Question

Which of the following represent conjugate acid-base pairs? For those pairs that are not conjugates, write the correct conjugate acid or base for each species in the pair. a. \(\mathrm{H}_{2} \mathrm{O}, \mathrm{OH}^{-}\) b. \(\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{SO}_{4}^{2-}\) c. \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) d. \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}, \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\)

Short answer

The conjugate acid-base pairs are: a. \(\mathrm{H}_{2} \mathrm{O}, \mathrm{OH}^{-}\) c. \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) d. \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}, \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\) For pair b, the correct conjugate base for \(\mathrm{H}_{2}\mathrm{SO}_{4}\) is \(\mathrm{HSO}_{4}^{-}\).

Step-by-step solution

Identify conjugate acid-base pairs

To find the conjugate acid of a base, add \(\mathrm{H}^{+}\) to the species, and to find the conjugate base of an acid, remove \(\mathrm{H}^{+}\) from the species. a. \(\mathrm{H}_{2} \mathrm{O}, \mathrm{OH}^{-}\) Here, \(\mathrm{OH}^{-}\) is the conjugate base of water \(\mathrm{(H}_{2}\mathrm{O)}\), as it differs by the presence or absence of one hydrogen ion. b. \(\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{SO}_{4}^{2-}\) Here, \(\mathrm{SO}_{4}^{2-}\) is not the conjugate base of \(\mathrm{H}_{2}\mathrm{SO}_{4}\). The correct conjugate base for \(\mathrm{H}_{2}\mathrm{SO}_{4}\) would be \(\mathrm{HSO}_{4}^{-}\) as it differs by the presence or absence of one hydrogen ion. c. \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) In this pair, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) is the conjugate base for \(\mathrm{H}_{3} \mathrm{PO}_{4}\), as they differ by the presence or absence of one hydrogen ion. d. $\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}, \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}$ Similarly, \(\mathrm{C}_{2}\mathrm{H}_{3} \mathrm{O}_{2}^{-}\) is the conjugate base for \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\), as they differ by the presence or absence of one hydrogen ion. As a conclusion, the given pairs in a, c, and d are conjugate acid-base pairs, while the given pair in b is not a conjugate acid-base pair.

Key concepts

Acid-Base Chemistry

Acid-base chemistry is a vital section of chemistry that deals with the reactions and mechanisms through which acids and bases interact. In simple terms, an acid is any species that can donate a proton (\(\mathrm{H}^+\)), while a base is any species that can accept a proton. This interaction is fundamental in aqueous solutions, impacting pH balance, biochemical reactions, and industrial processes. Many substances can act as either acids or bases, depending on the context of the reaction, and understanding this concept is crucial for predicting possible chemical outcomes.

In water, for example, the interaction of acids and bases can be seen as the \(\mathrm{H}^+\) ion transfers from one molecule to another. This forms the foundation for understanding conjugate acid-base pairs, which are directly involved in these interactions. Real-world applications of acid-base chemistry include titrations used to determine concentrations of acidic or basic solutions, and buffering systems, which keep biological and industrial systems at a specific pH range.

Conjugate Acids

A conjugate acid is what you get when a base gains a hydrogen ion. It is the acid member of a conjugate acid-base pair. For instance, when the base hydroxide ion ( \(\mathrm{OH}^{-}\) ) accepts a hydrogen ion, it becomes water (\(\mathrm{H}_{2}\mathrm{O}\)), forming its conjugate acid.

Here's how to identify a conjugate acid:

• Look for the original base that accepts a hydrogen ion.
• Add \(\mathrm{H}^+\) to this base.

In example solutions, consider acetic acid (\(\mathrm{HC}_{2}\mathrm{H}_{3}\mathrm{O}_{2}\)). The acetate ion (\(\mathrm{C}_{2}\mathrm{H}_{3}\mathrm{O}_{2}^{-}\)) would gain a hydrogen ion to form acetic acid, thereby becoming its conjugate acid. This concept is pivotal for understanding how acids can react under different conditions, and forms the crux of acid-base reaction mechanisms.

Conjugate Bases

The conjugate base is the species left over when an acid donates a hydrogen ion. It's essentially what remains of the original acid after the proton is removed. Each acid has a conjugate base, which is crucial in maintaining the equilibrium in acid-base reactions.

To identify a conjugate base:

• Start with the acid molecule.
• Remove one \(\mathrm{H}^+\) from it.

For instance, sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\)) loses a hydrogen ion and forms its conjugate base, \(\mathrm{HSO}_{4}^{-}\). In the given exercise, identifying \(\mathrm{C}_{2}\mathrm{H}_{3}\mathrm{O}_{2}^{-}\) as the conjugate base of acetic acid helps one understand how acids transform during reactions. These conversions are key in both lab environments and natural processes, determining how substances behave in the presence of acids or bases.

Hydrogen Ion Transfer

Hydrogen ion transfer, also known as proton transfer, is a foundational process in acid-base reactions. It involves the movement of \(\mathrm{H}^{+}\) ions from one molecule to another, and is essential for creating conjugate pairs. This transfer is what differentiates acids from bases in a solution, and it is crucial for maintaining chemical stability.

The key aspects of hydrogen ion transfer include:

• Acids donate \(\mathrm{H}^{+}\) ions.
• Bases accept \(\mathrm{H}^{+}\) ions.
• This transfer creates new bonds and alters electrical charges.

In the examples provided, water (\(\mathrm{H}_{2}\mathrm{O}\)) can act as an acid when it donates a hydrogen ion to form \(\mathrm{OH}^{-}\). Understanding this concept provides a clearer picture of how equilibrium in reactions is achieved, and why some reactions proceed faster than others due to easier \(\mathrm{H}^{+}\) transfers. This knowledge is valuable in a range of scientific and industrial applications, where control over chemical processes is essential.