Problem 16 - Chemistry The Central Science

Question

(a) Give the conjugate base of the following Bronsted-Lowry acids: (i) \(\mathrm{HCOOH},\) (ii) \(\mathrm{HPO}_{4}^{2-}\). (b) Give the conjugate acid of the following Bronsted-Lowry bases: (i) \(\mathrm{SO}_{4}^{2-}\), (ii) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\)

Step-by-Step Solution

Answer

(a) (i) \(\mathrm{HCOO}^-\), (ii) \(\mathrm{PO}_{4}^{3-}\) (b) (i) \(\mathrm{HSO}_{4}^-\), (ii) \(\mathrm{CH}_{3} \mathrm{NH}_{3}^+\)

Step 1: (a) Conjugate Base of HCOOH

To find the conjugate base of \(\mathrm{HCOOH}\), remove a proton (\(\mathrm{H}^{+}\)) from the molecule: \(\mathrm{HCOOH} - \mathrm{H}^+ = \mathrm{HCOO}^-\). Therefore, the conjugate base is \(\mathrm{HCOO}^-\).

Step 2: (a) Conjugate Base of HPO4^2-

To find the conjugate base of \(\mathrm{HPO}_{4}^{2-}\), remove a proton (\(\mathrm{H}^{+}\)) from the species: \(\mathrm{HPO}_{4}^{2-} - \mathrm{H}^+ = \mathrm{PO}_{4}^{3-}\). Therefore, the conjugate base is \(\mathrm{PO}_{4}^{3-}\).

Step 3: (b) Conjugate Acid of SO4^2-

To find the conjugate acid of \(\mathrm{SO}_{4}^{2-}\), add a proton (\(\mathrm{H}^{+}\)) to the species: \(\mathrm{SO}_{4}^{2-} + \mathrm{H}^+ = \mathrm{HSO}_{4}^-\). Therefore, the conjugate acid is \(\mathrm{HSO}_{4}^-\).

Step 4: (b) Conjugate Acid of CH3NH2

To find the conjugate acid of \(\mathrm{CH}_{3} \mathrm{NH}_{2}\), add a proton (\(\mathrm{H}^{+}\)) to the molecule: \(\mathrm{CH}_{3} \mathrm{NH}_{2} + \mathrm{H}^+ = \mathrm{CH}_{3} \mathrm{NH}_{3}^+\). Therefore, the conjugate acid is \(\mathrm{CH}_{3} \mathrm{NH}_{3}^+\).

Key Concepts

Bronsted-Lowry Theory

The Bronsted-Lowry theory is a fundamental concept in the study of chemistry, particularly when it comes to understanding how acids and bases behave. According to this theory, an acid is a substance that can donate a proton (hydrogen ion, \(H^+\)), and a base is a substance that can accept a proton.

This theory allows us to view the behavior of acids and bases not just as static entities, but as participants in a dynamic process of proton exchange. This is a critical shift from the more traditional definition of acids and bases that focused only on the presence of hydrogen and hydroxide ions.

When we talk about a substance being an acid or a base, we're referring to its ability to donate or accept protons in a reaction. The flexibility of the theory also means it can be applied to a wider range of substances, including those that don't contain hydroxyl (\(OH^-\)) or hydrogen ions but can still participate in proton transfer.

Acid-Base Reactions

Acid-base reactions are at the heart of chemistry. They involve the transfer of protons from one species to another and are often depicted in the formation of water from hydrogen and hydroxide ions. In an acid-base reaction according to the Bronsted-Lowry theory, two conjugate pairs are formed—the conjugate acid of the base and the conjugate base of the acid.

Understanding the nature of acid-base reactions is crucial for predicting the outcome of chemical reactions, and it's especially important in fields such as biochemistry, where these reactions are prevalent in metabolic pathways. In our textbook example, the acid-base reactions are shown as simple proton transfers, demonstrating the change in charge and composition as protons are added or removed.

Conjugate Pairs

Conjugate pairs form the backbone of the Bronsted-Lowry approach to acid-base chemistry. A conjugate pair consists of two species that transform into one another by the gain or loss of a proton. In every acid-base reaction, an acid donates a proton to the base, resulting in the formation of their respective conjugates—the conjugate base of the acid and the conjugate acid of the base.

• For a given acid, its conjugate base is formed by the removal of a proton.
• Conversely, for a given base, its conjugate acid is formed by the addition of a proton.

The concept of conjugate pairs emphasizes the reversibility of acid-base reactions and helps in the understanding of chemical equilibrium. The example given in the exercise beautifully illustrates conjugate pairs in action, urging students to think in terms of these reacting pairs rather than in isolation.

Proton Transfer Reactions

Proton transfer reactions are a type of chemical reaction where a proton (\(H^+\)) is transferred from one molecule to another. These reactions are the core of acid-base chemistry under the Bronsted-Lowry theory. Proton transfer is crucial as it can significantly affect the physical and chemical properties of the substances involved.

In our textbook solution, we see direct examples of proton transfer: the acid (\(HCOOH\)) loses a proton to become the conjugate base (\(HCOO^-\)), while the base (\(CH_3NH_2\)) gains a proton to become its conjugate acid (\(CH_3NH_3^+\)). These reactions are fundamental to many biological systems and industrial processes. The understanding of proton transfer reactions provides insight into the strength of acids and bases, which can be quantified by their ability to donate or accept protons in these reactions.