Problem 50 - General Chemistry: Principles and Modern Applications

Question

The Lewis structure of nitric acid, \(\mathrm{HONO}_{2}\), is a resonance hybrid. How important do you think the contribution of the following structure is to the resonance hybrid? Explain.

Step-by-Step Solution

Answer

The importance of the resonance structure in question to \(\mathrm{HONO}_{2}\)'s hybrid depends largely on its characteristics compared to other potential structures. Factors include obeisance to the octet rule, overall charge separation, and the presence of any atoms with unusual electron counts. More stable structures significantly contribute to the overall resonance hybrid, while less stable structures contribute less. Without specific structures provided, a definitive answer is not possible.

Step 1: Identify the Resonance Structures

First, recognise that multiple Lewis structures can represent the molecule (nitric acid in this case). Each of these is a possible resonance structure. Remember that real molecules are resonance hybrids; that is, they are a weighted average of all the possible resonance structures. Note that the question did not provide the Lewis structure in question, so make sure to have clarity or assumption on which structure is being referred.

Step 2: Evaluate the Resonance Contributors

Evaluate the resonance contributors. Key factors to consider include: whether the structure obeys the octet rule (each atom should have 8 electrons in its valence shell with a few exceptions); whether the structure separates charge favourably, and whether the structure has any atoms with an unusual number of electrons. Structures with these features are generally less stable and therefore less important contributors to the resonance hybrid.

Step 3: Determine the Significance of the Controversial Resonance Structures

The significance of each resonance structure in contributing to the overall properties of the molecule is proportional to its stability. In the context of resonance theory, more stable structures contribute more to the overall hybrid. If the provided structure has desirable characteristics as discussed earlier, it would likely have a significant contribution. However, the exact extent of its contribution cannot be determined without comparison to other possible resonance structures for nitric acid. In general, structures with more formal charges, separation of charge, or violated octet rule are less stable and contribute less to the resonant hybrid.

Key Concepts

Lewis Structures

Lewis structures are a simple way to represent molecules and show how the atoms are bonded to one another by electrons. Each atom in a molecule is represented by its chemical symbol, surrounded by dots that represent its valence electrons. Lines are used to show shared pairs of electrons, or bonds, between atoms. This visualization helps us understand how molecules interact and bond.

To draw Lewis structures:

• Count the total number of valence electrons for all atoms in the molecule.
• Draw a skeleton structure with atoms connected by single bonds.
• Use remaining electrons to complete the octet for each atom.
• Transform lone pairs into double or triple bonds if necessary.

Lewis structures are the foundation for understanding resonance, as they allow us to create multiple possible configurations for the same molecule.

Octet Rule

The octet rule is a fundamental concept in chemistry. It states that atoms tend to form bonds until they are surrounded by eight valence electrons, like the noble gases, which are stable. This is because having a full set of eight valence electrons generally lowers the potential energy of a molecule, making it more stable.

While the octet rule is a useful tool, there are exceptions:

• Hydrogen, which can only hold two electrons.
• Elements in the third period and beyond that can have expanded octets.
• Certain molecules formed by transition metals.

Understanding the octet rule is crucial when evaluating possible Lewis structures and determining their stability.

Molecular Stability

Molecular stability refers to how likely a molecule is to stay intact and not undergo a chemical change. Stable molecules are generally lower in energy and thus less reactive. In the context of resonance structures, molecular stability can tell us a lot about how much each structure contributes to the overall resonance hybrid.

Factors affecting molecular stability include:

• Adherence to the octet rule.
• Formal charges being minimized or zero.
• Separation of charge being minimal to nonexistent.

More stable structures tend to dominate the resonance hybrid, thereby having more influence on the molecule's properties.

Resonance Theory

Resonance theory is a concept that explains how some molecules can be represented by two or more valid Lewis structures, known as resonance structures. These structures are not in equilibrium but rather, the real molecular structure is a hybrid, or a weighted average, of all the possible structures.

Key points about resonance theory:

• Resonance structures must have the same placement of atoms, only the electron arrangement can vary.
• Structures with fewer formal charges are usually more significant.
• Structures that obey the octet rule generally contribute more.

The resonance hybrid is usually more stable than any of the individual resonance structures alone, due to the delocalization of electrons across the molecule.