Question

Which of the following do \(n o t\) represent 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 in the pair. a. \(\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{SO}_{4}^{2-}\) b. \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}, \mathrm{HPO}_{4}^{2-}\) c. \(\mathrm{HClO}_{4}, \mathrm{Cl}^{-}\) d. \(\mathrm{NH}_{4}^{+}, \mathrm{NH}_{2}^{-}\)

Short answer

The incorrect conjugate acid-base pairs are: a. H2SO4 and SO4^2- (correct pairs: H2SO4 and HSO4^-, SO4^2- and HSO4^-) c. HClO4 and Cl^- (correct pairs: HClO4 and ClO4^-, Cl^- and HCl) d. NH4^+ and NH2^- (correct pairs: NH4^+ and NH3, NH2^- and NH3^-) The only correct conjugate acid-base pair is: b. H2PO4^- and HPO4^2-

Step-by-step solution

a. Analyzing the H2SO4, SO4^2- pair

The given species are H2SO4 and SO4^2-. Let's compare their molecular formulas. By removing a proton (H+) from H2SO4, we get HSO4^-, not SO4^2-. So, they are not a conjugate acid-base pair.

a. Writing the correct conjugate acid-base pair

For H2SO4, the conjugate base would be HSO4^-, and for SO4^2- the conjugate acid would be HSO4^-. So the correct conjugate acid-base pairs are: - H2SO4 and HSO4^- - SO4^2- and HSO4^-

b. Analyzing the H2PO4^-, HPO4^2- pair

The given species are H2PO4^- and HPO4^2-. Let's compare their molecular formulas. By removing one proton (H+) from H2PO4^-, we get HPO4^2-. So, they are a conjugate acid-base pair.

c. Analyzing the HClO4, Cl^- pair

The given species are HClO4 and Cl^-. Let's compare their molecular formulas. By removing a proton (H+) from HClO4, we get ClO4^-, not Cl^-. So, they are not a conjugate acid-base pair.

c. Writing the correct conjugate acid-base pair

For HClO4, the conjugate base would be ClO4^-, and for Cl^- the conjugate acid would be HCl. So the correct conjugate acid-base pairs are: - HClO4 and ClO4^- - Cl^- and HCl

d. Analyzing the NH4^+, NH2^- pair

The given species are NH4^+ and NH2^-. Let's compare their molecular formulas. By removing one proton (H+) from NH4^+, we get NH3, not NH2^-. So, they are not a conjugate acid-base pair.

d. Writing the correct conjugate acid-base pair

For NH4^+, the conjugate base would be NH3, and for NH2^- the conjugate acid would be NH3^-. So the correct conjugate acid-base pairs are: - NH4^+ and NH3 - NH2^- and NH3^-

Key concepts

Acid-Base Chemistry

Acid-base chemistry is a foundational concept in chemistry that explores the reactions between acids and bases. These reactions are essential to understand because they are ubiquitous, not only in laboratories but also in biological systems and our everyday lives.

An acid is a substance that can donate a proton (a hydrogen ion, H+), whereas a base is a substance that can accept a proton. The strength of acids and bases is variable, with strong acids completely dissociating in water, while weak acids partially dissociate. Similarly, strong bases dissociate fully, whereas weak bases do not.

In the context of our exercise, when an acid donates a proton, it becomes its conjugate base; conversely, when a base accepts a proton, it becomes its conjugate acid. These two species form what is known as a 'conjugate acid-base pair' and are directly related through the gain or loss of a proton.

For example, when HCl (hydrochloric acid) donates a proton, it forms Cl- (chloride ion), its conjugate base. The reverse reaction would involve Cl- accepting a proton, turning it back into HCl. This demonstrates the reversible nature of acid-base reactions and emphasizes the importance of understanding conjugate pairs in predicting the outcomes of such reactions.

Brønsted-Lowry Acids and Bases

The Brønsted-Lowry theory defines an acid as a proton donor and a base as a proton acceptor. This theory enhances the understanding of acid-base reactions by focusing on the transfer of protons.

According to the Brønsted-Lowry concept, water can act as both an acid and a base, making it amphiprotic. This duality is demonstrated in reactions where water donates a proton to become hydroxide (OH-), or it can accept a proton to become hydronium (H3O+).

Identifying Conjugate Acid-Base Pairs

It's crucial to identify conjugate acid-base pairs correctly to comprehend the reaction dynamics. The exercise provided highlighted this by showing that certain pairs listed did not actually represent conjugate acid-base pairs because they did not differ by just a single proton. For instance, the pair NH4+, NH2- cannot be a conjugate pair since NH4+ would become NH3 after losing a proton, not NH2-.

The correct understanding of these pairs allows students to predict the course and products of acid-base reactions, which is a valuable skill in both academic studies and practical chemistry applications.

Molecular Formulas

Molecular formulas provide the exact number and type of atoms in a molecule. They are written as a combination of elemental symbols and numerical subscripts indicating the number of each atom. Molecular formulas are pivotal in studying chemical reactions, including acid-base reactions, as they allow us to visualize how atoms are reorganized during chemical processes.

For example, the molecular formula of sulfuric acid is H2SO4. When it loses a proton, the result is not SO42- as one might incorrectly assume by just removing the 'H' symbol. Instead, the correct product is HSO4-, which indicates that only one hydrogen ion was removed, and the remaining molecule is still composed of the same atoms with a single hydrogen less.

The careful interpretation of molecular formulas allows us to identify which species can correctly form conjugate acid-base pairs as they transform during the chemical reaction. In the exercise, it was critical to notice that the alteration of a molecular formula by adding or removing a proton results in a conjugate acid or base. This knowledge enables students to construct accurate representations of chemical reactions on a molecular level.