Problem 8 - Chemistry and Chemical Reactivity

Question

Decide whether each of the following is water-soluble. If soluble, tell what ions are produced. (a) \(\mathrm{NiCl}_{2}\) (c) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}\) (d) \(\mathrm{BaSO}_{4}\)

Step-by-Step Solution

Answer

(a) Soluble: Ni²⁺, 2Cl⁻; (b) Soluble: Cr³⁺, 3NO₃⁻; (c) Soluble: Pb²⁺, 2NO₃⁻; (d) Insoluble.

Step 1: Determine solubility of NiCl2

According to the solubility rules, chlorides (Cl⁻) are generally soluble in water except for those formed with Ag⁺, Pb²⁺, and Hg₂²⁺. Since NiCl₂ does not fall under any of these exceptions, it is soluble in water. The ions produced when NiCl₂ dissolves are Ni²⁺ and 2Cl⁻.

Step 2: Determine solubility of Cr(NO3)3

Nitrates (NO₃⁻) are always soluble in water, irrespective of the metal they are bonded to. Therefore, Cr(NO₃)₃ is soluble in water. The ions produced are Cr³⁺ and 3NO₃⁻.

Step 3: Determine solubility of Pb(NO3)2

As in the previous step, nitrates are always soluble. Thus, Pb(NO₃)₂ dissolves in water. The ions produced are Pb²⁺ and 2NO₃⁻.

Step 4: Determine solubility of BaSO4

Sulfates (SO₄²⁻) are generally soluble, except for those formed with Ba²⁺, Pb²⁺, and Ca²⁺. Since BaSO₄ includes Ba²⁺, it is an insoluble compound in water and does not dissociate into ions.

Key Concepts

Ionic Compounds

Ionic compounds are chemical compounds composed of positively charged ions (cations) and negatively charged ions (anions). These ions are held together in a lattice structure by strong electrostatic forces. Ionic compounds often form between metals and non-metals.
Some interesting characteristics of ionic compounds include:

• They have high melting and boiling points due to strong ionic bonds.
• They are usually solid at room temperature.
• They can conduct electricity when molten or dissolved in water because the ions are free to move.

In the context of the provided exercise, we see ionic compounds like \( \mathrm{NiCl_2} \), \( \mathrm{Pb(NO_3)_2} \), and \( \mathrm{Cr(NO_3)_3} \). These substances, when dissolved, break apart into individual ions. This is crucial to understand when analyzing their solubility and the ions they produce.

Dissolution Process

The dissolution process is the method by which an ionic compound dissolves in a solvent, like water, to form a solution. This occurs when the solvent particles surround and separate the ions from the crystal lattice structure of the solid.
Here's what happens in a simple dissolution process:

• The solvent molecules, typically water, interact with the surface ions of the crystal lattice.
• The solvent molecules overcome the lattice energy of the ionic compound, allowing the ions to separate.
• The ions become surrounded by water molecules and disperse throughout the solution.

In the exercise, it's noted that compounds like \( \mathrm{NiCl_2} \) dissolve in water to give \(\mathrm{Ni^{2+}} \) and \(\mathrm{2Cl^{-}} \) ions. Each ion becomes solvated and fully integrated into the water, forming an aqueous solution.

Solubility Exceptions

Solubility exceptions are specific conditions under which general solubility rules do not apply. Solubility rules help predict whether a compound will dissolve in water, but there are notable exceptions.
Let's look at some key solubility exceptions from the exercise:

• Chlorides (Cl⁻) are usually soluble, but not with Ag⁺, Pb²⁺, and Hg₂²⁺. Thus, \(\mathrm{NiCl_2}\) is soluble as Ni²⁺ isn't among the exceptions.
• Nitrates (NO₃⁻) are always soluble, without exceptions. That's why \(\mathrm{Cr(NO_3)_3}\) and \(\mathrm{Pb(NO_3)_2}\) dissolve easily.
• Sulfates (SO₄²⁻) are generally soluble but not with Ba²⁺, Pb²⁺, and Ca²⁺. \(\mathrm{BaSO_4}\) is insoluble, showing how these exceptions work.

Understanding these exceptions helps determine the solubility and resulting ions when these compounds are placed in water.

Aqueous Solutions

An aqueous solution is a solution in which the solvent is water. It's a vital concept in chemistry, as many reactions occur in aqueous solutions. When substances dissolve, they spread evenly in the water, leading to a homogeneous mixture.
Key features of aqueous solutions include:

• Ions or molecules disperse uniformly in the solution.
• The solution can conduct electricity if ions are present, due to their ability to move and carry charge.
• Concentration of dissolved substances can vary, affecting the solution's properties.

In the exercise example, aqueous solutions form when ionic compounds like \(\mathrm{NiCl_2}\) dissolve and dissociate into ions such as \(\mathrm{Ni^{2+}}\) and \(\mathrm{Cl^{-}} \). This transformation is critical for many chemical processes and reactions in both natural and industrial settings.