Problem 30 - General Chemistry

Question

\( \mathrm{~A}\) friend of yours has performed three titrations: strong acid with a strong base, weak acid with a strong base, and weak base with a strong acid. He hands you the three titration curves, saying he has forgotten which is which. What attributes of the curves would you look at to identify each curve correctly?

Step-by-Step Solution

Answer

Identify curves by pH behavior: equivalence points at pH 7, >7, and <7 for strong acid-strong base, weak acid-strong base, and weak base-strong acid, respectively.

Step 1: Recognizing Strong Acid-Strong Base Titration Curve

Strong acid-strong base titration curves are characterized by a very steep vertical segment around the equivalence point. This is because both the acid and the base fully dissociate, leading to a rapid pH change. The equivalence point occurs at pH 7, where the acid and base completely neutralize each other. Look for a curve that reflects this steep rise with the midpoint around pH 7.

Step 2: Identifying Weak Acid-Strong Base Titration Curve

In a weak acid-strong base titration, the curve typically begins at a pH higher than that of a strong acid-strong base titration because the weak acid does not fully dissociate in solution. The equivalence point, where the curve has the steepest slope, will occur at a pH greater than 7 due to the formation of a basic salt. Additionally, notice the buffer region before the vertical segment, which is indicative of the weak acid's ability to resist pH changes initially.

Step 3: Determining Weak Base-Strong Acid Titration Curve

For a weak base-strong acid titration, the curve starts at a relatively high pH (reflecting the weak base) and decreases as the strong acid is added. The equivalence point will be at a pH less than 7 because the resulting salt solution is acidic. The initial part of the curve might show a buffer region as well, similar to weak acid titrations, but in this case, it is due to the weak base resisting pH changes.

Key Concepts

Strong Acid-Strong Base Titration

A titration involving a strong acid and a strong base is characterized by its distinct titration curve. When identifying this type of curve, there are key features to look for. Initially, the pH remains relatively low if you start with the strong acid. As the titration progresses and base is added, the pH increases gradually. However, the most significant aspect of this curve is the **steep vertical section** near the equivalence point.

This rapid pH change occurs because both strong acids and bases completely dissociate in solution. The equivalence point in a strong acid-strong base titration is typically **around pH 7**. This neutral point results from the complete neutralization of the acid by the base. Hence, if you examine a titration curve and find a sharp increase in pH reaching its steepest change near pH 7, it most likely represents a strong acid-strong base titration.

• Look for a steep slope at the equivalence point.
• The equivalence point is generally at pH 7.
• Both acid and base fully dissociate.

Weak Acid-Strong Base Titration

For a titration between a weak acid and a strong base, the curve showcases its own unique traits. At the outset, one might notice a **starting pH higher** than that of a strong acid-strong base titration. This occurs because weak acids only partially dissociate, making the initial solution less acidic.

As you conduct the titration and add a strong base, the pH increases gradually until the buffer zone is reached. This **buffer region** reflects the weak acid's ability to stabilize the pH by resisting drastic changes; this is indeed a key identifier of the curve.

The equivalence point is noteworthy as it happens **at a pH greater than 7** due to the production of a basic salt. The section of the curve near this point shows the steepest incline. Therefore, if you spot a buffer region followed by a steep section peaking above pH 7, it signifies a weak acid-strong base titration.

• Initial pH is higher compared to strong acid-strong base titrations.
• Presence of a buffer zone that stabilizes pH fluctuations.
• The equivalence point is at a pH greater than 7.

Weak Base-Strong Acid Titration

During a titration involving a weak base and a strong acid, the curve reflects different characterizations compared to the previous ones. Initially, expect a **high starting pH**, indicative of a weak base's less acidic nature. As the strong acid is added, the pH gradually decreases. This includes a **buffer region** where the weak base continues to neutralize some of the added acid, helping to avoid drastic drops in pH.

The curve steepens considerably around the equivalence point, and at this juncture, the pH is **lower than 7**. This is due to the acidic nature of the solution formed by the salt produced from the weak base and strong acid. Thus, when the curve starts high and ends with a substantial descent below pH 7 at the equivalence point, it is indicative of a weak base-strong acid titration.

• Starts with a relatively high pH due to weak base.
• The presence of a buffer region indicates the base's resistance to pH changes.
• The equivalence point falls at a pH lower than 7.