Problem 51 - Chemistry: An Atoms-Focused Approach

Question

For each titration, predict whether the pH of the equivalence point is less than, equal to, or greater than 7 a. Quinine titrated with nitric acid b. Pyruvic acid titrated with calcium hydroxide c. Hydrobromic acid titrated with strontium hydroxide

Step-by-Step Solution

Answer

Question: Predict whether the pH of the equivalence point of the following titrations will be less than 7, equal to 7, or greater than 7: a. Quinine titrated with nitric acid, b. Pyruvic acid titrated with calcium hydroxide, c. Hydrobromic acid titrated with strontium hydroxide. Answer: a. The pH of the equivalence point is less than 7. b. The pH of the equivalence point is greater than 7. c. The pH of the equivalence point is equal to 7.

Step 1: Determine reactant properties

Quinine is a weak base, and nitric acid is a strong acid. These will react in the titration to form a salt.

Step 2: Determine pH at equivalence point

When a weak base is titrated with a strong acid, the reaction produces a weak acidic salt. Therefore, the pH at the equivalence point is expected to be less than 7. Answer: The pH of the equivalence point is less than 7. #b. Pyruvic acid titrated with calcium hydroxide#

Step 1: Determine reactant properties

Pyruvic acid is a weak acid, and calcium hydroxide is a strong base. These will react in the titration to form a salt.

Step 2: Determine pH at equivalence point

When a weak acid is titrated with a strong base, the reaction produces a weak basic salt. Therefore, the pH at the equivalence point is expected to be greater than 7. Answer: The pH of the equivalence point is greater than 7. #c. Hydrobromic acid titrated with strontium hydroxide#

Step 1: Determine reactant properties

Hydrobromic acid is a strong acid, and strontium hydroxide is a strong base. These will react in the titration to form a salt and water.

Step 2: Determine pH at equivalence point

When a strong acid is titrated with a strong base, the reaction produces a neutral salt and water. Therefore, the pH at the equivalence point is expected to be equal to 7. Answer: The pH of the equivalence point is equal to 7.

Key Concepts

Equivalence Point

In a titration, the equivalence point is a critical stage. It is the moment when the amount of titrant added is just enough to completely neutralize the analyte solution. This means that the moles of acid equal the moles of base in the titration mixture. However, the equivalence point does not always result in a neutral pH of 7. The actual pH at the equivalence point depends on the strength of the acids and bases involved in the reaction.

To assess the pH at the equivalence point, you need to consider the types of reactants:

• If a strong acid and a strong base react, the pH is neutral, around 7.
• If a strong acid reacts with a weak base, the pH is acidic (less than 7).
• If a weak acid reacts with a strong base, the pH is basic (greater than 7).

Understanding these interactions is essential to predict the pH behavior at the equivalence point during a titration.

Strong Acid and Base Reactions

Strong acid and base reactions are straightforward. They involve two strong reactants, such as hydrobromic acid (a strong acid) and strontium hydroxide (a strong base). During their reaction, they completely dissociate in water to release their ions:

• The acid donates hydrogen ions (\( H^+ \)).
• The base provides hydroxide ions (\( OH^- \)).

When these ions combine, they form water (\( H_2O \)) and an associated salt. At the equivalence point for strong acid and base reactions, such as hydrobromic acid titrated with strontium hydroxide, this complete neutralization results in a pH that is exactly 7. This occurs because the salt formed does not affect the pH of the solution.

Weak Acid and Base Reactions

Reactions involving weak acids and bases are less predictable in terms of pH at the equivalence point. This is because weak acids and bases do not completely dissociate in water. Let’s look at two types of reactions:

• Weak Acid with Strong Base: Pyruvic acid (a weak acid) reaction with calcium hydroxide (a strong base) produces a basic salt and water. At the equivalence point, the solution contains the salt of the weak acid's conjugate base, which tends to increase the pH, resulting in a pH greater than 7.
• Weak Base with Strong Acid: Quinine (a weak base) reaction with nitric acid (a strong acid) results in an acidic salt and water. Here, the salt of the weak base's conjugate acid is present at the equivalence point, which lowers the pH below 7.

Thus, the key takeaway is that the nature of the weak acid or base and its conjugate significantly influences the pH at the equivalence point. This is important when predicting the pH balance in these reactions.