't fully dissociate in water.
• KOH: A strong base that completely dissociates in solution.

During titration, KOH will neutralize the benzoic acid. At the equivalence point, excess benzoate ions (conjugate base) can react with H₂O to produce OH^-, resulting in a slightly basic solution.
This is why, for titrations involving a weak acid and strong base, the equivalence point pH is typically above 7.

Weak Base-Strong Acid

In the case of a weak base titrated with a strong acid, the situation is somewhat reversed. The equivalence point will be acidic due to the nature of the products in the reaction.
Consider ammonia and iodic acid as an illustrative example:

• Ammonia (NH₃): A weak base which accepts protons weakly.
• Iodic Acid (HIO₃): A strong acid that fully donates protons.

As the titration proceeds, the ammonia is protonated by the iodic acid, forming ammonium ions (NH₄⁺). At the equivalence point, the ammonium (the conjugate acid) further reacts with water to produce H₃O⁺, lowering the pH.
Thus, in weak base-strong acid titrations like with ammonia and iodic acid or hydroxylamine and hydrochloric acid, the equivalence point is usually below pH 7.