Why HNO₃ is Stronger than HNO₂

Acidity Strength: A Tale of Two Nitric Acids

In the realm of chemistry, acids play a crucial role, acting as proton donors and catalysts in numerous reactions. Among the diverse array of acids, nitric acid (HNO₃) and nitrous acid (HNO₂) stand out as two prominent members of the oxyacid family. Both possess unique properties and applications, but one question that often arises is: why is HNO₃ stronger than HNO₂? To delve into the answer, we must explore their molecular structures, bond characteristics, and the factors that govern their acidity strength.

Bond Characteristics: A Clash of Covalent and Dative Bonds

The molecular structures of HNO₃ and HNO₂ reveal intriguing differences that influence their acidity strength. HNO₃ exhibits a trigonal planar geometry, with three oxygen atoms and a hydrogen atom covalently bonded to the central nitrogen atom. This arrangement gives rise to three equivalent N–O bonds. In contrast, HNO₂ adopts a bent molecular structure, featuring two oxygen atoms and a hydrogen atom covalently bonded to the nitrogen atom. Additionally, there's a dative bond between the nitrogen and oxygen atoms, resulting in two distinct N–O bonds.

The presence of the dative bond in HNO₂ has a profound impact on its acidity strength. Dative bonds, also known as coordinate bonds, involve the sharing of electron pairs where one atom donates both electrons to form the bond. In HNO₂, the nitrogen atom donates a lone pair of electrons to the oxygen atom, creating the dative bond. This dative bond is weaker than the covalent N–O bonds found in HNO₃.

Delving into Acidity Strength: A Matter of Proton Dissociation

Acidity strength is fundamentally determined by the compound's ability to donate protons (H+ ions). The stronger the acid, the more readily it donates protons. Several factors influence acidity strength, including the stability of the conjugate base and the strength of the acid-base interaction.

In the case of HNO₃ and HNO₂, the stability of their conjugate bases plays a pivotal role. When HNO₃ donates a proton, it forms the nitrate ion (NO₃-), a highly stable and resonance-stabilized species. The delocalization of electrons across the three oxygen atoms in NO₃– distributes the negative charge, making it a weak base.

On the other hand, when HNO₂ donates a proton, it forms the nitrite ion (NO₂-), which is less stable than NO₃-. Nitrite ion lacks resonance stabilization and has a localized negative charge on the oxygen atom, making it a stronger base compared to nitrate ion. The weaker conjugate base of HNO₃ contributes to its higher acidity strength.

Resonance: A Symphony of Charge Distribution

Resonance, a phenomenon where electrons are delocalized across multiple chemical structures, plays a significant role in determining acidity strength. In HNO₃, the three oxygen atoms participate in resonance, allowing the negative charge of the conjugate base (NO₃-) to be distributed over multiple atoms. This dispersion of charge stabilizes the conjugate base, making HNO₃ a stronger acid.

In contrast, HNO₂ exhibits limited resonance due to the presence of the dative bond. The dative bond restricts the delocalization of electrons, resulting in a less stable conjugate base (NO₂-). Consequently, HNO₂ is a weaker acid compared to HNO₃.

Conclusion: A Stronger Acid Reigns Supreme

In conclusion, the higher acidity strength of HNO₃ compared to HNO₂ can be attributed to the stability of their conjugate bases, the strength of the acid-base interaction, and the extent of resonance. The interplay of these factors leads to HNO₃ being a stronger acid, readily donating protons and exhibiting a higher propensity to participate in acid-base reactions.

Frequently Asked Questions (FAQs)

• Q1: Why is the stability of the conjugate base important in determining acidity strength?

  • A1: A stable conjugate base can accommodate the negative charge more effectively, making the acid more willing to donate protons.

• Q2: How does resonance contribute to acidity strength?

  • A2: Resonance delocalizes the negative charge of the conjugate base, increasing its stability and, in turn, enhancing the acidity strength of the acid.

• Q3: What role does the dative bond play in the acidity strength of HNO₂?

  • A3: The dative bond weakens the N–O bond, making it more difficult for HNO₂ to dissociate and donate protons, leading to its lower acidity strength compared to HNO₃.

• Q4: Can you provide an example of a reaction where HNO₃ is used as an acid?

  • A4: HNO₃ is commonly used in the nitration of organic compounds, where it introduces a nitro group (-NO₂) onto the molecule.

• Q5: What are some applications of HNO₂?

  • A5: HNO₂ finds applications in various industries, including the production of dyes, rubber, and certain food additives.