Question

(a) Which of the following is the stronger Brønsted-Lowry acid, ${\mathrm{HClO}}_3$ or ${\mathrm{HClO}}_2$ ? (b) Which is the stronger BronstedLowry base, ${\mathrm{HS}}^{-}$or ${\mathrm{HSO}}_4^{-}$?

Short answer

(a) The stronger Brønsted-Lowry acid is ${\mathrm{HClO}}_3$, as its conjugate base ${\mathrm{ClO}}_3^{-}$ is more stable due to a higher number of oxygen atoms. (b) The stronger Brønsted-Lowry base is ${\mathrm{HSO}}_4^{-}$, as its conjugate acid ${\mathrm{H}}_2{\mathrm{SO}}_4$ is more stable due to more electronegative oxygen atoms.

Step-by-step solution

Compare the acidic strengths of ${\mathrm{HClO}}_3$ and ${\mathrm{HClO}}_2$

In order to compare their acidic strengths, let's analyze the conjugate bases of both acids after losing a proton: - Conjugate base of ${\mathrm{HClO}}_3$: ${\mathrm{ClO}}_3^{-}$ - Conjugate base of ${\mathrm{HClO}}_2$: ${\mathrm{ClO}}_2^{-}$ The stability of the conjugate base is determined by its ability to distribute the negative charge among the more electronegative atoms. Electronegativity of oxygen is high, so the higher the number of oxygen atoms attached to the central chlorine atom, the more stable the conjugate base will be due to electron delocalization. In this case, ${\mathrm{ClO}}_3^{-}$ has more oxygen atoms than ${\mathrm{ClO}}_2^{-}$. Therefore, the conjugate base of ${\mathrm{HClO}}_3$ is more stable than the conjugate base of ${\mathrm{HClO}}_2$. Since a more stable conjugate base corresponds to a stronger acid, we can conclude that ${\mathrm{HClO}}_3$ is the stronger Brønsted-Lowry acid.

Compare the basic strengths of ${\mathrm{HS}}^{-}$ and ${\mathrm{HSO}}_4^{-}$

In order to compare their basic strengths, let's analyze the conjugate acids of both bases after gaining a proton: - Conjugate acid of ${\mathrm{HS}}^{-}$: ${\mathrm{H}}_2\mathrm{S}$ - Conjugate acid of ${\mathrm{HSO}}_4^{-}$: ${\mathrm{H}}_2{\mathrm{SO}}_4$ The stability of the conjugate acid is determined by its ability to hold the extra proton while distributing the positive charge. In this case, ${\mathrm{H}}_2\mathrm{S}$ has only one oxygen atom, while ${\mathrm{H}}_2{\mathrm{SO}}_4$ has four oxygen atoms. Since oxygen is more electronegative than sulfur, the positive charge is more effectively distributed among the highly electronegative oxygen atoms in ${\mathrm{H}}_2{\mathrm{SO}}_4$, making it a more stable conjugate acid. Since a more stable conjugate acid corresponds to a stronger base, we can conclude that ${\mathrm{HSO}}_4^{-}$ is the stronger Brønsted-Lowry base. So, the final answers are: (a) The stronger Brønsted-Lowry acid is ${\mathrm{HClO}}_3$. (b) The stronger Brønsted-Lowry base is ${\mathrm{HSO}}_4^{-}$.

Key concepts

Understanding Acidic Strength

The acidic strength of a compound is an essential aspect of chemistry, especially in understanding how substances behave in various reactions. Acidic strength refers to an acid's ability to donate a proton (${\text{H}}^{+}$), and the stronger the acid, the more readily it will lose this proton.
One way to determine acidic strength is to consider the stability of its conjugate base after the acid has donated a proton. If the conjugate base is stable, the acid tends to be strong. This is because stable conjugate bases are better at handling the negative charge resulting from the loss of a proton.
In our exercise, we see that ${\text{HClO}}_3$ is stronger compared to ${\text{HClO}}_2$, primarily due to the increased stability of its conjugate base.

The Role of Conjugate Bases

Conjugate bases are formed when an acid donates a proton. They play a crucial role in determining the acid's strength because a stable conjugate base indicates a strong acid. The stability of these bases can be attributed to several factors including:

• Electron delocalization: More electronegative atoms can disperse the negative charge.
• Structure: The presence of electronegative atoms, like oxygen, supports the spread of negative charges, making the conjugate base more stable.

The conjugate base of ${\text{HClO}}_3$, ${\text{ClO}}_3^{-}$, benefits from having more oxygen atoms compared to ${\text{ClO}}_2^{-}$, the conjugate base of ${\text{HClO}}_2$. This results in better charge distribution and increased stability of ${\text{ClO}}_3^{-}$, thereby making ${\text{HClO}}_3$ a stronger acid.

Electronegativity and Its Impact

Electronegativity is the tendency of an atom to attract shared electrons in a bond. This property significantly influences both acidic strength and the stability of conjugate bases and acids.
In acids and their conjugate bases, highly electronegative atoms like oxygen can stabilize negative charge through electron delocalization. A greater number of such atoms can lead to increased stability, as seen in the case of ${\text{HClO}}_3$. More electronegative elements result in a more stable conjugate base, rendering the corresponding acid stronger.

• Electronegativity is crucial for distributing charges effectively.
• The greater the electronegativity difference in a molecule, the more stable the resulting ion will be.

Understanding this concept is key to comprehending why certain acids behave differently in chemical reactions, as observed in the comparison between ${\text{HClO}}_3$ and ${\text{HClO}}_2$.

Conjugate Acid and Base Pairing

In any Brønsted-Lowry reaction, the acid and base each have a corresponding conjugate base and conjugate acid. This pairing is vital for understanding reactions. When a base gains a proton, it forms a conjugate acid, and similarly, when an acid loses a proton, it forms a conjugate base.
For example, in our exercise, ${\text{HS}}^{-}$ becomes ${\text{H}}_2\text{S}$ (its conjugate acid), and ${\text{HSO}}_4^{-}$ transforms into ${\text{H}}_2{\text{SO}}_4$, a much more stable acid due to oxygen's high electronegativity. The stability of the conjugate acid, ${\text{H}}_2{\text{SO}}_4$, shows why ${\text{HSO}}_4^{-}$ is a stronger base.
This interrelation is critical for predicting behavior in acid-base reactions, harnessing the concept of conjugate acid-base pairs as a framework for understanding chemical interactions.