Problem 123 - 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

HClO4 acts as an acid by donating a proton to H2SO4, which acts as a base. The balanced reaction is HClO4 + H2SO4 -> ClO4- + H3SO4+.

Step 1: Identify the Chemical Species

First, identify the chemical species involved in the proton transfer reaction. Perchloric acid is represented as \( \text{HClO}_4 \) and sulfuric acid as \( \text{H}_2\text{SO}_4 \). Both of these can participate in the exchange of protons.

Step 2: Identify the Role of Each Species

Determine the role of each compound in the reaction. In this scenario, \( \text{HClO}_4 \) donates a proton, hence acts as the acid, while \( \text{H}_2\text{SO}_4 \) accepts the proton, thus behaving like a base.

Step 3: Write the Acid-Base Reaction Equation

The balanced acid-base reaction equation is:\[\text{HClO}_4 + \text{H}_2\text{SO}_4 \rightarrow \text{ClO}_4^- + \text{H}_3\text{SO}_4^+\]In this equation, \( \text{HClO}_4 \) donates a proton to \( \text{H}_2\text{SO}_4 \), forming \( \text{ClO}_4^- \) and \( \text{H}_3\text{SO}_4^+ \).

Step 4: Understanding the Lewis Structure

Draw the Lewis structure for \( \text{H}_2\text{SO}_4 \). Sulfuric acid has a sulfur atom at the center with two double-bonded oxygen atoms, two single-bonded oxygen atoms each with a hydrogen atom, and the corresponding lone pair redistributions.

Step 5: How Sulfuric Acid Functions as a Base

Sulfuric acid functions as a base by accepting a proton, utilizing one of the pairs of electrons on the oxygen atoms to bond with the incoming proton. This process forms \( \text{H}_3\text{SO}_4^+ \), showcasing its role as a base in this reaction.

Key Concepts

Perchloric Acid

Perchloric acid, known scientifically as \( \text{HClO}_4 \), is a highly potent acid due to its high dissociation ability. It is often considered a superacid, which means it is stronger than 100% sulfuric acid. This compound is composed of one hydrogen atom, one chlorine atom, and four oxygen atoms. The presence of these oxygen atoms, especially the negatively charged ions they form when the acid dissociates, makes perchloric acid highly effective in donating protons. When dissolved in another acid, such as sulfuric acid, \( \text{HClO}_4 \) still exhibits its strong acidic properties by transferring its available proton to other reactants.

In reactions, perchloric acid is often seen playing the role of a proton donor, as in the equation:
\[\text{HClO}_4 + \text{H}_2\text{SO}_4 \rightarrow \text{ClO}_4^- + \text{H}_3\text{SO}_4^+\]
This shows that even in an environment with sulfuric acid present, perchloric acid can still function as an effective acid.

Sulfuric Acid

Sulfuric acid, \( \text{H}_2\text{SO}_4 \), is a widely used acid in chemistry due to its strong acidic nature and reactivity. The structure of sulfuric acid consists of a sulfur atom centrally located, surrounded by four oxygen atoms. Each oxygen is involved in a bonding arrangement where two of them form double bonds with sulfur, and the other two each carry a hydrogen atom, making them hydroxyl groups (\( \text{-OH} \)).

• The double-bonded oxygens and the available lone pairs on the hydroxyl groups play a significant role in sulfuric acid's ability to react with other substances.
• In environments like the one presented with perchloric acid, sulfuric acid can function as a base even though it is traditionally considered an acid itself.

When acting as a base, sulfuric acid accepts a proton using one of the lone pairs from the oxygen atoms. This behavior forms \( \text{H}_3\text{SO}_4^+ \), highlighting its unique ability to undergo proton acceptance despite its acidity.

Lewis Dot Structure

The Lewis dot structure is a powerful tool in visualizing the electron distribution in molecules. Understanding the Lewis structure of sulfuric acid is crucial to comprehending its reactivity and base functionality.
Here are some key points about drawing the Lewis structure of \( \text{H}_2\text{SO}_4 \):

• The sulfur atom, being the central atom, is surrounded by four oxygen atoms.
• Two of these oxygen atoms form double bonds with sulfur, indicating shared electron pairs.
• The remaining two oxygens engage with the sulfur atom through single bonds, each holding a hydrogen atom, contributing to the molecule's overall potential for hydrogen bonding.
• The lone pairs positioned on these oxygen atoms become critical when \( \text{H}_2\text{SO}_4 \) approaches its role as a base. They provide sites for proton acceptance, facilitating the formation of \( \text{H}_3\text{SO}_4^+ \).

Through the Lewis structure, we get a clear representation of chemical bonds and lone pairs, giving insights into both the molecule's stability and reactivity, particularly in acid-base reactions.