Question

Peroxonitrous acid is an unstable intermediate formed in the oxidation of ${\mathrm{HNO}}_2$ by ${\mathrm{H}}_2{\mathrm{O}}_2$. It has the same formula as nitric acid, HNO ${}_3$. Show how you would expect peroxonitrous and nitric acids to differ in structure.

Short answer

Nitric acid (HNO3) shows a planar structure, with the N atom in the center, attached to an OH group and two O atoms, with equal bond length, predominantly due to resonance structure. Alternatively, peroxonitrous acid, formed in the oxidation of HNO2 by H2O2, gives a structure where the N atom is connected to one O atom and an O-O group, thus incorporating peroxide in its structure.

Step-by-step solution

Structure of Nitric Acid

First, consider the structure of HNO3 (nitric acid). It forms a planar structure with an N atom at the center attached to an OH group and to two O atoms. The molecule has resonance structures which means the double bond can swap places between the two O atoms and the bond lengths are the same, revealing the equal sharing of electrons.

Structure of Peroxonitrous Acid

Next, consider the structure of peroxonitrous acid. In the oxidation of HNO2 by H2O2, the HNO2 gives up an H+ ion, resulting in a nitrite ion ($N{O}_2^{-}$). Meanwhile, H2O2 can form O-O with its O atoms, leading to a peroxide ($O_2^{2-}$) ion. During the reaction, the nitrite ion and the peroxide ion combine, making the N atom to be as above, attached to one O atom and to an O-O group. This results in the formation of peroxonitrous acid.

Key concepts

Nitric Acid

Nitric acid, commonly known by its chemical formula ${\text{HNO}}_3$, is a well-known and widely used strong acid in chemistry. Structurally, this compound consists of a nitrogen atom at the center. It's worth noting that this nitrogen atom is attached to three oxygen atoms.

One of these is connected via a single bond, forming an $\text{OH}$ group, while the other two are linked to the nitrogen atom by alternating single and double bonds. This configuration leads to multiple resonance forms for nitric acid.

• The double bond location can alternate between the two oxygen atoms bonded to nitrogen.
• This movement results in bond lengths that are equal, as the electrons are delocalized between these bonds.
• The presence of resonance structures contributes to the molecule's stability.

Understanding the resonance structure in ${\text{HNO}}_3$ helps in grasping the distribution of electrons in the molecule and how it influences chemical reactivity.

Peroxonitrous Acid

Peroxonitrous acid is an intriguing compound formed through the oxidation reaction of nitrous acid (${\text{HNO}}_2$) with hydrogen peroxide (${\text{H}}_2{\text{O}}_2$). Despite sharing the formula ${\text{HNO}}_3$ with nitric acid, they are structurally distinct.

During the reaction that forms peroxonitrous acid, nitrite ions ${\text{NO}}_2^{-}$ release a hydrogen ion ${\text{H}}^{+}$ and combine with peroxide ions ${\text{O}}_2^{2-}$. Consequently, the resulting structure for peroxonitrous acid features an $\text{N-O-O}$ group.

• The nitrogen atom in peroxonitrous acid is bonded to an oxygen that forms part of an $\text{O-O}$, peroxide-like linkage.
• This structural formation makes it distinctly different from nitric acid.
• Due to the presence of the $\text{O-O}$ linkage, peroxonitrous acid is less stable and more susceptible to decomposition.

Understanding these structural differences is crucial for predicting the reactivity and stability of this intermediate.

Resonance Structures

Resonance structures are a vital concept in understanding the stability and reactivity of certain molecules, including nitric acid. They are alternative ways to depict the electron distribution in a molecule, which can't be represented by a single static structural formula.

In the case of nitric acid, resonance forms arise from the movement of electrons between the nitrogen and the oxygen atoms. This explains the equal bond lengths observed in the molecule, even though one bond might initially appear as a double bond and another as a single bond in separate structures.

• Resonance enhances the stability of a molecule by allowing electrons to be delocalized or spread out over several atoms.
• This delocalization can reduce the molecule's potential energy, lending it more stability.
• In analytical chemistry, correctly identifying resonance forms is fundamental to anticipating molecular behavior.

Utilizing resonance structures allows chemists to better predict and explain chemical reactions and the physical properties of molecules such as nitric acid.