Problem 15 - Chemistry: The Central Science

Question

(a) Give the conjugate base of the following Bronsted-Lowry acids: (i) \(\mathrm{HIO}_{3}\), (ii) \(\mathrm{NH}_{4}^{+}\). (b) Give the conjugate acid of the following Bronsted-Lowry bases: (i) \(\mathrm{O}^{2-}\), (ii) \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\).

Step-by-Step Solution

Answer

(a) Conjugate bases: (i) \(IO_3^{-}\), (ii) \(NH_3\) (b) Conjugate acids: (i) \(OH^{-}\), (ii) \(H_3PO_4\)

Step 1: Find the conjugate bases of given Bronsted-Lowry acids.

To find the conjugate base, remove a proton (H+) from the acid and write the molecular formula. (a) (i) \(HIO_3\) (Perchloric acid): Remove one proton to find the conjugate base. Conjugate base: \(IO_3^{-}\) (Iodate ion) (ii) \(NH_4^+\) (Ammonium ion): Remove one proton to find the conjugate base. Conjugate base: \(NH_3\) (Ammonia)

Step 2: Find the conjugate acids of given Bronsted-Lowry bases.

To find the conjugate acid, add a proton (H+) to the base and write the molecular formula. (b) (i) \(O^{2-}\) (Oxide ion): Add one proton to find the conjugate acid. Conjugate acid: \(OH^{-}\) (Hydroxide ion) (ii) \(H_2PO_4^{-}\) (Dihydrogen phosphate ion): Add one proton to find the conjugate acid. Conjugate acid: \(H_3PO_4\) (Phosphoric acid)

Key Concepts

Conjugate Base

In Bronsted-Lowry acid-base theory, the **conjugate base** is what remains after an acid donates a proton. This means that when an acid loses a hydrogen ion (H\(^+\)), it becomes its conjugate base.
For instance, in the case of **HIO\(_3\)**, when it donates a proton, it becomes **IO\(_3^{-}\)**, its conjugate base, the iodate ion. Similarly, **NH\(_4^+\)**, which is the ammonium ion, becomes **NH\(_3\)**, or ammonia, after losing a proton. This transformation demonstrates how acids and bases are linked in pairs called conjugate pairs.
Recognizing conjugate bases is crucial for understanding how substances can act in different chemical environments and is a cornerstone of chemical equilibrium.

Conjugate Acid

A **conjugate acid** is formed when a base gains a proton. This process highlights the reversible nature of proton exchange in the Bronsted-Lowry framework.
For example, the **oxide ion (O\(^{2-}\))** gains a proton to form **hydroxide ion (OH\(^-\))**, while **dihydrogen phosphate (H\(_2\)PO\(_4^{-}\))** becomes **phosphoric acid (H\(_3\)PO\(_4\))** after accepting a proton.

• This transformation is vital because it illustrates the dual nature of compounds, which can act either as acids or as bases depending on the reactions they undergo.
• Understanding conjugate acids helps predict the behavior of substances in acidic or basic solutions.

Proton Transfer

**Proton transfer** is at the heart of Bronsted-Lowry acid-base reactions. It involves the movement of protons between molecules, facilitating the transformation of an acid into its conjugate base and a base into its conjugate acid.
This transfer is crucial in biochemical processes and industrial applications, influencing pH levels, reaction rates, and equilibria.

• For instance, in the reaction where **HIO\(_3\)** donates a proton, this proton moves to another molecule or ion, allowing **IO\(_3^{-}\)** to form.
• Conversely, when **O\(^{2-}\)** gains a proton, it transforms into **OH\(^-\)**.

In nature and technology, proton transfer underpins many pivotal reactions, such as energy production in cells and the creation of pharmaceuticals.

Bronsted-Lowry Theory

The **Bronsted-Lowry theory** is a foundational concept in chemistry that explains acids and bases through their ability to donate or accept protons. This theory is broader than the classical concept since it doesn’t require the presence of water to define an acid or a base.
According to this theory:

• An **acid** is a proton donor.
• A **base** is a proton acceptor.

Using this definition, many reactions, including those in non-aqueous solutions, can be understood and predicted.
The Bronsted-Lowry theory enriches our comprehension of chemical reactions by characterizing substances not solely based on pH but on their dynamic potential to exchange protons. This flexibility in understanding facilitates the study of complex reactions in both laboratory and natural environments.