Problem 35 - Chemistry

Question

For each of the following aqueous reactions, identify the acid, the base, the conjugate base, and the conjugate acid. a. \(\mathrm{H}_{2} \mathrm{O}+\mathrm{H}_{2} \mathrm{CO}_{3} \rightleftharpoons \mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{HCO}_{3}\) b. \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}+\mathrm{H}_{3} \mathrm{O}^{+}\) c. \(\mathrm{HCO}_{3}^{-}+\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+} \rightleftharpoons \mathrm{H}_{2} \mathrm{CO}_{3}+\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\)

Step-by-Step Solution

Answer

a. Acid: \(\mathrm{H}_{2} \mathrm{CO}_{3}\), Base: \(\mathrm{H}_{2} \mathrm{O}\), Conjugate Base: \(\mathrm{HCO}_{3}^{-}\), Conjugate Acid: \(\mathrm{H}_{3} \mathrm{O}^{+}\) b. Acid: \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}\), Base: \(\mathrm{H}_{2} \mathrm{O}\), Conjugate Base: \(\mathrm{C}_{5} \mathrm{H}_{5}\mathrm{N}\), Conjugate Acid: \(\mathrm{H}_{3} \mathrm{O}^{+}\) c. Acid: \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}\), Base: \(\mathrm{HCO}_{3}^{-}\), Conjugate Base: \(\mathrm{C}_{5} \mathrm{H}_{5}\mathrm{N}\), Conjugate Acid: \(\mathrm{H}_{2} \mathrm{CO}_{3}\)

Step 1: Identify the acid

As an acid is a substance that donates a proton, we can identify \(\mathrm{H}_{2} \mathrm{CO}_{3}\) as the acid as it donates a proton to \(\mathrm{H}_{2} \mathrm{O}\). -

Step 2: Identify the base

A base is a substance that accepts a proton, so \(\mathrm{H}_{2} \mathrm{O}\) is the base as it accepts a proton from \(\mathrm{H}_{2} \mathrm{CO}_{3}\). -

Step 3: Identify the conjugate base

The conjugate base is the species that the acid forms after donating its proton, which in this case is \(\mathrm{HCO}_{3}^{-}\). -

Step 4: Identify the conjugate acid

The conjugate acid is the species that the base forms after accepting the proton, which in this case is \(\mathrm{H}_{3} \mathrm{O}^{+}\). b. $\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}+\mathrm{H}_{3} \mathrm{O}^{+}$ -

Step 5: Identify the acid

Here, the acid is \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}\) as it donates a proton to \(\mathrm{H}_{2} \mathrm{O}\). -

Step 6: Identify the base

The base in this reaction is \(\mathrm{H}_{2} \mathrm{O}\) as it accepts the proton from \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}\). -

Step 7: Identify the conjugate base

The conjugate base is the species that the acid forms after donating its proton, which in this case is \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\). -

Step 8: Identify the conjugate acid

The conjugate acid is the species that the base forms after accepting the proton, which in this case is \(\mathrm{H}_{3} \mathrm{O}^{+}\). c. $\mathrm{HCO}_{3}^{-}+\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+} \rightleftharpoons \mathrm{H}_{2} \mathrm{CO}_{3}+\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}$ -

Step 9: Identify the acid

In this reaction, the acid is \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}\) as it donates a proton to \(\mathrm{HCO}_{3}^{-}\). -

Step 10: Identify the base

The base is \(\mathrm{HCO}_{3}^{-}\) as it accepts a proton from \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}\). -

Step 11: Identify the conjugate base

The conjugate base is the species that the acid forms after donating its proton, which in this case is \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\). -

Step 12: Identify the conjugate acid

The conjugate acid is the species that the base forms after accepting the proton, which in this case is \(\mathrm{H}_{2} \mathrm{CO}_{3}\).

Key Concepts

Acid and Base Identification

Understanding how to identify acids and bases in a reaction is essential for students studying chemistry. An acid can be defined as a substance that donates a proton (H⁺), while a base is a substance that accepts a proton.

Looking at the reactions presented in the exercise, we can see that the acid and base are identified by their ability to donate and accept protons, respectively. For example, in reaction a, \(\mathrm{H}_{2}\mathrm{CO}_{3}\) is identified as the acid because it donates a proton to water (\(\mathrm{H}_{2}\mathrm{O}\)), which acts as the base. It's important to note that water is often involved in acid-base reactions and can act as either an acid or a base, a phenomenon known as amphoteric behavior.

This identification is crucial for understanding the rest of the acid-base reactions. Once students grasp this concept, they can use it to identify the roles of different compounds in various chemical reactions, providing a foundation for more complex chemistry concepts.

Conjugate Acid-Base Pairs

In an acid-base reaction, the conjugate acid and the conjugate base are two sides of the same coin. A conjugate base is formed when an acid loses a proton, whereas a conjugate acid is formed when a base gains a proton.

For instance, when \(\mathrm{H}_{2}\mathrm{O}\) accepts a proton from \(\mathrm{H}_{2}\mathrm{CO}_{3}\), it forms \(\mathrm{H}_{3}\mathrm{O}^{+}\), its conjugate acid. Conversely, after \(\mathrm{H}_{2}\mathrm{CO}_{3}\) loses a proton, it becomes \(\mathrm{HCO}_{3}^{-}\), its conjugate base. Here, \(\mathrm{H}_{2}\mathrm{O}\) and \(\mathrm{H}_{3}\mathrm{O}^{+}\), and \(\mathrm{H}_{2}\mathrm{CO}_{3}\) and \(\mathrm{HCO}_{3}^{-}\) form two conjugate acid-base pairs.

The concept of conjugate acid-base pairs helps students understand that acid-base reactions are reversible and that the roles of acids and bases can switch depending on the direction of the reaction. This interplay is fundamental in chemical equilibrium and various biological systems where buffer solutions play a role.

Proton Transfer Reactions

Proton transfer reactions are the heart of acid-base chemistry. These reactions involve the transfer of a proton from an acid to a base. The direction in which the proton is transferred determines which substances are the acids and bases.

For example, in reaction b, \(\mathrm{C}_{5}\mathrm{H}_{5}\mathrm{NH}^{+}\) donates a proton to water, so it is acting as the acid, and water is the base. This transfer of a proton is what forms new substances: the conjugate base \(\mathrm{C}_{5}\mathrm{H}_{5}\mathrm{N}\) and the conjugate acid \(\mathrm{H}_{3}\mathrm{O}^{+}\).

To help students improve their understanding of proton transfer reactions, it is helpful to visualize the reaction as a two-way street, where the proton can 'move' from the acid to the base and vice versa. This bidirectional nature is pivotal when explaining why these reactions are reversible and stressing the dynamic equilibrium that often exists in acid-base chemistry. Such visualization aids in grasping the concept that chemical reactions are not always one-way processes but instead can reach a state where reactants and products exist in balance.