Problem 19 - Chemistry: The Central Science

Question

(a) The hydrogen sulfite ion \(\left(\mathrm{HSO}_{3}^{-}\right)\)is amphiprotic. Write a balanced chemical equation showing how it acts as an acid toward water and another equation showing how it acts as a base toward water. (b) What is the conjugate acid of \(\mathrm{HSO}_{3}^{-}\)? What is its conjugate base?

Step-by-Step Solution

Answer

The hydrogen sulfite ion (HSO3-) acts as an acid towards water in the following equation: \[ \mathrm{HSO}_{3}^{-} + \mathrm{H_2O} \rightarrow \mathrm{SO}_{3}^{2-} + \mathrm{H_3O}^{+} \] As a base, it reacts with water as follows: \[ \mathrm{HSO}_{3}^{-} + \mathrm{H_2O} \rightarrow \mathrm{H_2SO}_{3} + \mathrm{OH}^{-} \] The conjugate acid of HSO3- is H2SO3, and its conjugate base is SO3^(2-).

Step 1: HSO3- acting as an acid

As an acid, HSO3- will donate a proton (H+) to water. Therefore, it will look like this: \[ \mathrm{HSO}_{3}^{-} + \mathrm{H_2O} \rightarrow \mathrm{SO}_{3}^{2-} + \mathrm{H_3O}^{+} \]

Step 2: HSO3- acting as a base

As a base, HSO3- will accept a proton (H+) from water. Therefore, it will look like this: \[ \mathrm{HSO}_{3}^{-} + \mathrm{H_2O} \rightarrow \mathrm{H_2SO}_{3} + \mathrm{OH}^{-} \]

Step 3: Finding the conjugate acid of HSO3-

To find the conjugate acid, we need to add a proton (H+) to the species. Therefore, the conjugate acid would be: \[ \mathrm{H_2SO}_{3} \]

Step 4: Finding the conjugate base of HSO3-

To find the conjugate base, we need to remove a proton (H+) from the species. Therefore, the conjugate base would be: \[ \mathrm{SO}_{3}^{2-} \]

Key Concepts

Chemical Equations

When we refer to chemical equations, we're talking about a symbolic representation of a chemical reaction. For students diving into chemistry, mastering the art of writing and balancing chemical equations is crucial. These equations depict the reactants (substances going into the reaction) on the left and the products (substances produced by the reaction) on the right, separated by an arrow symbolizing the direction of the reaction. For instance, when hydrogen sulfite ion, \( \mathrm{HSO}_{3}^{-} \), acts as an acid in water, it donates a proton (\(H^+\)) and transforms into sulfate ion (\( \mathrm{SO}_{3}^{2-} \)) and hydronium ion (\( \mathrm{H_3O}^{+} \)). This reaction is represented by the balanced equation:
\[ \mathrm{HSO}_{3}^{-} + \mathrm{H_2O} \rightarrow \mathrm{SO}_{3}^{2-} + \mathrm{H_3O}^{+} \]
A well-balanced chemical equation obeys the Law of Conservation of Mass, meaning that atoms are neither created nor destroyed during a chemical reaction. Thus, each element must have the same number of atoms on both sides of the equation.

Acid-Base Reactions

Understanding acid-base reactions is central to the study of chemistry. In simplest terms, an acid is a substance that can donate a proton, while a base is a substance that can accept a proton. This exchange of protons characterizes acid-base reactions. One remarkable feature of the hydrogen sulfite ion (\( \mathrm{HSO}_{3}^{-} \)) is that it can act as both an acid and a base – this is known as amphiprotic behavior. As an acid, hydrogen sulfite donates a proton to water, producing sulfate ion and hydronium ion, as shown by the first equation in the original exercise. Conversely, as a base, it accepts a proton from water, making the uncharged hydrogen sulfite (\( \mathrm{H_2SO}_{3} \)) and hydroxide ion (\( \mathrm{OH}^{-} \)), which can be encapsulated in the following reaction:
\[ \mathrm{HSO}_{3}^{-} + \mathrm{H_2O} \rightarrow \mathrm{H_2SO}_{3} + \mathrm{OH}^{-} \]
This dual ability to donate and accept a proton makes the understanding of amphiprotic substances and their role in acid-base reactions both intriguing and complex.

Conjugate Acid-Base Pairs

The concept of conjugate acid-base pairs is integral to acid-base chemistry. A conjugate acid-base pair consists of two species that transform into each other by the gain or loss of a proton. When a base accepts a proton, it becomes its conjugate acid; conversely, when an acid donates a proton, it forms its conjugate base. For example, the hydrogen sulfite ion (\( \mathrm{HSO}_{3}^{-} \)) can act as a base and accept a proton, becoming its conjugate acid (\( \mathrm{H_2SO}_{3} \)). This is demonstrated in the original solution when \( \mathrm{HSO}_{3}^{-} \) reacts with water. Similarly, when \( \mathrm{HSO}_{3}^{-} \) acts as an acid and donates a proton, it forms its conjugate base (\( \mathrm{SO}_{3}^{2-} \)). This pair of reactions shows the interconvertible nature of conjugate acid-base pairs:
\[ \mathrm{HSO}_{3}^{-} \text{ (as a base)} + \mathrm{H_2O} \rightarrow \mathrm{H_2SO}_{3} \text{ (conjugate acid)} + \mathrm{OH}^{-} \]
\[ \mathrm{HSO}_{3}^{-} \text{ (as an acid)} + \mathrm{H_2O} \rightarrow \mathrm{SO}_{3}^{2-} \text{ (conjugate base)} + \mathrm{H_3O}^{+} \]
The understanding of conjugate acid-base pairs helps students predict the direction of acid-base reactions and comprehend their equilibrium dynamics.