Problem 115 - Chemistry and Chemical Reactivity

Question

Perchloric acid behaves as an acid, even when it is dissolved in sulfuric acid. (a) Write a balanced equation showing how perchloric acid can transfer a proton to sulfuric acid. (b) Draw a Lewis electron dot structure for sulfuric acid. How can sulfuric acid function as a base?

Step-by-Step Solution

Answer

(a) \( \text{HClO}_4 + \text{H}_2\text{SO}_4 \rightarrow \text{ClO}_4^- + \text{H}_3\text{SO}_4^+ \). (b) Sulfuric acid can act as a base by accepting a proton.

Step 1: Understanding Perchloric Acid as an Acid

Perchloric acid (\( \text{HClO}_4 \)) is a strong acid, meaning it donates protons easily. When dissolved in another acid such as sulfuric acid (\( \text{H}_2\text{SO}_4 \)), it still donates protons.

Step 2: Balanced Equation for Proton Transfer

In this reaction, perchloric acid donates a proton (\( \text{H}^+ \)) to sulfuric acid. The equation can be represented as:\[\text{HClO}_4 + \text{H}_2\text{SO}_4 \rightarrow \text{ClO}_4^- + \text{H}_3\text{SO}_4^+.\]Here, the perchlorate ion (\( \text{ClO}_4^- \)) is formed as perchloric acid loses a proton, and the hydronium ion of sulfuric acid (\( \text{H}_3\text{SO}_4^+ \)) is formed.

Step 3: Drawing Lewis Structure for Sulfuric Acid

The sulfuric acid molecule can be visualized with sulfur as the central atom, bonded to four oxygen atoms. Two of these oxygens hold double bonds, while two are single-bonded with one hydrogen each: - S: Central atom - O: Double-bonded (x2), Single-bonded with H (x2) Here is a simplified Lewis structure: ``` O || O=S-OH || OH ```

Step 4: Understanding Sulfuric Acid's Basic Function

Although sulfuric acid is typically an acid, when it accepts a proton (\( \text{H}^+ \)) from another acid (like perchloric acid), it acts as a Bronsted-Lowry base. During the reaction with perchloric acid, sulfuric acid accepts a proton and becomes the hydrosulfate ion (\( \text{H}_3\text{SO}_4^+ \)).

Key Concepts

Proton Transfer in Acid-Base Reactions

Proton transfer is a fundamental concept in understanding how acids and bases interact. When perchloric acid (HClO_4) donates a proton (H^+) to sulfuric acid (H_2SO_4), it showcases a classic acid-base reaction. In this process, the proton moves from the perchloric acid to the sulfuric acid, highlighting the tendency of strong acids like perchloric acid to give away protons easily.

The balanced equation for this reaction is represented as:\[\text{HClO}_4 + \text{H}_2\text{SO}_4 \rightarrow \text{ClO}_4^- + \text{H}_3\text{SO}_4^+.\]

• Donor: Perchloric acid (HClO_4) acts as the proton donor.
• Acceptor: Sulfuric acid (H_2SO_4) accepts the proton.
• Products: The perchlorate ion (ClO_4^-) and the hydrosulfate ion (H_3SO_4^+) are formed.

This transfer of protons is integral to the function of acids and is a key feature of their behavior in chemical reactions.

Lewis Structure of Sulfuric Acid

The Lewis structure provides a visual representation of the bonding in molecules. For sulfuric acid ( H_2SO_4 ), sulfur is the central atom surrounded by four oxygen atoms.

• Double Bonds: Two of the oxygen atoms form double bonds with sulfur.
• Single Bonds with Hydrogen: The other two oxygen atoms are single-bonded to both sulfur and a hydrogen atom.

This can be depicted with the structure:
O
||
O=S-OH
||
OH

Through this structure:

• The use of oxygen leads to a charge distribution that allows sulfuric acid to act in various roles, depending on the reaction context.
• The presence of hydrogen bound to oxygen atoms enforces its acidity, as they can easily release protons.

Understanding the Lewis structure is crucial for visualizing how sulfuric acid can shift roles in different chemical scenarios.

Bronsted-Lowry Base Behavior

The Bronsted-Lowry theory expands the definition of acids and bases beyond typical examples. In this model, an acid is a proton donor and a base is a proton acceptor. Although sulfuric acid is traditionally an acid, it can behave as a base when it accepts a proton from another acid like perchloric acid.

During this interaction:

• Sulfuric Acid as Base: It accepts a proton (H^+ ), forming the hydrosulfate ion ( H_3SO_4^+ ).
• Reaction Insight: This gives sulfuric acid the ability to participate in reactions not just as an acid but also a base under the right conditions.

This flexibility is instrumental in various chemical processes and is pivotal to understanding the dynamics of proton-transfer reactions. The ability of sulfuric acid to function in dual roles depending on the reacting species reflects the dynamic nature of chemical reactivity.