Problem 16 - Chemistry The Molecular Science

Question

Which are conjugate acid-base pairs? (a) \(\mathrm{O}^{2-}\) and \(\mathrm{H}_{3} \mathrm{O}^{+}\) (b) \(\mathrm{H}_{3} \mathrm{O}^{+}\) and \(\mathrm{O}^{2-}\) (c) \(\mathrm{NH}_{2}^{-}\) and \(\mathrm{NH}_{3}\) (d) \(\mathrm{NH}_{3}\) and \(\mathrm{NH}_{4}^{+}\) (e) \(\mathrm{O}^{2-}\) and \(\mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

Pairs (c) and (d) are conjugate acid-base pairs.

Step 1: Understand Conjugate Acid-Base Pairs

A conjugate acid-base pair consists of two species that differ by a proton ( H^+ ). The acid is the species with the extra proton, whereas the base is the species that loses a proton. When an acid loses a proton, it becomes the conjugate base, and when a base gains a proton, it becomes the conjugate acid.

Step 2: Analyze Each Option

Let's evaluate each given pair to determine if they conform to the concept of conjugate acid-base pairs as defined in Step 1.

Step 3: Assess Pair (a)

Pair (a) is O^{2-} and H_3O^+ . The molecules differ by much more than one proton, and thus they are not conjugate acid-base pairs.

Step 4: Assess Pair (b)

Pair (b) is H_3O^+ and O^{2-} . This is the same as pair (a) but reversed. They do not differ by just a single proton, so they are not conjugate acid-base pairs.

Step 5: Assess Pair (c)

Pair (c) is NH_2^- and NH_3 . NH_3 can gain a proton ( H^+ ) to form NH_4^+ , but NH_2^- and NH_3 differ by a single proton with NH_2^- being the conjugate base of NH_3 , making them a conjugate pair.

Step 6: Assess Pair (d)

Pair (d) is NH_3 and NH_4^+ . NH_4^+ is a protonated form of NH_3 . They differ by exactly one proton, making them a conjugate acid-base pair.

Step 7: Assess Pair (e)

Pair (e) is O^{2-} and H_2O . The oxyanion O^{2-} would need to gain two protons to become water, thus O^{2-} and H_2O do not differ by one proton, making them not a conjugate acid-base pair.

Step 8: Conclusion

From the analysis, pairs (c) and (d) are conjugate acid-base pairs. In these pairs, each differs by a single proton ( H^+ ), which is consistent with the definition of conjugate acid-base pairs.

Key Concepts

Acid-Base Chemistry

Acid-base chemistry is a fundamental concept in chemistry that explores how acids and bases interact with each other. It revolves around the idea of proton transfer, where acids donate protons ( H^+ ) and bases accept them. This interaction can be better understood through the Bronsted-Lowry theory, which defines acids as proton donors and bases as proton acceptors.

When an acid loses a proton, it turns into its conjugate base; conversely, when a base gains a proton, it becomes its conjugate acid. This concept is essential in understanding chemical reactions, as it helps predict the outcome when acids and bases come into contact with each other. For instance, when HCl (an acid) reacts with NaOH (a base), H_2O and NaCl are formed due to the proton exchange.

Understanding acid-base chemistry allows us to predict how substances will behave in a variety of environments, from industrial processes to biological systems.

Proton Transfer

Proton transfer is a critical aspect of many chemical reactions, particularly those involving acids and bases. In simple terms, it is the movement of a proton from an acid to a base. This transfer results in the formation of new conjugate acid-base pairs.

For example, when NH_3 accepts a proton from H_2O , it transforms into NH_4^+ , turning water into its conjugate base, the hydroxide ion ( OH^- ). This type of exchange is the key operation in any acid-base reaction, facilitating transformations that lead to new products.

Proton transfer not only drives these reactions but also influences numerous natural and engineered processes, such as the regulation of pH in our bodies or the functioning of batteries.

NH3 and NH4+ Conjugate Pair

The conjugate pair of ammonia ( NH_3 ) and ammonium ion ( NH_4^+ ) is a classic example of acid-base chemistry in action. NH_3 acts as a base because it can accept a proton. When it does accept a proton, it becomes NH_4^+ , making NH_4^+ the conjugate acid.

This proton transfer is reversible, demonstrating dynamic equilibrium between the two species in solution. The significance of this pair lies in its role in biological and chemical systems. In the environment, this system helps regulate nitrogen levels, and in living organisms, it plays a role in cellular metabolism.

• Ammonia ( NH_3 ): Proton acceptor, turns into ammonium.
• Ammonium ( NH_4^+ ): Proton donor, turns back into ammonia when it loses a proton.

The ability of NH_3 to switch between its base and acid forms exemplifies conjugate acid-base pairs' importance for maintaining chemical balance.

NH2- and NH3 Conjugate Pair

The pair of amide ion ( NH_2^- ) and ammonia ( NH_3 ) also represents an important conjugate acid-base relationship. In this case, NH_2^- acts as a base by accepting a proton to become NH_3 , its conjugate acid.

This conversion illustrates how bases are linked to their conjugate acids via proton transfer. This reaction is crucial because it demonstrates the versatility of nitrogen-containing compounds in proton exchange processes.

• Amide ion ( NH_2^- ): Conjugate base, becomes ammonia by gaining a proton.
• Ammonia ( NH_3 ): Conjugate acid, reverts back to amide ion when it loses a proton.

Understanding this pair is crucial in fields like organic chemistry and biochemistry, where nitrogen-containing groups frequently undergo proton exchanges to drive reactions.