Question

On the same graph, sketch pH titration curves for the titration of \((1)\) a strong acid with a strong base and (2) a weak acid with a strong base. How do the two curves differ with respect to the following? (a) The initial \(\mathrm{pH}\) (b) The \(\mathrm{pH}\) in the region between the start of the titration and the equivalence point (c) The \(\mathrm{pH}\) at the equivalence point (d) The \(\mathrm{pH}\) beyond the equivalence point (e) The volume of base required to reach the equivalence point

Short answer

Strong acid titration starts with low pH and has a neutral equivalence point at pH 7, while weak acid starts at higher pH and has a basic equivalence point.

Step-by-step solution

Understanding Titration Curves

Titration curves represent the change in pH as a titrant is added to a solution. For a strong acid titrated with a strong base, the curve starts at a low pH, sharply rises at the equivalence point, and then levels off high. For a weak acid titrated with a strong base, the curve starts at a higher pH, rises more gradually at first, and then becomes steep at the equivalence point.

Analyzing Initial pH

A strong acid, like HCl, has a low initial pH (around 1) as it is completely ionized. A weak acid, like acetic acid, has a higher initial pH (around 2-3) because it does not ionize completely.

pH Before Equivalence Point

Between the start and the equivalence point, the pH for the strong acid titration increases steadily as more base is added. For the weak acid, the pH increases more slowly initially due to the buffer region created by the partially ionized acid and its conjugate base.

pH at the Equivalence Point

The pH at the equivalence point for a strong acid-strong base titration is 7, as the resulting solution is neutral. For a weak acid-strong base titration, the pH is greater than 7 due to the presence of the conjugate base of the weak acid, which partially dissociates.

pH Beyond the Equivalence Point

After the equivalence point, both curves gradually approach a high pH value, as excess strong base dominates the solution. The pH rises more sharply for the weak acid titration as the ionized base adds more hydroxide ions.

Volume of Base to Reach Equivalence Point

The volume of base required to reach the equivalence point is determined by the initial concentration and volume of the acid. For both strong and weak acid titrations, this volume is theoretically the same if the acids start with the same initial moles.

Key concepts

Strong Acid vs Weak Acid

When discussing titration curves, it's crucial to distinguish between strong acids and weak acids. A strong acid, such as hydrochloric acid (HCl), completely dissociates in water. This means that it releases all its hydrogen ions, leading to a very low initial pH, often around 1.
In contrast, a weak acid, like acetic acid (CH₃COOH), only partially ionizes in solution. This partial ionization means that the initial pH is higher, usually between 2 and 3.
The difference in dissociation impacts how these acids behave in titrations. While a strong acid immediately reacts with a base to form a more neutral solution, a weak acid only gradually reacts, forming a buffer solution, which consists of the weak acid and its conjugate base. This buffer resists changes in pH more effectively than a solution of a strong acid would.

Equivalence Point

The equivalence point in a titration is pivotal as it signifies the moment when the amount of titrant base added equals the amount of acid initially present in the solution.
For a titration involving a strong acid and a strong base, the pH at the equivalence point is generally 7. This neutrality occurs because the acid and base fully neutralize each other, forming water and a salt.
However, when titrating a weak acid with a strong base, the equivalence point results in a pH greater than 7. This is due to the presence of the weak acid's conjugate base, which forms as a result of the reaction, partially dissociating and increasing the pH above neutral.
Understanding this difference is key for correctly interpreting titration curves and preparing solutions with the desired pH.

pH Changes in Titration

Titration curves visually depict the pH changes during a titration and can vary greatly between strong and weak acids. Initially, for strong acids, the pH is very low due to complete dissociation. As you add titrant base, the pH climbs steeply, especially near the equivalence point.
In contrast, for a weak acid, the initial pH is not as low. As titrant is added, the pH rises more slowly initially, due to the buffer region formed by the weak acid and its conjugate base.
This buffer region is one of the main differences between titrations of strong versus weak acids. It extends from the start of the titration up to the equivalence point and is crucial for stability in the pH, slowing its change as titration progresses. After reaching the equivalence point, both curves become similar, showing a rapid increase in pH with the continued addition of the base.

Acid-Base Reactions

Acid-base reactions are central to understanding titration curves. These reactions occur when an acid donates a proton to a base. In a titration, this usually involves a strong or weak acid reacting with a strong base.
In the reaction of a strong acid with a strong base, complete neutralization happens, forming water, leading to a final product that is more neutral. Conversely, when a weak acid reacts with a strong base, not all molecules are neutralized immediately.
The formation of a buffer solution during weak acid titrations is a direct outcome of acid-base reactions. Here, the weak acid loses hydrogen ions gradually, and its conjugate base captures these changes over time. This buffering action is essential for controlling pH changes in various chemical and biological systems.
These acid-base reactions not only determine the shape of the titration curve but also provide insight into the chemistry of the substances involved, making them a fundamental concept in analytical chemistry.