Problem 15 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) Give the conjugate base of the following Brønsted Lowry acids: (i) \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-},\) (ii) HBr. (b) Give the conjugate acid of the following Bronsted-Lowry bases: (i) \(\mathrm{CN}^{-},\) (ii) \(\mathrm{HSO}_{4}^{-}\).

Step-by-Step Solution

Answer

The conjugate bases of the given Brønsted-Lowry acids are: (i) \(\mathrm{HPO}_{4}^{2-}\) and (ii) \(\mathrm{Br}^{-}\). The conjugate acids of the given Brønsted-Lowry bases are: (i) \(\mathrm{HCN}\) and (ii) \(\mathrm{H}_{2}\mathrm{SO}_{4}\).

Step 1: (a) Conjugate base of H2PO4- (i)

To find the conjugate base of H2PO4-, we need to remove a proton (H+) from its structure. Removing a proton results in the following formula: \(\mathrm{HPO}_{4}^{2-}\). So the conjugate base of H2PO4- is HPO4²⁻.

Step 2: (a) Conjugate base of HBr (ii)

Similarly, to find the conjugate base of HBr, we need to remove a proton (H+) from its structure. Removing a proton results in the following formula: \(\mathrm{Br}^{-}\). So the conjugate base of HBr is Br⁻.

Step 3: (b) Conjugate acid of CN- (i)

To find the conjugate acid of CN-, we need to add a proton (H+) to its structure. Adding a proton results in the following formula: \(\mathrm{HCN}\). So the conjugate acid of CN- is HCN.

Step 4: (b) Conjugate acid of HSO4- (ii)

To find the conjugate acid of HSO4-, we need to add a proton (H+) to its structure. Adding a proton results in the following formula: \(\mathrm{H}_{2}\mathrm{SO}_{4}\). So the conjugate acid of HSO4- is H2SO4.

Key Concepts

Understanding Conjugate Acids

In the Brønsted-Lowry acid-base theory, a conjugate acid is what a base becomes when it accepts a proton (H⁺). Consider it in terms of a partnership where a base gains a proton.
Let's use the cyanide ion ( CN⁻ ) as an example. When CN⁻ accepts a proton, it transforms into its conjugate acid, HCN (hydrogen cyanide). In this change, the cyanide ion takes an H⁺ ion from its surroundings, altering its formula and nature to HCN .

Another illustration involves HSO_4^- , which transforms into its conjugate acid, H_2SO_4 , by accepting a proton. This change illustrates the dynamic interactions during acid-base reactions, where species transform by accepting protons.
Understanding this process is essential, as it helps to predict the outcomes of various chemical reactions, allowing chemists to control reaction pathways effectively.

Deciphering Conjugate Bases

The concept of conjugate bases in the context of Brønsted-Lowry theory focuses on what an acid becomes after it loses a proton (H⁺). Imagine an acid donating a part of itself — that's the proton removal story unfolding.
Take H_2PO_4^− , for instance. When it donates a proton, it evolves into its conjugate base, HPO_4^{2-} . This transformation underscores the reduction in positive charge and highlights the acid's capacity of donating protons.

Similarly, when hydrobromic acid (HBr) sheds its proton, it results in the formation of its conjugate base, Br^− . This showcases the inherent balance in proton transfer, aiding in maintaining equilibrium in natural and synthetic processes alike.
Gaining insight into conjugate bases helps one predict how substances will behave in different environments, especially in biological and environmental systems.

Exploring Proton Transfer Reactions

Proton transfer reactions lie at the heart of the Brønsted-Lowry acid-base framework. These reactions focus on the movement of protons (H⁺) from one molecule to another, driving the reactions that dictate chemical behavior.
In such reactions, an acid donates a proton to a base, evident with the reaction between HBr (an acid) and H_2O (a base), where HBr donates a proton to become a Br^- ion. This results in H_3O^+ , a conjugate acid of water.

Similarly, understanding how H_2PO_4^− transforms by losing a proton to form HPO_4^{2-} elucidates these elemental swaps. This ability to donate or accept protons is critical in many applications ranging from industrial to biological systems.

• They regulate pH in organisms by controlling acidic and basic conditions.
• Facilitate key enzymatic activities by ensuring correct conditions for biochemical reactions.

Understanding and manipulating proton transfer reactions are core to advancements in both scientific research and daily life applications.