Problem 15 - Chemistry The Central Science

Question

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

Step-by-Step Solution

Answer

The conjugate bases and acids of the given Bronsted-Lowry acids and bases are: (a) Conjugate bases: (i) HIO₃ ➞ IO₃⁻ (ii) NH₄⁺ ➞ NH₃ (b) Conjugate acids: (i) O²⁻ ➞ OH⁻ (ii) H₂PO₄⁻ ➞ H₃PO₄

Step 1: Identify the acid as HIO₃

The given Bronsted-Lowry acid is HIO₃.

Step 2: Remove a proton from the acid

To find the conjugate base, we need to remove a proton (H⁺) from the given acid, which means we are left with IO₃⁻.

Step 3: Write down the conjugate base

The conjugate base of HIO₃ is IO₃⁻. (ii) NH₄⁺

Step 1: Identify the acid as NH₄⁺

The given Bronsted-Lowry acid is NH₄⁺.

Step 2: Remove a proton from the acid

To find the conjugate base, we need to remove a proton (H⁺) from the given acid, which means we are left with NH₃.

Step 3: Write down the conjugate base

The conjugate base of NH₄⁺ is NH₃. (b) Give the conjugate acid of the following Bronsted-Lowry bases: (i) O²⁻

Step 1: Identify the base as O²⁻

The given Bronsted-Lowry base is O²⁻.

Step 2: Add a proton to the base

To find the conjugate acid, we need to add a proton (H⁺) to the given base, which means we end up with OH⁻.

Step 3: Write down the conjugate acid

The conjugate acid of O²⁻ is OH⁻. (ii) H₂PO₄⁻

Step 1: Identify the base as H₂PO₄⁻

The given Bronsted-Lowry base is H₂PO₄⁻.

Step 2: Add a proton to the base

To find the conjugate acid, we need to add a proton (H⁺) to the given base, which means we end up with H₃PO₄.

Step 3: Write down the conjugate acid

The conjugate acid of H₂PO₄⁻ is H₃PO₄.

Key Concepts

Conjugate Base

In the world of Bronsted-Lowry acids and bases, understanding the concept of a conjugate base is central. When an acid donates a proton, or hydrogen ion ( H^+ ), it transforms into its conjugate base. This transformation is crucial to acid-base reactions. For example, when HIO_3 , an acid, loses a proton, it becomes the conjugate base, IO_3^- . Similarly, NH_4^+ , when losing a proton, turns into NH_3 , its conjugate base.

To recognize a conjugate base:

• Start by identifying the acid in the reaction.
• Remove one proton from this acid.
• The resulting species is the conjugate base.

This process showcases the reversible nature of acid-base reactions, where the conjugate base can often absorb a proton to revert to the original acid, highlighting the complementary relationship between acids and bases.

Conjugate Acid

Just as acids have conjugate bases, bases have conjugate acids. The concept of a conjugate acid comes into play when a base accepts a proton, resulting in a newly formed acid. Consider the base O^{2-} ; when it gains a proton, it transforms into the conjugate acid OH^- . Another example is H_2PO_4^- , which, upon accepting a proton, becomes H_3PO_4 .

To understand conjugate acids, follow these steps:

• Identify the base involved in the reaction.
• Add one proton to the identified base.
• The new species formed is the conjugate acid.

This exchange signifies the dynamic nature of acid-base chemistry, where bases turn into acids through proton acceptance, showcasing the balance and interplay between conjugate acid-base pairs.

Proton Transfer

At the heart of Bronsted-Lowry acid-base theory is the concept of proton transfer. This process involves the movement of a proton from an acid to a base. Such transfers are fundamental to many chemical reactions and drive the formation of conjugate acid-base pairs.

Consider the transformation of HIO_3 to IO_3^- and NH_4^+ to NH_3 , where protons are transferred from these acids, converting them to their respective conjugate bases. Similarly, the transition of O^{2-} to OH^- and H_2PO_4^- to H_3PO_4 involves an addition of protons, forming conjugate acids.

Important points to remember about proton transfer:

• Proton transfer creates two new entities: a conjugate base and a conjugate acid.
• It demonstrates the reversible reactions where an acid becomes a base and vice versa.

Mastering proton transfer helps in understanding the nuanced interactions in acid-base chemistry, forming the basis for predicting reaction outcomes and understanding their equilibrium states.