Problem 20 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) Write an equation for the react ion in which \(\mathrm{HSO}_{4}^{-}(a q)\) acts as a base in \(\mathrm{H}_{2} \mathrm{O}(l) .(\mathbf{b})\) Write an equation for the reaction in which \(\mathrm{HSO}_{4}^{-}(a q)\) acts as an acid in \(\mathrm{H}_{2} \mathrm{O}(I) \cdot(\mathbf{c})\) What is the conjugate acid of \(\mathrm{HSO}_{4}^{-}(a q) ?\) What is its conjugate base?

Step-by-Step Solution

Answer

When HSO4- acts as a base in H2O, the following reaction occurs: \( \mathrm{HSO}_{4}^{-}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{SO}_{4}^{2-}(a q) + \mathrm{H}_{3} \mathrm{O}^{+}(a q) \). When HSO4- acts as an acid in H2O, the reaction is: \( \mathrm{HSO}_{4}^{-}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{H}_{2}\mathrm{SO}_{4}(a q) + \mathrm{OH}^{-}(a q) \). The conjugate acid of HSO4- is H2SO4, and its conjugate base is SO4 2-.

Step 1: Part A: HSO4- as a base in H2O

When HSO4- acts as a base in H2O, it means the ion is accepting a proton (H+) from water (H2O). The following reaction takes place: \[ \mathrm{HSO}_{4}^{-}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{SO}_{4}^{2-}(a q) + \mathrm{H}_{3} \mathrm{O}^{+}(a q) \]

Step 2: Part B: HSO4- as an acid in H2O

When HSO4- acts as an acid in H2O, the ion is donating a proton to water (H2O). This reaction can be represented as follows: \[ \mathrm{HSO}_{4}^{-}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{H}_{2}\mathrm{SO}_{4}(a q) + \mathrm{OH}^{-}(a q) \]

Step 3: Part C: Conjugate Acid and Conjugate Base

In the first reaction (Part A), HSO4- is acting as a base, meaning it accepts a proton. When HSO4- accepts a proton, it becomes H2SO4. So, the conjugate acid of HSO4- is H2SO4. In the second reaction (Part B), HSO4- is acting as an acid, meaning it donates a proton. When HSO4- donates a proton, it becomes SO4 2-. Therefore, the conjugate base of HSO4- is SO4 2-.

Key Concepts

Conjugate Acid

In chemistry, a conjugate acid is formed when a base accepts a proton, also known as a hydrogen ion (H^{+}). This is a central idea in studying acid-base reactions. When a base gains a proton, it transitions into its conjugate acid form. For example, consider the ion sulfate hydrogensulfate, \( \text{HSO}_4^- \):

• As it acts as a base, it accepts a proton (H^{+}) from water, transitioning into its conjugate acid \( \text{H}_2\text{SO}_4 \).

This concept becomes apparent in Step 1 of the original solution, where \( \text{HSO}_4^- \) acts as a base in water, forming \( \text{H}_3\text{O}^+ \) and transforming into its conjugate acid, \( \text{H}_2\text{SO}_4 \).
This transformation illustrates how a species can switch roles from a base to its conjugate acid through protonation.
Understanding these relationships helps in predicting the outcomes of equilibria in acid-base reactions.

Conjugate Base

A conjugate base forms when an acid donates a proton to another species. It is what remains of the acid after the donation of a hydrogen ion (H^{+}). In essence, it is the deprotonated form of the acid.
In the reaction processes involving hydrogensulfate, \( \text{HSO}_4^- \), mentioned in Step 2 of the original solution:

• When \( \text{HSO}_4^- \) acts as an acid, it donates a proton to water, thus becoming \( \text{SO}_4^{2-} \).

This transformation demonstrates the production of the conjugate base, \( \text{SO}_4^{2-} \), as a result of this proton donation.
Understanding how conjugate bases work helps in analyzing the strengths of acids and their tendencies to donate protons. Typically, the stronger the acid, the weaker its conjugate base, because a strong acid gives up protons easily, and the resulting conjugate base has little affinity for re-association with the proton.

Sulfate Chemistry

Sulfate chemistry plays a significant role in various chemical processes, particularly in acid-base reactions. The sulfate ion, \( \text{SO}_4^{2-} \), serves as the conjugate base for several reactions involving sulfuric acid and its variants.

• In reactions where \( \text{HSO}_4^- \) acts as either a base or an acid, we encounter it transitioning to \( \text{H}_2\text{SO}_4 \) or \( \text{SO}_4^{2-} \) respectively.

These transitions are critical in understanding the versatility of sulfate ions in buffering systems and their equilibrium setups.
Sulfate ions in aqueous solutions play a vital part in maintaining a balance between acidity and alkalinity. Since \( \text{HSO}_4^- \) can act both as an acid and a base, it exemplifies amphiprotic behavior, which is helpful in environmental and biological systems, such as regulating the pH of natural waters and biological fluids.
Studying the unique properties and reactions of sulfate ions provides deeper insights into their role in chemistry and their functionality within larger chemical equations or reactions.