Problem 52 - 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. HCN titrated with \(\mathrm{Ca}(\mathrm{OH})_{2}\) b. LiOH titrated with HI c. \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{N}\) titrated with \(\mathrm{KOH}\)

Step-by-Step Solution

Answer

Question: Predict the pH at the equivalence point for each of the following titrations: a. HCN titrated with Ca(OH)2 b. LiOH titrated with HI c. C5H5N titrated with KOH Answer: a. The pH at the equivalence point for HCN titrated with Ca(OH)2 is greater than 7. b. The pH at the equivalence point for LiOH titrated with HI is equal to 7. c. The pH at the equivalence point for C5H5N titrated with KOH is less than 7.

Step 1: Titration a: HCN titrated with Ca(OH)2

In this titration, HCN (a weak acid) is titrated with Ca(OH)2 (a strong base). At the equivalence point, HCN has reacted completely with Ca(OH)2, generating the conjugate base CN- and water. The conjugate base (CN-) of the weak acid will possess basic properties, causing the pH of the solution at the equivalence point to be greater than 7.

Step 2: Titration b: LiOH titrated with HI

LiOH is a strong base, and HI is a strong acid. When a strong base is titrated with a strong acid, the resulting solution at the equivalence point is neutral, which means that the pH of the solution at the equivalence point is equal to 7.

Step 3: Titration c: C5H5N titrated with KOH

C5H5N, also known as pyridine, is a weak base. It is titrated with KOH, a strong base. At the equivalence point, the weak base has reacted completely with the strong base, producing the conjugate acid (C5H5NH+) and water. The conjugate acid (C5H5NH+) of the weak base will have acidic properties, causing the pH of the solution at the equivalence point to be less than 7. In summary: a. The pH at the equivalence point for HCN titrated with Ca(OH)2 is greater than 7. b. The pH at the equivalence point for LiOH titrated with HI is equal to 7. c. The pH at the equivalence point for C5H5N titrated with KOH is less than 7.

Key Concepts

Titration

Titration is a process used in chemistry to determine the concentration of an unknown acid or base by adding a solution of known concentration until the reaction reaches the equivalence point. At the equivalence point, the number of moles of acid equals the number of moles of base. Titrations can occur in different combinations, such as strong acid with strong base or weak acid with strong base.

• Strong acid-strong base titration results in a neutral solution at the equivalence point.
• Weak acid-strong base or weak base-strong base titrations result in a solution that is either basic or acidic at the equivalence point.

Understanding the nature of the acid and base involved helps predict the pH at the equivalence point.

Weak Acid-Strong Base Reaction

In a weak acid-strong base titration, a weak acid like HCN reacts with a strong base such as Ca(OH)₂. As the titration progresses, the weak acid is neutralized by the strong base, forming a salt and water. At the equivalence point, the acid is fully neutralized, producing the conjugate base, which has basic characteristics.
The key here is the formation of the conjugate base (like CN⁻) that affects the pH:

• The strong base completely neutralizes the weak acid.
• The conjugate base created has a tendency to accept protons from water, making the solution basic.

Thus, the pH at the equivalence point is greater than 7, indicating a basic solution.

Strong Acid-Strong Base Reaction

When a strong acid is titrated with a strong base, such as in the case of HI and LiOH, the reaction produces a neutral solution of salt and water at the equivalence point. Both reactants ionize completely in solution, ensuring all the acid molecules meet their base counterparts, which makes the reaction straightforward.
Some crucial points about this reaction:

• Complete ionization ensures no excess of H⁺ or OH⁻ ions at the equivalence point.
• The resulting solution is neutral, with a pH of 7.

This neutralization results in water and a salt with no residual acidic or basic properties.

Weak Base-Strong Base Reaction

A weak base-strong base titration involves a weak base, like pyridine (C₅H₅N), reacting with a strong base such as KOH. At the equivalence point, the weak base has completely reacted, resulting in the formation of a conjugate acid (C₅H₅NH⁺) that makes the solution acidic.
In this scenario:

• The strong base does not affect the conjugate acid's acidic nature.
• The solution remains acidic due to the presence of the conjugate acid.

Therefore, the pH of the solution at the equivalence point is less than 7, indicating an acidic environment. This is due to the conjugate acid's ability to donate protons back into the water.