Question

In which of the following acid-base titration, \(\mathrm{pH}\) is greater than 8 at equivalence point? (a) Acetic acid vs ammonia (b) Acetic acid vs sodium hydroxide (c) Hydrochloric acid vs ammonia (d) Hydrochloric acid vs sodium hydroxide

Short answer

Acetic acid vs sodium hydroxide (option b) has a pH greater than 8 at equivalence.

Step-by-step solution

Understand Titration Components

In an acid-base titration, we need to determine the nature of the reactants. Acetic acid is a weak acid, ammonia is a weak base, hydrochloric acid is a strong acid, and sodium hydroxide is a strong base.

Analyze the Equivalence Point for Weak Acid and Strong Base

In a titration between a weak acid (like acetic acid) and a strong base (like sodium hydroxide), the equivalence point occurs at a pH greater than 7 because the solution contains a weak acid's conjugate base, which is basic.

Consider Other Titration Scenarios

For acetic acid vs ammonia, both are weak, leading to a pH close to neutral at equivalence. Hydrochloric acid vs ammonia involves a strong acid, leading to a pH less than 7. Hydrochloric acid vs sodium hydroxide involves a strong acid and a strong base, leading to a neutral pH of 7.

Select the Correct Scenario

The titration giving a pH greater than 8 at equivalence point involves a weak acid and a strong base. Thus, acetic acid vs sodium hydroxide is the correct choice.

Key concepts

Equivalence Point

The equivalence point in an acid-base titration is a fundamental concept.
It occurs when the amount of acid equals the amount of base, indicating that the reactants have completely reacted with each other.
This point is essential because it signifies the complete neutralization of the acid and base.
In a solution, the equivalence point is typically identified by a sharp change in pH.
This change is monitored using a pH indicator or a pH meter.
Traditionally, titrations involving strong acids and bases result in a neutral pH of 7 at the equivalence point.
However, if the titration involves a weak acid and a strong base, the pH at the equivalence point will usually be greater than 7.
Understanding the characteristics of the equivalence point is crucial in predicting the outcome of a titration.

Weak Acid and Strong Base

The interaction between a weak acid and a strong base offers intriguing results in titration.
Acetic acid is a classic example of a weak acid, while sodium hydroxide represents a strong base.
When the titration occurs, acetic acid is partially neutralized because it's only weakly ionized in solution.
The strong base, sodium hydroxide, completely dissociates, pushing the reaction forward.
As a result, a conjugate base, such as acetate ion in the case of acetic acid, remains in solution.
The presence of these conjugate bases essentially makes the solution slightly basic.
The chemistry between weak acids and strong bases creates interesting dynamics and can significantly affect the pH at the equivalence point, shifting it above neutral.
This scenario highlights the complexity and beauty of acid-base chemistry.

pH Greater Than 7

Measuring the pH at the equivalence point provides insightful data about the reaction's nature.
In the case of a weak acid-strong base titration, the pH at equivalence point typically exceeds 7.
This phenomenon occurs because, at equivalence, the solution primarily contains the conjugate base formed from the weak acid.
As mentioned earlier, even though the initial weak acid is neutralized, the resultant conjugate base can slightly raise the pH.
For example, in a titration involving acetic acid and sodium hydroxide, the acetate ion raises the pH beyond the neutral point.
Understanding this outcome demonstrates how the nature of reactants determines the pH level during titration.

Acetic Acid and Sodium Hydroxide Titration

The titration between acetic acid and sodium hydroxide is a textbook example, often encountered in chemistry classes.
As acetic acid reacts with sodium hydroxide, the base neutralizes the acid, and an exothermic reaction occurs.
This process forms water and sodium acetate, a salt.
At the equivalence point, sodium acetate dominates the solution, providing an observable pH change.
During this titration, since acetic acid is a weak acid, and sodium hydroxide is a strong base, the pH at the equivalence point is greater than 7.
This detail is crucial in many calculations and considerations when analyzing titration results.
Exploring this specific titration can help students grasp more complex titration scenarios and concepts.