15.8

Mixtures of Acids

16.6K views03:27 min

In a mixture of a strong acid and a weak acid, the strong acid dissociates completely and significantly increases the concentration of hydronium ions, whereas the weak acid only partially dissociates. Similarly, in a mixture of two weak acids, the acid that is relatively stronger produces more hydronium ions than the weaker acid.

In both cases, the dissociation of the weaker acid is suppressed when it is in the presence of a stronger acid. Le Châtelier’s principle explains that the formation of hydronium ions by the stronger acid shifts the equilibrium towards the reactants, thereby reducing the dissociation of the weak acid.

Thus, the pH of a mixture of acids is mainly determined by the concentration of the stronger acid. For example, in a mixture that contains 0.15 molar hydrochloric acid and 0.30 molar hydrocyanic acid, hydrochloric acid–a strong acid–produces a hydronium ion concentration of 0.15 molar.

In contrast, hydrocyanic acid–a weak acid–only partially dissociates. The concentration of hydronium ions produced by hydrocyanic acid can be calculated from its acid dissociation constant, K_a, and an ICE table.

The initial concentration of hydronium ions is equal to the initial concentration of hydrochloric acid, 0.15 molar, and the initial concentration of cyanide ions is zero. The change in concentration of hydronium ions and cyanide ions is denoted by x.

As x is a relatively small number, 0.30 minus x can be approximated to 0.30, and 0.15 plus x can be approximated to 0.15 using the 5% rule.

The K_a for hydrocyanic acid is 4.9 × 10⁻¹⁰, and it is equal to the concentration of hydronium ions times the concentration of cyanide ions divided by the concentration of hydrocyanic acid.

Substituting the values from the ICE table into the K_a expression gives the concentration of hydronium ions produced by hydrocyanic acid, which is negligible compared to the concentration of hydronium ions produced by hydrochloric acid.

The pH can be calculated by taking the negative log of the concentration of hydronium ions: 0.15 molar. Therefore, the pH of the mixture is solely determined by the concentration of hydrochloric acid, the strong acid.

Similarly, the pH of a mixture of two weak acids present in equal amounts will be primarily determined by the concentration of the relatively stronger acid.

For example, in a mixture of hydrofluoric acid and hydrocyanic acid, hydrofluoric acid will be the major determinant of the mixture’s pH, as it has a K_a of 3.5 × 10⁻⁴, which is almost a million times higher than the K_a of hydrocyanic acid.

Overview

Loading...

$$\rightleftharpoonup{xx}$$ $$\longleftharp{xx}$$, $$\longrightharp{xx}$$,

The pH of a solution containing an acid can be determined using its acid dissociation constant and its initial concentration. If a solution contains two different acids, then its pH can be determined using one of several methods depending upon the relative strength of the acids and their dissociation constants.

A Mixture of a Strong Acid and a Weak Acid

In a mixture of a strong acid and a weak acid, the strong acid dissociates completely and becomes a source of almost all the hydronium ions present in the solution. In contrast, the weak acid shows partial dissociation and produces a negligible concentration of hydronium ions. The high concentration of hydronium ions produced by the strong acid further reduces the dissociation of the weak acid. This happens because, according to Le Chatelier’s principle – “When a chemical system at equilibrium is disturbed, the system shifts in a direction that minimizes the disturbance.” The excess hydronium ions produced by the strong acid disturbs the equilibrium, and thus the reaction will move in the reverse direction until the equilibrium is established. This leads to a decrease in the dissociation of the weak acid. Because of this decrease, a pH of a mixture of a strong and weak acid can be calculated from the concentration of the strong acid only. For example, the pH of a mixture with an equal concentration of hydrochloric acid (HCl), a strong acid, and formic acid (HCHO₂), a weak acid, can be determined from the concentration of HCl only. If the concentration of the HCl in the mixture is 0.0020 M, its pH can be calculated as follows.

Protein secondary structure, alpha helix, beta sheet, diagram, molecular configuration, structural analysis.

Here, the concentration of hydronium ions produced by HCHO₂ and the autoionization of water is negligible and thus can be ignored.

A Mixture of Two Weak Acids with Different Dissociation Constants

In a mixture of two weak acids, the pH of a mixture will be determined by the stronger acid if its dissociation constant is significantly higher than the weaker acid. For example, in a mixture with an equal concentration of nitrous acid (HNO₂) and hypochlorous acid (HClO), the HNO₂will be the main determinant of the pH of the mixture as its K_a(4.6 × 10⁻⁴) is approximately 10,000 times higher than the K_a(2.9 × 10⁻⁸) of HClO. According to Le Chatelier’s principle, HClO shows decreased dissociation in the presence of HNO₂.

Transcript

Loading...

$$\rightleftharpoonup{xx}$$ $$\longleftharp{xx}$$, $$\longrightharp{xx}$$,

In a mixture of a strong acid and a weak acid, the strong acid dissociates completely and significantly increases the concentration of hydronium ions, whereas the weak acid only partially dissociates. Similarly, in a mixture of two weak acids, the acid that is relatively stronger produces more hydronium ions than the weaker acid.

In both cases, the dissociation of the weaker acid is suppressed when it is in the presence of a stronger acid. Le Châtelier’s principle explains that the formation of hydronium ions by the stronger acid shifts the equilibrium towards the reactants, thereby reducing the dissociation of the weak acid.

Thus, the pH of a mixture of acids is mainly determined by the concentration of the stronger acid. For example, in a mixture that contains 0.15 molar hydrochloric acid and 0.30 molar hydrocyanic acid, hydrochloric acid–a strong acid–produces a hydronium ion concentration of 0.15 molar.

In contrast, hydrocyanic acid–a weak acid–only partially dissociates. The concentration of hydronium ions produced by hydrocyanic acid can be calculated from its acid dissociation constant, K_a, and an ICE table.

The initial concentration of hydronium ions is equal to the initial concentration of hydrochloric acid, 0.15 molar, and the initial concentration of cyanide ions is zero. The change in concentration of hydronium ions and cyanide ions is denoted by x.

As x is a relatively small number, 0.30 minus x can be approximated to 0.30, and 0.15 plus x can be approximated to 0.15 using the 5% rule.

The K_a for hydrocyanic acid is 4.9 × 10⁻¹⁰, and it is equal to the concentration of hydronium ions times the concentration of cyanide ions divided by the concentration of hydrocyanic acid.

Substituting the values from the ICE table into the K_a expression gives the concentration of hydronium ions produced by hydrocyanic acid, which is negligible compared to the concentration of hydronium ions produced by hydrochloric acid.

The pH can be calculated by taking the negative log of the concentration of hydronium ions: 0.15 molar. Therefore, the pH of the mixture is solely determined by the concentration of hydrochloric acid, the strong acid.

Similarly, the pH of a mixture of two weak acids present in equal amounts will be primarily determined by the concentration of the relatively stronger acid.

For example, in a mixture of hydrofluoric acid and hydrocyanic acid, hydrofluoric acid will be the major determinant of the mixture’s pH, as it has a K_a of 3.5 × 10⁻⁴, which is almost a million times higher than the K_a of hydrocyanic acid.

Explore More Videos

Acid Dissociation Constant

Le Chatelier Principle

Hydronium Ion Concentration

ICE Table Method

Acid Strength Comparison

Weak Acid Suppression