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Chapter 16: Problem 28

\(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a strong acid, but \(\mathrm{HSO}_{4}^{-}\) is a weak acid. Account for the difference in strength of these two related species.

Short Answer

Expert verified
The full dissociation of H_2SO_4 makes it a strong acid, unlike HSO_4^- , which only partially dissociates.

Step by step solution

01

Understanding Acid Strength

A strong acid is one that completely dissociates in water. Sulfuric acid ( H_2SO_4 ) is considered a strong acid because it fully dissociates into its ions, releasing two protons ( H^+ ) in the process.
02

First Dissociation of H_2SO_4

When H_2SO_4 is dissolved in water, it dissociates into HSO_4^- and H^+ . The equation is H_2SO_4 ightarrow H^+ + HSO_4^- . This step is nearly complete, contributing to the strong acidic behavior of H_2SO_4 .
03

Second Dissociation of HSO_4^-

The bisulfate ion ( HSO_4^- ) represents the second dissociation step of H_2SO_4 . When HSO_4^- dissociates, it forms SO_4^{2-} and another H^+ : HSO_4^- ightarrow H^+ + SO_4^{2-} . This dissociation is incomplete, making HSO_4^- a weak acid.
04

Reason for Difference in Acid Strength

The difference in strengths is due to the first dissociation of H_2SO_4 being complete, as it is a strong acid, whereas HSO_4^- does not fully dissociate. The conjugate base SO_4^{2-} is too stable, and therefore, the backward reaction is favored, limiting further proton release.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Strong Acid
A strong acid is one of the most powerful components in chemistry due to its ability to completely dissociate in water. This means that when a strong acid like sulfuric acid \(\mathrm{H}_2\mathrm{SO}_4\) is added to water, it breaks down entirely into its constituent ions.
Sulfuric acid is an example that fully dissociates into \(\mathrm{H}^+\) ions and \(\mathrm{HSO}_4^-\). This behavior showcases why strong acids are incredibly effective in donating protons (\(\mathrm{H}^+\)).
  • Complete dissociation means no original acid molecules are left intact in the solution.
  • Strong acids have high conductivity due to the abundance of free ions in the solution.
  • They tend to have a low pH value, often below 3.
This characteristic of complete dissociation gives strong acids their unique properties in chemical reactions, where they can completely drive a reaction to completion by providing the necessary protons.
Weak Acid
Unlike strong acids, weak acids do not dissociate completely in water. This means only a partial disassociation occurs, where some molecules of the acid remain intact.
An example of a weak acid is the bisulfate ion \(\mathrm{HSO}_4^-\), which results from the partial dissociation of sulfuric acid. In solution, only a portion of \(\mathrm{HSO}_4^-\) dissociates into \(\mathrm{H}^+\) ions and \(\mathrm{SO}_4^{2-}\).
  • The partial release of \(\mathrm{H}^+\) ions results in a less acidic solution compared to strong acids.
  • Weak acids maintain an equilibrium between dissociated ions and the undissociated molecules.
  • This leads to a higher pH level than strong acids, usually between 3 and 7.
Understanding the behavior of weak acids helps predict their efficacy in chemical processes where gradual proton donation is beneficial.
Dissociation
Dissociation in chemistry refers to the process where molecules split into smaller particles, such as ions, usually when dissolved in water. Dissociation is a key concept when studying acids.
For instance, sulfuric acid \(\mathrm{H_2SO_4}\) undergoes two stages of dissociation. The first dissociation is complete, yielding \(\mathrm{HSO}_4^-\) and \(\mathrm{H}^+\). The partial second dissociation transforms \(\mathrm{HSO}_4^-\) into \(\mathrm{H}^+\) and \(\mathrm{SO_4^{2-}}\).
  • The extent of dissociation is a key determinant of the acid's strength.
  • Complete dissociation correlates with strong acids, while partial dissociation is typical of weak acids.
  • The equilibrium constant (\(K_a\)) provides a measure of the extent of dissociation in weak acids.
Keenly understanding dissociation provides insights into how acids will behave in different solutions and reactions.
Conjugate Base
When an acid donates a proton, it forms a conjugate base. This concept is integral to understanding acid-base equilibria.
In the case of sulfuric acid, its first dissociation step produces the conjugate base \(\mathrm{HSO}_4^-\). Similarly, the second dissociation of \(\mathrm{HSO}_4^-\) produces the conjugate base \(\mathrm{SO}_4^{2-}\).
  • A conjugate base is the remainder of an acid molecule after the proton is removed.
  • The strength and stability of a conjugate base influence the acid's ability to donate further protons.
  • This is because a stable conjugate base (like \(\mathrm{SO}_4^{2-}\)) resists converting back to its acid form, limiting additional dissociations.
Conjugate bases help determine the acidity or basicity of a substance in solution, providing a balance that is essential for many natural and industrial chemical processes.

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