Problem 19 - Chemistry The Molecular Nature of Matter

Question

Explain why nitric acid is a stronger acid than nitrous acid. $$ \mathrm{HO}-\mathrm{NO}_{2} \quad \mathrm{HO}-\mathrm{NO} $$ nitric acid nitrous aci

Step-by-Step Solution

Answer

Nitric acid \(\mathrm{HNO}_{3}\) is stronger than nitrous acid \(\mathrm{HNO}_{2}\) because its conjugate base, \(\mathrm{NO}_{3}^{-}\), is more stable due to greater delocalization of negative charge across its oxygen atoms.

Step 1: Understanding Acid Strength

The strength of an acid can be understood by its ability to donate a proton (H+ ion) to form its conjugate base. Stronger acids dissociate to a greater extent in water, which is often due to the stability of their conjugate bases. A stable conjugate base implies a stronger acid.

Step 2: Comparing Conjugate Base Stability

For nitric acid \(\mathrm{HNO}_{3}\), the conjugate base is \(\mathrm{NO}_{3}^{-}\). For nitrous acid \(\mathrm{HNO}_{2}\), the conjugate base is \(\mathrm{NO}_{2}^{-}\). The structural difference is that the conjugate base of nitric acid has an additional oxygen atom.

Step 3: Electron Delocalization in Conjugate Bases

The \(\mathrm{NO}_{3}^{-}\) ion has a resonance structure that allows for the negative charge to be delocalized across all oxygen atoms, which stabilizes the ion. Conversely, the \(\mathrm{NO}_{2}^{-}\) ion has less resonance stabilization because there are fewer oxygen atoms to delocalize the negative charge.

Step 4: Conclusion on Acid Strength

Nitric acid is a stronger acid than nitrous acid because its conjugate base \(\mathrm{NO}_{3}^{-}\) is more stable due to the delocalization of negative charge across more oxygen atoms compared to the conjugate base of nitrous acid \(\mathrm{NO}_{2}^{-}\). Greater stabilization of the conjugate base leads to increased acidity.

Key Concepts

Conjugate Base Stability

Understanding conjugate base stability is essential when examining acid strength. The stability of a conjugate base—a species that remains after an acid donates its proton—is a key factor in the acidity of the original compound.

The more stable the conjugate base, the more willingly the acid will give up its proton, and thus, the stronger the acid. Stability of the conjugate base can be influenced by several factors, including the presence of electronegative atoms, charge distribution, and the molecule's ability to delocalize negative charge through resonance.

For example, nitric acid (HNO₃) with its conjugate base nitrate ion (NO₃⁻), shows high stability due to the effective delocalization of the negative charge across three oxygen atoms. This distribution of charge decreases the energy of the ion, favoring deprotonation and signifying strong acidity. On the other hand, nitrous acid (HNO₂) forms a less stable conjugate base, nitrite ion (NO₂⁻), because the negative charge is less delocalized, being spread out over only two oxygen atoms. Hence, it is a weaker acid compared to nitric acid.

Students may improve their understanding of this concept by drawing Lewis structures of the acids and their conjugate bases and identifying factors that contribute to base stability, such as inductive effects and the availability of resonance structures.

Resonance Structures in Chemistry

The concept of resonance structures in chemistry explains how certain molecules have the ability to distribute their electron density across multiple positions, leading to a stabilization of the molecule or ion. Resonance structures are different forms of a molecule where electrons are located in different positions, yet the overall placement of atoms remains the same.

These structures are not real and cannot be isolated; rather, the actual molecule exists as a hybrid of these structures, sometimes referred to as a resonance hybrid. The more resonance structures that can be drawn for a molecule or ion, the more stable it is considered to be due to the delocalization of electron density.

In the context of acid strength, resonance can greatly influence the stability of conjugate bases. For instance, the nitrate ion (NO₃⁻) has several resonance forms that contribute to the distribution of its negative charge, resulting in greater stability. Students can visualize this by drawing all possible resonance structures and noting how the negative charge is shared among the different oxygen atoms. This visualization helps in appreciating why acids with conjugate bases that are capable of resonance are generally stronger acids.

Acid Dissociation

The process of acid dissociation is fundamental to understanding acid strength. When an acid dissociates in water, it releases hydrogen ions (H⁺, also called protons) into the solution. The measure of an acid's tendency to dissociate is called its acid dissociation constant, Ka. A larger Ka value indicates a stronger acid, as it implies a greater degree of ionization in water.

The dissociation process is influenced by the nature of the acid itself, including its molecular structure and the stability of the conjugate base it forms upon losing a proton. Strong acids, like sulfuric or hydrochloric acid, dissociate completely in aqueous solution, releasing all of their available protons.

Comparatively, weaker acids like nitrous acid (HNO₂) do not dissociate fully, with only a fraction of their molecules losing a proton to form nitrite ions (NO₂⁻). Students are encouraged to explore various acids by comparing their Ka values and examining the degree to which they dissociate in solution. This will enhance their understanding of why some acids are stronger than others and the role that complete or partial dissociation plays in that determination.