Problem 19 - Chemistry The Central Science

Question

(a) The hydrogen oxalate ion \(\left(\mathrm{HC}_{2} \mathrm{O}_{4}^{-}\right)\) is amphiprotic. Write a balanced chemical equation showing how it acts as an acid toward water and another equation showing how it acts as a base toward water. (b) What is the conjugate acid of \(\mathrm{HC}_{2} \mathrm{O}_{4}\) ? What is its conjugate base?

Step-by-Step Solution

Answer

The hydrogen oxalate ion (\(\mathrm{HC}_{2} \mathrm{O}_{4}^{-}\)) acts as an acid towards water and forms \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-} + \mathrm{H}_{3}\mathrm{O}^{+}\), with a balanced equation: \(\mathrm{HC}_{2}\mathrm{O}_{4}^{-} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{C}_{2}\mathrm{O}_{4}^{2-} + \mathrm{H}_{3}\mathrm{O}^{+}\). It also acts as a base towards water and forms \(\mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4} + \mathrm{OH}^{-}\), with a balanced equation: \(\mathrm{HC}_{2}\mathrm{O}_{4}^{-} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4} + \mathrm{OH}^{-}\). The conjugate acid of \(\mathrm{HC}_{2} \mathrm{O}_{4}^{-}\) is \(\mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4}\), while its conjugate base is \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\).

Step 1: 1. Hydrogen oxalate as an acid

When an amphiprotic substance behaves as an acid, it donates a proton (H\(^{+}\)) to the water molecule. The hydrogen oxalate ion (\(\mathrm{HC}_{2}\mathrm{O}_{4}^{-}\)) will lose a proton, forming \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\) ion. The balanced chemical equation for this reaction is given by: \(\mathrm{HC}_{2}\mathrm{O}_{4}^{-} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{C}_{2}\mathrm{O}_{4}^{2-} + \mathrm{H}_{3}\mathrm{O}^{+}\)

Step 2: 2. Hydrogen oxalate as a base

As a base, the amphiprotic hydrogen oxalate ion (\(\mathrm{HC}_{2}\mathrm{O}_{4}^{-}\)) will accept a proton from the water molecule. This results in the formation of the ion \(\mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4}\). The balanced chemical equation for this reaction is given by: \(\mathrm{HC}_{2}\mathrm{O}_{4}^{-} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4} + \mathrm{OH}^{-}\)

Step 3: 3. Conjugate acid of \(\mathrm{HC}_{2}\mathrm{O}_{4}^{-}\)

The conjugate acid of a substance is formed when it accepts a proton. In the case of \(\mathrm{HC}_{2}\mathrm{O}_{4}^{-}\), it becomes \(\mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4}\) after accepting a proton, according to the reaction in step 2. Therefore, the conjugate acid of \(\mathrm{HC}_{2}\mathrm{O}_{4}^{-}\) is \(\mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4}\).

Step 4: 4. Conjugate base of \(\mathrm{HC}_{2}\mathrm{O}_{4}^{-}\)

The conjugate base of a substance is formed when it donates a proton. In the case of \(\mathrm{HC}_{2}\mathrm{O}_{4}^{-}\), it becomes \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\) after donating a proton, according to the reaction in step 1. Therefore, the conjugate base of \(\mathrm{HC}_{2}\mathrm{O}_{4}^{-}\) is \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\).

Key Concepts

Acid-Base Reactions

Acid-base reactions are fundamental chemical processes involving the transfer of protons (\( ext{H}^+ \)) between reactants. In these reactions, an acid is typically defined as a substance that donates a proton, while a base accepts a proton.
Water is a versatile participant in acid-base reactions because it can act as either an acid or a base, depending on the substances it interacts with.
When considering an acid-base reaction, it is useful to think of a seesaw, where the balance of protons is shifted between the acid and base.

• Acid donating a proton: When an acid experiences a loss of a proton, it transforms into its conjugate base.
• Base accepting a proton: When a base gains a proton, it transforms into its conjugate acid.

This exchange of protons is what characterizes and defines acid-base reactions, making them pivotal in chemical studies.

Conjugate Acid-Base Pairs

Conjugate acid-base pairs are crucial to understanding the complete picture of acid-base reactions. These pairs allow us to see what remains after an acid has donated a proton and what is formed when a base accepts a proton.
Consider the hydrogen oxalate ion \( \mathrm{HC}_{2}\mathrm{O}_{4}^{-} \). This ion illustrates how substances contain both a conjugate acid, when gaining a proton, and a conjugate base when losing one.

• Conjugate Acid: When the hydrogen oxalate ion acts as a base, it accepts a proton to form \( \mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4} \).
• Conjugate Base: When the hydrogen oxalate ion acts as an acid, it loses a proton and forms \( \mathrm{C}_{2}\mathrm{O}_{4}^{2-} \).

Remember that in every acid-base reaction, two conjugate acid-base pairs are at work, allowing for intricate interactions within solutions.

Hydrogen Oxalate Ion

The hydrogen oxalate ion \( \mathrm{HC}_{2}\mathrm{O}_{4}^{-} \) is a fascinating example of an amphiprotic substance. This means it has the exceptional ability to either donate or accept a proton, acting as both an acid and a base depending on the environment.

• Acting as an Acid: In the presence of water, it can donate a proton, transforming into the \( \mathrm{C}_{2}\mathrm{O}_{4}^{2-} \) ion. The reaction is as follows: \( \mathrm{HC}_{2}\mathrm{O}_{4}^{-} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{C}_{2}\mathrm{O}_{4}^{2-} + \mathrm{H}_{3}\mathrm{O}^{+} \).
• Acting as a Base: Conversely, it can accept a proton from water, thereby becoming \( \mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4} \) and releasing hydroxide ions. This reaction goes: \( \mathrm{HC}_{2}\mathrm{O}_{4}^{-} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{H}_{2}\mathrm{C}_{2}\mathrm{O}_{4} + \mathrm{OH}^{-} \).

By understanding how the hydrogen oxalate ion behaves in different scenarios, we can appreciate the dual nature of amphiprotic substances and their significance in chemistry.