Problem 14 - Chemistry: An Atoms-Focused Approach

Question

When \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}\) dissolves in water, which of the following species are nearest the \(\mathrm{Cr}^{3+}\) ions? a. Other \(\mathrm{Cr}^{3+}\) ions b. \(\mathrm{NO}_{3}^{-}\) ions c. \(\mathrm{H}_{2} \mathrm{O}\) molecules with the \(\mathrm{O}\) atoms closest to the \(\mathrm{Cr}^{3+}\) d. \(\mathrm{H}_{2} \mathrm{O}\) molecules with the \(\mathrm{H}\) atoms closest to the \(\mathrm{Cr}^{3+}\)

Step-by-Step Solution

Answer

Answer: H2O molecules.

Step 1: Write the dissolution reaction of Cr(NO3)3 in water

When Cr(NO3)3 dissolves in water, it breaks down into its constituent ions. The dissolution reaction can be written as: Cr(NO3)3 (s) -> Cr^{3+} (aq) + 3 NO3^{-} (aq)

Step 2: Identify possible interactions between Cr^{3+} ions and other species

When Cr(NO3)3 dissolves, the Cr^{3+} ions will interact with other species in the solution. The possible species are: 1. Other Cr^{3+} ions 2. NO3^{-} ions 3. H2O molecules

Step 3: Determine the strongest interaction between Cr^{3+} ions and other species

Cr^{3+} ions have a positive charge, so they will be attracted to species with a negative charge. The two possible negatively charged species in the solution are NO3^{-} ions and the oxygen atoms of H2O molecules.

Step 4: Compare the strength of the interactions between Cr^{3+} ions and negatively charged species

Due to the high charge density of the Cr^{3+} ions, they will form stronger attractions with the negatively charged oxygen atoms in H2O molecules, as these atoms are more electronegative than the nitrate ions. So, the Cr^{3+} ions will be surrounded by H2O molecules.

Step 5: Confirm which species are nearest to the Cr^{3+} ions

Based on the analysis of the interactions, we can conclude that the species nearest to the Cr^{3+} ions when Cr(NO3)3 dissolves in water are H2O molecules, with the O atoms closest to the Cr^{3+}. Therefore, the correct answer is option c.

Key Concepts

Dissolution Reaction

When a compound like \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}\) dissolves in water, the process it undergoes is called a dissolution reaction. This reaction involves the breakup of the compound into its individual ions in the solution. The solid \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}\), when added to water, dissociates into \(\mathrm{Cr}^{3+}\) ions and \(\mathrm{NO}_{3}^{-}\) ions. This can be represented by the equation:

• \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3} (s) \rightarrow \mathrm{Cr}^{3+} (aq) + 3 \mathrm{NO}_{3}^{-} (aq)\)

This reaction is crucial because it sets the stage for various interactions between ions and molecules in the solution. Dissolution reactions are fundamental to understanding aqueous solutions, as they illustrate how compounds form distinct ionic entities that interact with the solvent molecules, primarily water in this case.
These interactions influence the properties such as conductivity and stability of the resulting solution.

Ionic Interaction

Ionic interaction in aqueous solutions is a central concept that describes how ions like \(\mathrm{Cr}^{3+}\) and \(\mathrm{NO}_{3}^{-}\) behave once dissolved. The

• \(\mathrm{Cr}^{3+}\)
• \(\mathrm{NO}_{3}^{-}\)

ions each have specific patterns of interaction based on their charges. The positive charge of \(\mathrm{Cr}^{3+}\) makes it highly attracted to negatively charged species.
In our scenario, the negative charge is found in \(\mathrm{NO}_{3}^{-}\) ions and the partially negative oxygen atoms in water molecules. While \(\mathrm{NO}_{3}^{-}\) ions do present an interaction prospect, the oxygen atoms in water are more electronegative, allowing for stronger electrostatic attractions.
This setup leads the positively charged \(\mathrm{Cr}^{3+}\) ions to interact more robustly with the negatively charged ends of water molecules, creating a stabilizing environment that dissociates the original solid compound effectively.

Hydration Shell

The formation of a hydration shell is an essential aspect of how ions such as \(\mathrm{Cr}^{3+}\) behave when they are dissolved in water. Water is a polar molecule, meaning that it has a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. When \(\mathrm{Cr}^{3+}\) ions are released into water, the oxygen atoms' partial negative charge attracts and attaches around these ions.
This phenomenon creates a hydration shell, a layer of water molecules that cluster around the ion. These shells are vital because they stabilize the ions within the solution, preventing them from rejoining into a solid state.
Hydration shells are a key strategy by which nature maintains different ions solvated and in motion.

• They play a fundamental role in dissolving ionic compounds.
• They influence the solution's chemical reactivity and physical properties, such as boiling and freezing points.

In our exercise, this is why the \(\mathrm{Cr}^{3+}\) ions are closest to the oxygen atoms of \(\mathrm{H}_{2}\mathrm{O}\) molecules, forming stable ionic solutions which are a hallmark of effective dissolution in water.