Problem 38 - Chemistry The Central Science In Si Units

Question

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

Step-by-Step Solution

Answer

(a) above pH 7, (b) below pH 7, (c) below pH 7.

Step 1: Analyze the Titration of Benzoic Acid with KOH

Benzoic acid is a weak acid and KOH is a strong base. The equivalence point for a weak acid-strong base titration is above pH 7 due to the formation of the conjugate base, which hydrolyzes to yield some OH\(^-\) ions.

Step 2: Analyze the Titration of Ammonia with Iodic Acid

Ammonia is a weak base while iodic acid is a strong acid. In a weak base-strong acid titration, the equivalence point is below pH 7. This happens because the conjugate acid formed in the solution releases H\(^+\) ions, contributing to increased acidity.

Step 3: Analyze the Titration of Hydroxylamine with Hydrochloric Acid

Hydroxylamine is a weak base and hydrochloric acid is a strong acid. Similar to the titration of ammonia with a strong acid, the equivalence point will be below pH 7 as the conjugate acid of hydroxylamine will release H\(^+\) ions in the solution.

Key Concepts

Equivalence Point

When you perform a titration, finding the equivalence point is crucial. This is the stage where the amount of acid equals the amount of base, resulting in a complete neutralization. However, don't always expect a pH of 7 at this point.
In titrations involving weak acids with strong bases, or weak bases with strong acids, the equivalence point is influenced by the strength of the acid and base involved.

• For weak acid-strong base titrations, the equivalence point tends to be above pH 7.
• For weak base-strong acid titrations, it usually falls below pH 7.

Understanding the equivalence point helps you predict the pH behavior of the solution as it transitions from acidic to basic, or vice versa.

pH Level

The pH level is a measure of acidity or basicity of a solution. It ranges from 0 (very acidic) to 14 (very alkaline or basic), with 7 being neutral like pure water.
During titration, the pH level will vary as you add an acid or a base to the solution.

• In a titration involving a weak acid and a strong base, the pH rises drastically after the equivalence point. More pH-sensitive indicators (or pH meters) are often used to closely track the changes.
• In a weak base-strong acid titration, the pH drops significantly past the equivalence point.

Monitoring the pH level during titration is vital to determine the precise moment of neutralization and to analyze the chemical interactions occurring within the solution.

Weak Acid-Strong Base Titration

In a weak acid-strong base titration, like benzoic acid with KOH, the base neutralizes the acid, which leads to the formation of water and a salt. However, what's interesting here is the salt formed tends to create a basic solution.
That's why the equivalence point ends up with a pH greater than 7.

• The weak acid partially dissociates in water.
• As more strong base is added, the solution becomes more basic.

This is what causes the characteristic pH jump after the equivalence point, making the endpoint alkaline rather than neutral.

Weak Base-Strong Acid Titration

In a weak base-strong acid titration, such as when ammonia is mixed with iodic acid, something different happens. Here, the strong acid dominates.
The significance of this interaction is the formation of a conjugate acid, making the resulting solution more acidic.

• The weak base does not sufficiently counteract the added strong acid.
• Once at the equivalence point, the pH is usually below 7.

Understanding this balance helps predict the resulting pH and tailor the choice of indicators or analytical methods accordingly.