Problem 38 - Chemistry The Central Science

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 \(\mathrm{NaOH},(\mathbf{b})\) calcium hydroxide titrated with perchloric acid, (c) pyridine titrated with nitric acid.

Step-by-Step Solution

Answer

(a) The equivalence point for formic acid titrated with NaOH will be at a pH above 7, as it involves a weak acid and a strong base, forming a weak conjugate base. (b) The equivalence point for calcium hydroxide titrated with perchloric acid will be at a pH of 7, as it involves a strong base and a strong acid, forming a neutral salt. (c) The equivalence point for pyridine titrated with nitric acid will be at a pH below 7, as it involves a weak base and a strong acid, forming a weak conjugate acid.

Step 1: (a) Formic Acid Titrated with NaOH

Formic acid (HCOOH) is a weak acid, and sodium hydroxide (NaOH) is a strong base. At the equivalence point, the weak acid will have reacted with a strong base, forming a weak conjugate base (formate ion, HCOO⁻). Because the base is weak, this titration's equivalence point will be at a pH above 7.

Step 2: (b) Calcium Hydroxide Titrated with Perchloric Acid

Calcium hydroxide (Ca(OH)₂) is a strong base, and perchloric acid (HClO₄) is a strong acid. When a strong base reacts with a strong acid, a neutral salt and water are formed. This means the equivalence point for this titration will be at a pH of exactly 7.

Step 3: (c) Pyridine Titrated with Nitric Acid

Pyridine (C₅H₅N) is a weak base, and nitric acid (HNO₃) is a strong acid. At the equivalence point, the weak base will have reacted with a strong acid, forming a weak conjugate acid (pyridinium ion, C₅H₅NH+). As the conjugate acid is weak, the equivalence point for this titration will be at a pH below 7. In summary: (a) Formic acid with NaOH: pH above 7 (b) Calcium hydroxide with Perchloric Acid: pH 7 (c) Pyridine with Nitric Acid: pH below 7

Key Concepts

Acid-Base Titration

Acid-base titration is a process used by chemists to determine the concentration of an unknown acid or base solution by adding a known volume and concentration of an acid or base (titrant) until the reaction reaches what is known as the equivalence point. At the equivalence point, the number of moles of hydrogen ions (H+) from the acid equates to the number of moles of hydroxide ions (OH-) from the base, indicating that the reaction has been completely neutralized.

Let's take the example of formic acid being titrated with sodium hydroxide. Formic acid (HCOOH) is a weak acid, and its reaction with the strong base NaOH won't be complete until a sufficient volume of NaOH is added to fully neutralize it. As a result of this chemical reaction, water and the conjugate base of formic acid, formate (HCOO⁻), are formed. Since the conjugate base has a tendency to accept protons, it will slightly increase the pH of the solution above 7 at the equivalence point.

On the contrary, when calcium hydroxide, a strong base, is titrated with a strong acid like perchloric acid, the neutralization produces a salt and water without any tendency to alter the pH, hence the equivalence point here is at pH 7. In each titration, indicators or pH meters can be used to determine when the equivalence point has been reached, which is crucial for accurate calculations.

pH Scale

The pH scale is a measure of how acidic or basic a solution is, ranging from 0 to 14. A pH of 7 is considered neutral, which is the pH of pure water. Readings below 7 indicate acidic solutions, while readings above 7 indicate basic solutions. It's important to note that the pH scale is logarithmic; each whole number on the scale represents a tenfold increase or decrease in acidity. For instance, a solution with pH 4 is ten times more acidic than a solution with pH 5.

The significance of the pH scale during a titration cannot be overstated. When a weak base like pyridine is titrated with a strong acid such as nitric acid, the resultant solution at the equivalence point has a pH below 7 due to the formation of a weak conjugate acid (pyridinium ion) which can donate H+ to the solution. Conversely, titrating a weak acid with a strong base will result in a basic solution at the equivalence point, with a pH above 7. Hence, monitoring the pH during the titration provides a clear indication of the reaction progression and when the equivalence point has been reached.

Conjugate Acid-Base Pairs

Conjugate acid-base pairs are two substances that transform into each other by the gain or loss of a proton (H+). In an acid-base reaction, the acid donates a proton and becomes its conjugate base, while the base accepts a proton and becomes its conjugate acid. This concept helps explain the behavior of acids and bases during a titration.

For example, during the titration of formic acid with NaOH, the formic acid (HCOOH) loses a proton to become its conjugate base, the formate ion (HCOO⁻). Because formate is a weak conjugate base, it only weakly accepts protons from water, resulting in a basic solution at the equivalence point. In contrast, when pyridine reacts with nitric acid, it gains a proton to become pyridinium (C₅H₅NH+), a weak conjugate acid, which then weakly donates protons to the solution, creating an acidic solution at the equivalence point.

Understanding conjugate acid-base pairs is fundamental for predicting the pH at the equivalence point of a titration. Generally, a strong acid will have a weak conjugate base, and a strong base will have a weak conjugate acid. This relationship is central to determining whether the equivalence point will be acidic, basic, or neutral.