Question

Predict whether the equivalence point of each of the following titrations is below, above, or at \(\mathrm{pH} 7:\) (a) formic acid titrated with \(\mathrm{NaOH},\) (b) calcium hydroxide titrated with perchloric acid, (c) pyridine titrated with nitric acid.

Short answer

In summary, the equivalence points for the given titrations will have the following pH values: (a) Formic acid titrated with NaOH (weak acid, strong base): Above 7 (b) Calcium hydroxide titrated with perchloric acid (strong base, strong acid): At 7 (c) Pyridine titrated with nitric acid (weak base, strong acid): Below 7

Step-by-step solution

Case (a): Formic acid titrated with NaOH

Formic acid (\(HCOOH\)) is a weak acid, and sodium hydroxide (\(NaOH\)) is a strong base. During the titration, a salt, sodium formate (\(NaHCOO\)), and water (\(H_2O\)) will be formed: \[HCOOH + NaOH \rightarrow NaHCOO + H_2O\] Sodium formate is a salt of a weak acid and a strong base. Such salts can make the solution either acidic or basic depending on the nature of the anion. In this case, the anion is the formate ion, which will produce a basic solution. Thus, the equivalence point's pH will be above 7.

Case (b): Calcium hydroxide titrated with perchloric acid

Calcium hydroxide (\(Ca(OH)_2\)) is a strong base, and perchloric acid (\(HClO_4\)) is a strong acid. When calcium hydroxide reacts with perchloric acid, calcium perchlorate (\(Ca(ClO_4)_2\)) and water (\(H_2O\)) form: \[Ca(OH)_2 + 2HClO_4 \rightarrow Ca(ClO_4)_2 + 2H_2O\] Calcium perchlorate is the salt of a strong acid and a strong base. In this case, the resulting solution will be neutral, which means the equivalence point's pH will be equal to 7.

Case (c): Pyridine titrated with nitric acid

Pyridine (\(C_5H_5N\)) is a weak base, and nitric acid (\(HNO_3\)) is a strong acid. When pyridine reacts with nitric acid, pyridinium nitrate (\(C_5H_5NHNO_3\)) forms: \[C_5H_5N + HNO_3 \rightarrow C_5H_5NHNO_3\] Pyridinium nitrate is a salt of a weak base and a strong acid. Such salts make the solution acidic because the weak base cannot efficiently absorb the excess protons from the acid. The solution will be acidic, and the equivalence point's pH will be below 7. In summary, the equivalence points for the given titrations will have the following pH values: (a) Formic acid titrated with NaOH: Above 7 (b) Calcium hydroxide titrated with perchloric acid: At 7 (c) Pyridine titrated with nitric acid: Below 7

Key concepts

Equivalence Point

Understanding the equivalence point in an acid-base titration is essential for grasping the course of the reaction. It is that precise moment where the amount of acid equals the amount of base in a solution. At this juncture, all the titrant (acid or base being added) has reacted with the substance being titrated, leading to the complete neutralization of the acid by the base, or vice versa.

As an example, when titrating a weak acid like formic acid with a strong base like sodium hydroxide, the equivalence point occurs when enough base has been added to react completely with all the acid. The pH at the equivalence point is not always neutral (pH 7), and varies depending on the strengths of the acids and bases involved. For instance, when a weak acid is titrated with a strong base, the equivalence point pH will be above 7 due to the production of a basic salt and water. Conversely, titrating a weak base with a strong acid, as with pyridine and nitric acid, yields an equivalence point with a pH below 7, indicating an acidic solution.

pH Level

The pH level is a measure of how acidic or basic a solution is, on a scale from 0 to 14. A pH of 7 is considered neutral, below 7 is acidic, and above 7 is basic or alkaline. Throughout a titration, monitoring the pH informs you about the ongoing chemical reactions. When the pH changes drastically with a very small addition of titrant, it often indicates that the equivalence point has been reached.

The pH of the solution is particularly important at the equivalence point; for example, when a strong base like calcium hydroxide reacts with a strong acid like perchloric acid, the pH at the equivalence point is neutral. This information is valuable in predicting the behavior of the titration and designing appropriate indicators to signify the end point of the titration. Understanding pH is not only critical for titrations but also for numerous biological and chemical processes where the acidity or alkalinity of an environment dictates the outcome.

Weak and Strong Acids and Bases

Acids and bases are described as either 'strong' or 'weak' based on their ability to dissociate in water. Strong acids and bases dissociate completely, releasing all their hydrogen ions (H+) or hydroxide ions (OH−) into the solution. Examples include hydrochloric acid (HCl) and sodium hydroxide (NaOH). On the other hand, weak acids and bases only partially dissociate and exist as a mixture of the undissociated species and the ions they release, like formic acid (HCOOH) or pyridine (C₅H₅N).

The strength of the acids and bases involved affects the pH of the solution not just at the equivalence point but throughout the entire titration process. For instance, titrating a weak base with a strong acid will result in a solution where the pH decreases more sharply as it approaches the equivalence point. It is this intrinsic nature of the weak and strong acids and bases that determines the pH curve's shape during titration, making it a fundamental concept in understanding acid-base reactions.