Problem 73 - Introductory Chemistry Atoms First

Question

Nitric acid consists of \(\mathrm{NO}_{3}^{-}\) and a proton, \(\mathrm{H}^{+}\). (a) Draw a Lewis dot diagram for the nitrate ion. (b) Draw a Lewis dot diagram for nitric acid. (Hint: Nitric acid contains an \(\mathrm{O}-\mathrm{H}\) bond. \()\) (c) Which species, nitrate ion or nitric acid, is more likely to be found in aqueous solution? Explain.

Step-by-Step Solution

Answer

In summary, the nitrate ion, \(\mathrm{NO}_{3}^{-}\), has a Lewis dot diagram with a single electron and a negative charge on the central nitrogen atom, connected to three oxygen atoms (one with a double bond to nitrogen and two with single bonds). For nitric acid, the central nitrogen atom is connected to two oxygen atoms (one with a double bond and one with a single bond) and an \(\mathrm{O}-\mathrm{H}\) bond. In an aqueous solution, the nitrate ion is more likely to be found since nitric acid will readily donate its proton, forming nitrate ions and hydronium ions.

Step 1: Drawing Lewis structure for Nitrate Ion

Let's start by drawing the Lewis structure for the nitrate ion, \(\mathrm{NO}_{3}^{-}\). 1. Count the total number of valence electrons. Nitrogen (N) has 5 valence electrons, oxygen (O) has 6 valence electrons, and considering the extra electron from the negative charge, we have a total of \(5+(3\times6)+1=24\) valence electrons. 2. Place the central atom, which is the one that has the least electronegativity. In this case, nitrogen (N) will be the central atom. 3. Distribute the electrons such that the octet rule is satisfied for every atom, placing the remaining electrons around the central atom. 4. Check the octets. If an atom doesn't have an octet, form double or triple bonds as required. Applying the above rules for the nitrate ion, we get the following structure: ..N.. .| | O=N-O (with a single electron and a negative charge on the central nitrogen atom)

Step 2: Drawing Lewis structure for Nitric Acid

Now, let's draw the Lewis structure for nitric acid, considering that there is an O-H bond. 1. Count the total number of valence electrons. In this case, we have one hydrogen atom (H) as well. For nitric acid, we have a total of \(5+(3\times6)+1=24\) valence electrons (the same as for the nitrate ion). 2. Place the central atom, which, once again, is nitrogen (N). 3. First, form the O-H bond. 4. Distribute the rest of the electrons such that the octet rule is satisfied for every atom, placing the remaining electrons around the central atom. Applying the above rules, we get the following structure for nitric acid: ...O ...\| ...N=O ...|. ...O ...\ ... H

Step 3: Identifying the more stable species in aqueous solution

To determine which species - nitrate ion or nitric acid - is more likely to be found in an aqueous solution, we need to consider the behavior of each species in water. Nitric acid, being a strong acid, will readily donate its proton (H\(^{+}\)) to water to form the nitrate ion (\(\mathrm{NO}_{3}^{-}\)) and a hydronium ion (\(\mathrm{H}_{3}\mathrm{O}^{+}\)). Thus, in an aqueous solution, more nitrate ions and hydronium ions will be present compared to nitric acid molecules. This means that the nitrate ion is more likely to be found in aqueous solution than nitric acid. In conclusion, (a) The Lewis dot diagram for the nitrate ion is: ..N.. .| | O=N-O (with a single electron and a negative charge on the central nitrogen atom) (b) The Lewis dot diagram for nitric acid is: ...O ...\| ...N=O ...|. ...O ...\ ... H (c) The nitrate ion is more likely to be found in aqueous solution, as the nitric acid will readily donate its proton to water to form nitrate ions and hydronium ions.

Key Concepts

Nitrate Ion

The nitrate ion, represented by the chemical formula \(\mathrm{NO}_3^{-}\), is an important component in educational chemistry exercises.It consists of one nitrogen (N) atom centrally connected to three oxygen (O) atoms.The negative charge indicates that there is one extra electron compared to the neutral molecule.

When drawing the Lewis dot diagram for the nitrate ion, we account for a total of 24 valence electrons:

• Nitrogen contributes 5 electrons.
• Each oxygen contributes 6 electrons, totalling 18 from oxygen.
• Plus one extra electron due to the negative charge, resulting in 24 valence electrons overall.

To satisfy the octet rule, the electrons are arranged such that each oxygen is double-bonded to nitrogen, while one is single-bonded with an additional lone pair, allowing resonance structures.

This causes the nitrate ion to have a symmetrical planar structure, contributing to its stability in various chemical processes.

Nitric Acid

Nitric acid, with the chemical formula \(\mathrm{HNO}_3\), plays a significant role in many industrial and laboratory settings.In its Lewis structure, a notable component is the presence of an O-H bond (hydroxyl group), which readily participates in reactions.

Similar to the nitrate ion, it consists of 24 valence electrons:

• 5 electrons from nitrogen.
• 18 electrons from the three oxygen atoms.
• 1 electron from the hydrogen atom.

The formation of an O-H bond distinguishes nitric acid from the nitrate ion, making it a strong acid.This acid readily dissociates in water, losing its hydrogen ion \(\mathrm{H}^{+}\) to form the nitrate ion \(\mathrm{NO}_3^{-}\) and a hydronium ion \(\mathrm{H}_3\mathrm{O}^{+}\).

This ability to donate a proton in reactions is a key characteristic of nitric acid, influencing its behavior in aqueous environments.

Aqueous Solution

An aqueous solution is a solution in which the solvent is water, and various solutes, such as acids, bases, and salts, can be dissolved.When considering whether the nitrate ion or nitric acid is more stable in an aqueous environment, it's essential to understand the chemical behavior of both in water.

Nitric acid, as a strong acid, tends to dissociate quickly in water, donating its proton \(\mathrm{H}^{+}\) to form the nitrate ion \(\mathrm{NO}_3^{-}\) and hydronium ion \(\mathrm{H}_3\mathrm{O}^{+}\).

• This reaction occurs readily because nitric acid completely ionizes in an aqueous medium.

As a result, the concentration of nitrate ions in an aqueous solution is generally higher than that of undissociated nitric acid molecules.

Thus, students will observe that in water, the nitrate ion is the predominant species, showcasing how chemical equilibria and solubility rules are applied in practice.