Question

Which of the following form a conjugate acid-base pair? For those pairs that are not conjugate acid-base pairs, write the correct conjugate acid-base pair for each species. a. \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{PO}_{4}^{3-}\) b. \(HBr, BrO\) c.\( HCN, CN -\) d. \(HF, F"\)

Short answer

The correct conjugate acid-base pairs are: a. \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) and \(\mathrm{H}\mathrm{PO}_{4}^{2-}, \mathrm{PO}_{4}^{3-}\) b. \(HBr, Br^{-}\) and \(HBrO, BrO^{-}\) c. \(HCN, CN^-\) d. \(HF, F^-\)

Step-by-step solution

Identify Conjugate Acid-Base Pairs

For each given pair, compare the species and determine if they only differ by one proton (H+). a. \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{PO}_{4}^{3-}\) Here, the difference between the two species is 3 protons. So, this is not a conjugate acid-base pair. b. \(HBr, BrO\) Here, the difference between the species is not only in protons but also in the presence of an oxygen atom. So, this is not a conjugate acid-base pair. c. \(HCN, CN^-\) The difference between the two species is exactly 1 proton (H+). So, this is a conjugate acid-base pair. d. \(HF, F^-\) Here, the difference between the two species is exactly 1 proton (H+). So, this is a conjugate acid-base pair.

Correct the Non-Conjugate Pairs

For those pairs that are not conjugate acid-base pairs, write the correct conjugate acid-base pair for each species. a. \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{PO}_{4}^{3-}\) The correct conjugate base for \(\mathrm{H}_{3} \mathrm{PO}_{4}\) is \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\), and the correct conjugate acid for \(\mathrm{PO}_{4}^{3-}\) is \(\mathrm{H}\mathrm{PO}_{4}^{2-}\). The corrected pair is: \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) and \(\mathrm{H}\mathrm{PO}_{4}^{2-}, \mathrm{PO}_{4}^{3-}\). b. \(HBr, BrO\) The correct conjugate base for \(HBr\) is \(Br^{-}\), and the correct conjugate acid for \(BrO\) is \(HBrO\). The corrected pair is: \(HBr, Br^{-}\) and \(HBrO, BrO^{-}\). The exercise is now complete. The correct conjugate acid-base pairs are: a. \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) and \(\mathrm{H}\mathrm{PO}_{4}^{2-}, \mathrm{PO}_{4}^{3-}\) b. \(HBr, Br^{-}\) and \(HBrO, BrO^{-}\) c. \(HCN, CN^-\) d. \(HF, F^-\)

Key concepts

Acid-Base Chemistry

Acid-base chemistry is a fundamental concept in chemistry that involves the transfer of protons (H+) between reactants. It's central to understanding chemical reactions, especially those in aqueous solutions. According to the Arrhenius definition, an acid is a substance that increases the concentration of hydrogen ions (H+) in a solution, while a base increases the concentration of hydroxide ions (OH-).

However, this definition was broadened by the Bronsted-Lowry theory, which identifies acids as proton donors and bases as proton acceptors. This theory is particularly powerful because it explains acid-base reactions in both aqueous and non-aqueous systems. For students tackling acid-base exercises, it's crucial to look for species that differ by a single proton to identify a conjugate acid-base pair. If you're facing a problem, always check the number of protons that each species can donate or accept, and remember the role of water as a potential partner in these reactions.

Bronsted-Lowry Acids and Bases

The Bronsted-Lowry theory further refines our understanding of acids and bases. Under this theory, an acid is a substance that can donate a proton (H+), and a base is a substance that can accept a proton. The conjugate base is what remains after an acid has donated a proton, and conversely, the conjugate acid is formed when a base accepts a proton.

This pairing of an acid with its conjugate base, or a base with its conjugate acid, is known as a conjugate acid-base pair. This concept is vital in solving exercises about acid-base pairs. For instance, when identifying if two species are conjugate acid-base pairs, the presence or absence of that extra proton is what defines their relationship. If the difference is not just one proton, then further analysis is needed to find the correct conjugate pairs.

Proton Transfer Reactions

Proton transfer reactions are at the core of acid-base chemistry. They involve the movement of protons from one species (the acid) to another (the base). When a proton is transferred, the acid becomes its conjugate base, and the base becomes its conjugate acid, forming a conjugate acid-base pair.

For students seeking clarity on this concept, it's helpful to visualize a proton hopping from one molecule to another. Every proton transfer reaction in an aqueous solution includes water as either the donating or accepting species. Understanding how these reactions work helps with predicting the outcome of acid-base reactions and writing balanced chemical equations for such processes. Remember that acid-base reactions can be reversible, and equilibrium can be established between the acid and conjugate base or the base and conjugate acid.