Question

Compare the electron dot structures of the hydrogen carbonate ion and nitric acid. (a) Are these species isoelectronic? (b) How many resonance structures does each species have? (c) What are the formal charges of each atom in these species? (d) Compare the two species with respect to their acid-base behavior. (Can either or both species behave as a base and form a bond to \(\mathrm{H}^{+} ?\) )

Short answer

(a) Yes, they are isoelectronic. (b) Both have 2 resonance structures. (c) HCO₃⁻: O single-bond -1, others 0; HNO₃: N +1, single O -1, others 0. (d) HCO₃⁻ can be a base; HNO₃ is a strong acid.

Step-by-step solution

Draw Lewis Structures

First, draw the Lewis structure for each species. The hydrogen carbonate ion (HCO₃⁻) has a bonded structure of H-O-C(=O)-O⁻. For nitric acid (HNO₃), the structure is H-O-N(=O)-O.

Check for Isoelectronicity (Part a)

Calculate the total number of electrons for each species. HCO₃⁻ has 24 valence electrons (H=1, C=4, O₃=18, +1 for the negative charge). HNO₃ also has 24 valence electrons (H=1, N=5, O₃=18). Both species have the same number of valence electrons.

Determine Resonance Structures (Part b)

The hydrogen carbonate ion has two major resonance structures due to the possibility of different O-C-O bonding arrangements. Nitric acid has also two resonance structures, as the double bond between nitrogen and oxygen can shift.

Calculate Formal Charges (Part c)

In HCO₃⁻, assign formal charges: the single bonded oxygen has a charge of -1, while the other atoms have a formal charge of 0. In HNO₃, the formal charges are: the nitrogen is +1, the single bonded oxygen (not bonded to H) is -1, and others are 0.

Analyze Acid-Base Behavior (Part d)

Hydrogen carbonate ion can act as a base and accept \( ext{H}^{+} \) to form carbonic acid (H₂CO₃), whereas nitric acid is a strong acid and not typically a base. Thus, HCO₃⁻ can behave as a base and form a bond with \( ext{H}^{+} \).

Key concepts

Isoelectronic Species

Isoelectronic species are molecules or ions that have the same number of electrons. To determine if two species are isoelectronic, you calculate the total number of valence electrons they contain, including any charges. For instance, hydrogen carbonate ion (HCO₃⁻) and nitric acid (HNO₃) both contain 24 valence electrons. The hydrogen carbonate ion contains hydrogen (1 electron), carbon (4 electrons), and oxygen (6 electrons each from three oxygen atoms) along with an additional electron due to its negative charge. Nitric acid has hydrogen (1 electron), nitrogen (5 electrons), and oxygen (6 electrons each from three oxygen atoms). Since both species have the same total number of electrons, they are considered isoelectronic, despite having different atomic compositions and structures.
Although these species are isoelectronic, it is key to note that being isoelectronic doesn't mean they are chemically or structurally similar. It simply indicates they have the same number of electrons, which is a basis for comparison in chemistry.

Resonance Structures

Resonance structures are different ways to draw the structure of a molecule or ion that can't be accurately represented with just one Lewis structure. For these structures, electrons can be placed in different positions to show the resonance modes of the molecule. It's important to remember that resonance structures are not real separate entities but a way to depict possible configurations.
In the case of hydrogen carbonate ion (HCO₃⁻), there are two major resonance structures, which arise from the possible configurations of the double bonds between carbon and oxygen atoms. Similarly, nitric acid (HNO₃) also has two principal resonance structures. In these structures, the double bond between the nitrogen and one of the oxygen atoms can be placed in different positions among the oxygen atoms.

• Resonance helps in understanding the distribution of electron density.
• It also explains certain properties like molecular stability.

These forms are crucial in explaining behaviors and reactions of molecules, such as acidity and bonding preferences.

Formal Charge

Formal charge is a concept used in chemistry to estimate the elementary charge on an atom within a molecule. It helps in identifying the most stable structure of a molecule by comparing the distribution of electrons.
To calculate the formal charge on an atom, use the formula:
\[ \text{Formal Charge} = \text{Valence electrons on-free atom} - \text{Non-bonding electrons} - \frac{\text{Bonding electrons}}{2} \]
In the hydrogen carbonate ion (HCO₃⁻), the oxygen with the single bond holds a formal charge of -1, while other atoms have a formal charge of 0. In contrast, nitric acid (HNO₃) has a nitrogen atom with a formal charge of +1, and a single bonded oxygen not connected to hydrogen with a formal charge of -1. The sum of formal charges helps determine if the drawn structure is valid and reflects the actual charge distribution in a molecule.

• Formal charges are critical for determining which resonance structures are major contributors.
• They assist in predicting the reactivity of molecules.

Understanding formal charge can thus guide one in predicting the preferred form of a molecule in nature.

Acid-Base Behavior

Acid-base behavior of molecules is crucial in chemistry. It describes how species can donate or accept protons. This concept is often explained through the Brønsted-Lowry theory, where acids are proton donors and bases are proton acceptors.
Comparing the hydrogen carbonate ion (HCO₃⁻) with nitric acid (HNO₃), we observe distinct behaviors. The hydrogen carbonate ion can act as a base and accept a proton to form carbonic acid (H₂CO₃). This shows that it can accept an additional proton, enhancing its base character. Conversely, nitric acid is a strong acid and does not usually act as a base. It tends to donate a proton rather than accept one.

• Hydrogen carbonate ion, being amphoteric, can also function as an acid in other reactions.
• Nitric acid readily releases protons, contributing to its acidic nature.

Understanding these behaviors is critical for predicting reaction outcomes, especially buffer solutions and titration scenarios.