Problem 4 - Chemistry and Chemical Reactivity

Question

What are the products of each of the following acid-base reactions? Indicate the acid and its conjugate base and the base and its conjugate acid. (a) \(\mathrm{HClO}_{4}+\mathrm{H}_{2} \mathrm{O} \rightarrow\) (b) \(\mathrm{NH}_{4}^{+}+\mathrm{H}_{2} \mathrm{O} \rightarrow\) (c) \(\mathrm{HCO}_{3}^{-}+\mathrm{OH}^{-} \rightarrow\)

Step-by-Step Solution

Answer

(a) \( \mathrm{ClO}_{4}^{-} + \mathrm{H}_{3} \mathrm{O}^{+} \); (b) \( \mathrm{NH}_{3} + \mathrm{H}_{3} \mathrm{O}^{+} \); (c) \( \mathrm{CO}_{3}^{2-} + \mathrm{H}_{2} \mathrm{O} \)."}

Step 1: Understand Acid-Base Reaction

In an acid-base reaction, an acid donates a proton (H⁺) to a base. The acid forms its conjugate base by losing a proton, and the base forms its conjugate acid by gaining a proton.

Step 2: Predict the Products for (a)

In the reaction \( \mathrm{HClO}_{4} + \mathrm{H}_{2} \mathrm{O} \rightarrow \), \( \mathrm{HClO}_{4} \) is the acid and donates a proton to \( \mathrm{H}_{2} \mathrm{O} \), which acts as the base. This forms \( \mathrm{ClO}_{4}^{-} \) (the conjugate base of the acid) and \( \mathrm{H}_{3} \mathrm{O}^{+} \) (the conjugate acid of the base).

Step 3: Predict the Products for (b)

In the reaction \( \mathrm{NH}_{4}^{+} + \mathrm{H}_{2} \mathrm{O} \rightarrow \), \( \mathrm{NH}_{4}^{+} \) is the acid and donates a proton to \( \mathrm{H}_{2} \mathrm{O} \). This results in the formation of \( \mathrm{NH}_{3} \) (the conjugate base of the acid) and \( \mathrm{H}_{3} \mathrm{O}^{+} \) (the conjugate acid of the base).

Step 4: Predict the Products for (c)

In the reaction \( \mathrm{HCO}_{3}^{-} + \mathrm{OH}^{-} \rightarrow \), \( \mathrm{HCO}_{3}^{-} \) acts as an acid by donating a proton to \( \mathrm{OH}^{-} \), which acts as a base. This forms \( \mathrm{CO}_{3}^{2-} \) (the conjugate base of the acid) and \( \mathrm{H}_{2} \mathrm{O} \) (the conjugate acid of the base).

Step 5: Identify Acid-Base Pairs

- For (a): Acid: \( \mathrm{HClO}_{4} \), Conjugate Base: \( \mathrm{ClO}_{4}^{-} \); Base: \( \mathrm{H}_{2} \mathrm{O} \), Conjugate Acid: \( \mathrm{H}_{3} \mathrm{O}^{+} \).- For (b): Acid: \( \mathrm{NH}_{4}^{+} \), Conjugate Base: \( \mathrm{NH}_{3} \); Base: \( \mathrm{H}_{2} \mathrm{O} \), Conjugate Acid: \( \mathrm{H}_{3} \mathrm{O}^{+} \).- For (c): Acid: \( \mathrm{HCO}_{3}^{-} \), Conjugate Base: \( \mathrm{CO}_{3}^{2-} \); Base: \( \mathrm{OH}^{-} \), Conjugate Acid: \( \mathrm{H}_{2} \mathrm{O} \).

Key Concepts

Conjugate Acid-Base Pairs

In acid-base reactions, the concept of conjugate acid-base pairs is fundamental. An acid reacts by giving up a proton, turning it into its conjugate base. Meanwhile, the base accepts this proton, transforming itself into its conjugate acid. This reciprocal transformation is what ties the acid and base into pairs.

For example, in the reaction between perchloric acid \( \mathrm{HClO}_{4} \) and water \( \mathrm{H}_{2} \mathrm{O} \), \( \mathrm{HClO}_{4} \) donates a proton and becomes \( \mathrm{ClO}_{4}^{-} \), its conjugate base. Simultaneously, \( \mathrm{H}_{2} \mathrm{O} \) accepts the proton, forming \( \mathrm{H}_{3} \mathrm{O}^{+} \), its conjugate acid. This transformation illustrates how these pairs are interrelated and shift between each role depending on the reaction context.

Understanding these relationships is crucial because they predict how substances interact during a reaction and determine how the reaction progresses. Once an acid has turned into its conjugate base, it can no longer donate a proton. Likewise, the newly formed conjugate acid is now capable of donating a proton in certain contexts, completing the cycle of proton exchange.

Proton Transfer

Proton transfer is the heart of acid-base reactions. It's the process where a proton (often represented as \( \mathrm{H}^{+} \)) moves from an acid to a base. This movement of protons alters the structure and reactivity of the involved species.

Consider the transfer in the reaction between ammonium ion \( \mathrm{NH}_{4}^{+} \) and water \( \mathrm{H}_{2} \mathrm{O} \). Here, \( \mathrm{NH}_{4}^{+} \) transfers a proton to \( \mathrm{H}_{2} \mathrm{O}, \) which transforms \( \mathrm{NH}_{4}^{+} \) into \( \mathrm{NH}_{3} \), and \( \mathrm{H}_{2} \mathrm{O} \) into \( \mathrm{H}_{3} \mathrm{O}^{+}.\)

This action exemplifies the departure of a proton from the acid, and its subsequent acceptance by the base, leading to new species being generated. Understanding this mechanism is vital because it explains how materials undergo change in a solution, providing insight into the nature of chemical reactions at the molecular level.

Chemical Reactions

Chemical reactions, especially acid-base ones, are about transformations. They involve the rearrangement of atoms and molecules. In acid-base reactions, these transformations revolve around protons hopping from one molecule to another.

The initial substances, known as reactants, undergo direct changes forming new substances called products. For acids and bases, these changes clear up the pathways for proton exchange. Take the reaction of bicarbonate \( \mathrm{HCO}_{3}^{-} \) with hydroxide \( \mathrm{OH}^{-} \) for instance. Bicarbonate gives a proton to hydroxide, creating carbonate \( \mathrm{CO}_{3}^{2-} \) and water.

Being comfortable with how acids and bases will react with one another allows one to anticipate the products. It also explains how even minor proton shifts can generate significant changes in the resulting substances. Mastery over this concept provides clarity to the intricate dance of protons involved in such reactions.

Reaction Products

The final goal of studying acid-base reactions is to understand and predict the resulting products. These products are the outcome of the proton transfer process and tell us about the reaction's nature.

In the acid-base reaction of \( \mathrm{HCO}_{3}^{-} \) with \( \mathrm{OH}^{-} \), the products are carbonate \( \mathrm{CO}_{3}^{2-} \) and water \( \mathrm{H}_{2} \mathrm{O} \). Each product directly corresponds to a reactant transformed by the exchange of a proton.

Keen insight into acid-base reactions allows one to know the outcomes before the reaction proceeds. Aligning acids with their conjugates provides an analytical route to discern what happens during the exchange. Comprehending reaction products is not only about understanding what came forth, but also why it happened—and what could result from altering initial conditions.