Problem 13 - Chemistry and Chemical Reactivity

Question

The following compounds are water-soluble. What ions are produced by each compound in aqueous solution? (a) \(\mathrm{KOH}\) (c) \(\mathrm{LiNO}_{3}\) (b) \(\mathrm{K}_{2} \mathrm{SO}_{4}\) (d) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

(a) KOH: K⁺, OH⁻; (b) K₂SO₄: 2K⁺, SO₄²⁻; (c) LiNO₃: Li⁺, NO₃⁻; (d) (NH₄)₂SO₄: 2NH₄⁺, SO₄²⁻.

Step 1: Determine the Ions for KOH

When potassium hydroxide (\(\text{KOH}\)) dissolves in water, it splits into ions. Potassium (\(\text{K}^+\)) as a cation and hydroxide (\(\text{OH}^-\)) as an anion are formed. So, the ions produced are \(\text{K}^+\) and \(\text{OH}^-\).

Step 2: Determine the Ions for LiNO3

Lithium nitrate (\(\text{LiNO}_3\)) dissolves in water to form ions. Lithium ion (\(\text{Li}^+\)) is the cation, and the nitrate ion (\(\text{NO}_3^-\)) is the anion. Therefore, the ions in the solution are \(\text{Li}^+\) and \(\text{NO}_3^-\).

Step 3: Determine the Ions for K2SO4

When potassium sulfate (\(\text{K}_2\text{SO}_4\)) dissolves, it forms two potassium ions (\(2\text{K}^+\)) and one sulfate ion (\(\text{SO}_4^{2-}\)). Thus, the ions are \(2\text{K}^+\) and \(\text{SO}_4^{2-}\).

Step 4: Determine the Ions for (NH4)2SO4

Ammonium sulfate (\((\text{NH}_4)_2\text{SO}_4\)) dissociates in water into two ammonium ions (\(2\text{NH}_4^+\)) and one sulfate ion (\(\text{SO}_4^{2-}\)). The resulting ions are \(2\text{NH}_4^+\) and \(\text{SO}_4^{2-}\).

Key Concepts

Aqueous Solution

An aqueous solution is a mixture where a solute is dissolved in water. Water acts as the solvent, meaning it is the substance that does the dissolving. Why use water? Because it is a universal solvent, known for its ability to dissolve many substances. This happens due to water's polar nature, where one end is slightly positive and the other is slightly negative.

When ionic compounds like potassium hydroxide (KOH) or lithium nitrate (LiNO₃) are added to water, they dissolve, meaning they mix with water so perfectly that they are no longer visible. This forms an aqueous solution.

• The solute (in this case, the ionic compound) separates into individual ions when in water.
• These ions are then surrounded by water molecules, which stabilize them in solution.

Water's role is crucial in this process, as its polarity is key to pulling apart the ions of the solute and keeping them dissolved.

Water-Soluble Compounds

Water-soluble compounds inherently dissolve in water to produce a solution. Understanding what makes a compound water-soluble involves looking at its molecular structure and the bonds within it. Typically, compounds made of ionic bonds, such as salts, are water-soluble.

For example, potassium sulfate ( K₂SO₄) and ammonium sulfate ( (NH₄)₂SO₄) are considered salts with ionic bonds that easily dissolve in water:

• Ionic compounds dissociate into ions because water, as a solvent, separates these ions and stabilizes them in solution.
• This solubility is crucial for various biological and industrial processes.

Not all compounds are water-soluble, but many ionic compounds are due to their interaction with water molecules. This is central to understanding how substances like KOH and LiNO₃ behave in aqueous environments.

Dissociation of Ionic Compounds

Dissociation refers to the process by which an ionic compound breaks apart into its individual ions when dissolved in water. It's like the ions are freed from their solid crystal lattice and can move around independently. This is an essential concept in chemistry because it explains how ionic solutions conduct electricity.

Let's break down some examples:

• KOH dissociates into K⁺ and OH⁻ ions.
• LiNO₃ becomes Li⁺ and NO₃⁻ ions.
• K₂SO₄ splits into two K⁺ ions and one SO₄²⁻ ion.
• (NH₄)₂SO₄ releases two NH₄⁺ ions and one SO₄²⁻ ion.

During dissociation, the charge of the ions is important. It allows attraction between the ions and polar water molecules, pulling the crystal lattice apart.

Understanding dissociation helps explain why solutions of these ionic compounds can carry an electrical current, crucial for many practical applications.