Problem 34 - Chemistry The Central Science

Question

How does titration of a strong, monoprotic acid with a strong base differ from titration of a weak, monoprotic acid with a strong base with respect to the following: (a) quantity of base required to reach the equivalence point, (b) \(\mathrm{pH}\) at the beginning of the titration, \((\mathbf{c}) \mathrm{pH}\) at the equivalence point, \((\mathbf{d}) \mathrm{pH}\) after addition of a slight excess of base, (e) choice of indicator for determining the equivalence point?

Step-by-Step Solution

Answer

Both strong and weak monoprotic acids require the same amount of base to reach the equivalence point, while the pH differs at various stages of titration. Strong acids have a lower initial pH, a pH of 7 at the equivalence point, and a greater pH after excess base addition, compared to weak acids. Strong acids can use phenolphthalein or bromothymol blue as indicators, whereas weak acids should use phenolphthalein due to its pH range above 7.

Step 1: a) Quantity of base required to reach the equivalence point

In both cases, the quantity of base required to reach the equivalence point is the same. This is because the stoichiometry of the acid-base reaction remains the same. In other words, one mole of acid will react with one mole of base for both strong and weak monoprotic acids.

Step 2: b) pH at the beginning of the titration

The initial pH for a strong acid will be lower than that for a weak acid, as strong acids fully ionize in water, producing a higher concentration of \(\mathrm{H}^+\) ions, which results in a lower pH. On the other hand, weak acids only partially ionize, producing fewer \(\mathrm{H}^+\) ions, which results in a higher initial pH value.

Step 3: c) pH at the equivalence point

For the titration of a strong acid with a strong base, the pH at the equivalence point is typically 7, as the salt that forms will not hydrolyze. For the titration of a weak acid with a strong base, the pH at the equivalence point will be greater than 7, as the conjugate base of the weak acid will hydrolyze in water, producing a basic solution.

Step 4: d) pH after the addition of a slight excess of base

For both cases, the pH will be greater than 7 after the addition of a slight excess of base, as more base was added than required to reach the equivalence point. However, the pH will typically be higher for the titration of a weak acid with a strong base, as the conjugate base of the weak acid will continue to hydrolyze in water, producing an even more basic solution.

Step 5: e) Choice of indicator for determining the equivalence point

For the titration of a strong acid with a strong base, an indicator with a pH range around 7, like phenolphthalein or bromothymol blue, is suitable. For the titration of a weak acid with a strong base, an indicator with a pH range above 7, like phenolphthalein, should be chosen, as it will change color around the equivalence point, which will have a pH greater than 7.

Key Concepts

Equivalence Point

The equivalence point in a titration marks the stage where the number of moles of acid equals the number of moles of base. This point is significant for understanding reaction completion. For both strong and weak monoprotic acids titrated with a strong base, the mole ratio remains 1:1. This means that the quantity of base needed to reach the equivalence point remains the same for each case. The focus here is on the stoichiometry, which defines this crucial turning point of a titration process.

pH

In the context of titration, pH acts as a critical indicator of the acidity or basicity of a solution. At the start, a strong acid will have a lower pH compared to a weak acid because strong acids fully ionize, increasing the concentration of \(\mathrm{H}^+\) ions. For a weak acid, partial ionization means fewer \(\mathrm{H}^+\) ions, resulting in a higher initial pH.
As titration progresses and the equivalence point is reached, the pH reflects significant differences. For a strong acid with a strong base, the pH is around 7, forming a neutral salt. However, a weak acid with a strong base will have a pH above 7 at equivalence due to the basic nature of the conjugate base formed.

Acid-Base Reactions

Titration exemplifies classic acid-base reactions, where acids donate and bases accept \(\mathrm{H}^+\). During titration, acids and bases react to form water and a salt. The nature of the acid—strong vs. weak—plays a critical role. Strong acids completely ionize, ensuring all reactions are immediate and complete.
In contrast, weak acids only partially ionize, so they react more slowly. This difference impacts the solution’s initial pH and behavior at the equivalence point. Despite these variations, the quantity of base required stays consistent due to the fixed stoichiometric ratio.

Choice of Indicator

Choosing the right indicator is crucial for accurately determining the equivalence point. An indicator is a substance that changes color at a particular pH range. For strong acid-strong base titrations, indicators like phenolphthalein or bromothymol blue, which are active around pH 7, are ideal. Since the equivalence point is neutral, these indicators provide clear signal changes.
For weak acid-strong base titrations, the pH at the equivalence point is above 7 due to the basic salt formed. Thus, indicators that work in a higher pH range, such as phenolphthalein, are preferred. The choice ensures the color change accurately reflects the equivalence point for each acid type.