Question

Which of the following do not 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. HI \(, \mathrm{I}^{-}\) b. \(\mathrm{HClO}, \mathrm{HClO}_{2}\) c. \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{PO}_{4}^{3-}\) d. \(\mathrm{H}_{2} \mathrm{CO}_{3}, \mathrm{CO}_{3}^{2-}\)

Short answer

The pairs a, c, and d are not conjugate acid-base pairs. The correct conjugate acid-base pairs for each species are: a. (HI, I⁻) - Conjugate acid-base pair b. (HClO, ClO⁻) and (HClO₂, ClO₂⁻) c. (H₃PO₄, H₂PO₄⁻) and (HPO₄²⁻, PO₄³⁻) d. (H₂CO₃, HCO₃⁻) and (HCO₃⁻, CO₃²⁻)

Step-by-step solution

Analyzing the pair

HI is an acid that can donate a proton H⁺ to I⁻ to form I²⁻. I⁻ is thus the conjugate base of the acid HI. Therefore, HI and I⁻ represent a conjugate acid-base pair. b. HClO, HClO₂

Analyzing the pair

In this case, HClO₂ has one more proton (H⁺) compared to HClO. However, these two species are not a conjugate acid-base pair because they vary by nitrogen rather than a proton (H⁺). We can write the correct conjugate acid-base pair for each species as follows: For HClO: Acid - HClO, Conjugate Base - ClO⁻ For HClO₂: Acid - HClO₂, Conjugate Base - ClO₂⁻ c. H₃PO₄, PO₄³⁻

Analyzing the pair

H₃PO₄ is an acid that can donate a proton (H⁺) to form H₂PO₄⁻. Since the given ion PO₄³⁻ is not formed by the loss of a single proton (H⁺) from H₃PO₄, these are not a conjugate pair. The correct conjugate pairs are as follows: For H₃PO₄: Acid - H₃PO₄, Conjugate Base - H₂PO₄⁻ For PO₄³⁻: Acid - HPO₄²⁻, Conjugate Base - PO₄³⁻ d. H₂CO₃, CO₃²⁻

Analyzing the pair

Here, H₂CO₃ is an acid that can donate a proton (H⁺) to form HCO₃⁻. Since the given ion, CO₃²⁻, is formed by the loss of two protons (H⁺) from H₂CO₃, these are not a conjugate acid-base pair. The correct conjugate pairs are as follows: For H₂CO₃: Acid - H₂CO₃, Conjugate Base - HCO₃⁻ For CO₃²⁻: Acid - HCO₃⁻, Conjugate Base - CO₃²⁻

Key concepts

Acid-Base Chemistry

Acid-base chemistry is a fundamental concept that encompasses the reactions between acids and bases. In essence, it involves the exchange of protons (hydrogen ions, H⁺) from acids to bases. Acids are substances that can donate protons, whereas bases are substances that can accept them. When an acid loses a proton, it becomes a base, known as the conjugate base of the original acid. Conversely, when a base gains a proton, it becomes an acid, called the conjugate acid of the original base.

Understanding the behavior of acids and bases is crucial for a variety of chemical processes, ranging from industrial applications to biological systems. Even the water we drink is subjected to acid-base interactions, which affect its pH level and, consequently, its suitability for consumption. The identification of conjugate acid-base pairs, as seen in the exercise provided, is a key skill in mastering acid-base chemistry.

Proton Transfer Reactions

Proton transfer reactions are at the heart of acid-base chemistry, characterized by the movement of protons from one substance to another. These reactions are also known as 'proton hopping' and are a type of chemical reaction that involves the transfer of a proton from an acid to a base. A classic example is the reaction between hydrogen chloride (HCl) and water, where HCl donates a proton to water, forming hydronium ions (H₃O⁺) and chloride ions (Cl⁻).

In the context of the exercise, we analyzed various pairs to determine if they were conjugate acid-base pairs. This involved checking whether the species differ by exactly one proton. Not all given pairs met this criterion, highlighting the necessity to discern subtle differences between molecular structures to accurately predict and understand proton transfer reactions.

Brønsted-Lowry Theory

The Brønsted-Lowry theory is a widely accepted theory of acids and bases formulated by Johannes Brønsted and Thomas Lowry. This theory defines an acid as a proton donor and a base as a proton acceptor. It expands upon previous theories by accounting for the role of proton transfer in acid-base reactions and provides a broader perspective compared to other theories such as the Arrhenius theory, which limits acids and bases to aqueous solutions.

According to the Brønsted-Lowry theory, the chemical reaction between an acid and base results in the formation of a conjugate acid-base pair, which ties into the exercise where the concept was practically applied. It emphasized the importance of recognizing conjugate pairs and showed that not all species that differ in one atom, or even one proton, necessarily constitute a conjugate acid-base pair, reinforcing the need to closely examine the molecular changes that occur during proton transfer.