Question

Identifying Conjugate Bases Write the conjugate base for each acid: a. ${\mathrm{H}}_3{\mathrm{PO}}_4$ b. ${\mathrm{H}}_2{\mathrm{CO}}_3$ c. ${\mathrm{CH}}_3\mathrm{COOH}$ d. ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$

Short answer

a. ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$, b. ${\mathrm{HCO}}_3^{-}$, c. ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$, d. ${\mathrm{CH}}_3{\mathrm{NH}}_2$.

Step-by-step solution

Understanding Conjugate Bases

A conjugate base is formed when an acid donates a proton (H⁺) to a base. This means that the conjugate base of any acid is the species that remains after the acid has given up a proton.

Identifying the Conjugate Base of ${\mathrm{H}}_3{\mathrm{PO}}_4$

The acid ${\mathrm{H}}_3{\mathrm{PO}}_4$ loses one proton (H⁺) to form its conjugate base. Removing one H⁺ from ${\mathrm{H}}_3{\mathrm{PO}}_4$ leaves ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$. Thus, the conjugate base is ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$.

Identifying the Conjugate Base of ${\mathrm{H}}_2{\mathrm{CO}}_3$

The acid ${\mathrm{H}}_2{\mathrm{CO}}_3$ loses one proton (H⁺) to form its conjugate base. Removing one H⁺ from ${\mathrm{H}}_2{\mathrm{CO}}_3$ leaves ${\mathrm{HCO}}_3^{-}$. Thus, the conjugate base is ${\mathrm{HCO}}_3^{-}$.

Identifying the Conjugate Base of ${\mathrm{CH}}_3\mathrm{COOH}$

The acid ${\mathrm{CH}}_3\mathrm{COOH}$ loses one proton (H⁺) to form its conjugate base. Removing one H⁺ from ${\mathrm{CH}}_3\mathrm{COOH}$ leaves ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$. Thus, the conjugate base is ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$.

Identifying the Conjugate Base of ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$

The acid ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$ loses one proton (H⁺) to form its conjugate base. Removing one H⁺ from ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$ leaves ${\mathrm{CH}}_3{\mathrm{NH}}_2$. Thus, the conjugate base is ${\mathrm{CH}}_3{\mathrm{NH}}_2$.

Key concepts

Understanding Acid-Base Chemistry

Acid-base chemistry is all about the transfer of protons. An acid is a chemical species that can donate a proton ( H⁺ ), while a base is a species that can accept a proton. When an acid donates its proton, it transforms into what we call its conjugate base. This transformation is central to many reactions in chemistry. Acid-base chemistry is important not only in the laboratory but also in biological systems and industrial processes.

In any acid-base reaction, we have two pairs of conjugates:

• Acid 1 and its conjugate base.
• Base 1 and its conjugate acid.

Understanding these pairings allows chemists to predict the direction of reactions and their outcomes.
Overall, acid-base chemistry teaches us about balance and equilibrium, crucial concepts for mastering various chemical reactions.

Mechanism of Proton Donation

Proton donation is a key feature of acid behavior in chemistry. Essentially, when an acid donates a proton, it is left with one less hydrogen atom and usually gains a negative charge. This shift results in what is known as a conjugate base. The concept of proton donation can be simplified by considering it as a shift of a single proton from the acid molecule to the base.

Let's take examples from the exercise:

• For phosphoric acid, $H_{3}P{O}_4$, donating one proton forms $H_{2}P{O}_4^{-}$.
• Similarly, carbonic acid, $H_{2}C{O}_3$, changes to $HC{O}_3^{-}$.

These transitions illustrate that understanding proton donation involves tracking a proton's movement from one compound to another. This donor-acceptor interaction is fundamental to how acids and bases behave in solutions, affecting properties like pH and reactivity.

Chemical Species Involved in Conjugation

Chemical species, such as acids and bases, are composed of atoms and molecules that define their properties and reactions. In the context of conjugate bases, a chemical species undergoes a transformation upon losing a proton.
The remaining species, now known as the conjugate base, retains all other atoms of the acid but has one fewer proton.

Through the step-by-step breakdown, you can see how each acid transforms into a conjugate base:

• For acetic acid, $C{H}_{3}COOH$, its conjugate base is $C{H}_{3}CO{O}^{-}$.
• For methylammonium ion, $C{H}_{3}N{H}_3^{+}$, it becomes $C{H}_{3}N{H}_2$.

These examples highlight how chemical species adjust their structure to maintain stability after proton donation. Recognizing these changes is essential for predicting the behavior of substances and their reaction possibilities in different environments.