Problem 11 - Chemistry and Chemical Reactivity

Question

Which compound or compounds in each of the following groups is (are) expected to be soluble in water? (a) \(\mathrm{CuO}, \mathrm{CuCl}_{2}, \mathrm{FeCO}_{3}\) (b) \(\mathrm{AgI}, \mathrm{Ag}_{3} \mathrm{PO}_{4}, \mathrm{AgNO}_{3}\) (c) \(\mathrm{K}_{2} \mathrm{CO}_{3}, \mathrm{KI}, \mathrm{KMnO}_{4}\)

Step-by-Step Solution

Answer

(a) CuCl₂ is soluble; (b) AgNO₃ is soluble; (c) K₂CO₃, KI, KMnO₄ are soluble.

Step 1: Recalling Solubility Rules

To determine the solubility of compounds, we first need to recall common solubility rules. For instance, most nitrates (NO₃⁻), acetates (CH₃COO⁻), and perchlorates (ClO₄⁻) are soluble. Most salts of alkali metals (Li⁺, Na⁺, K⁺, etc.) and ammonium (NH₄⁺) are also soluble. Chlorides (Cl⁻), bromides (Br⁻), and iodides (I⁻) are generally soluble, except when paired with Ag⁺, Pb²⁺, and Hg₂²⁺. Carbonates (CO₃²⁻) are usually insoluble except those of alkali metals and NH₄⁺.

Step 2: Analyzing each compound group (a)

In group (a), - **CuO (copper(II) oxide)**: Most metal oxides are insoluble in water. CuO is no exception and is insoluble. - **CuCl₂ (copper(II) chloride)**: Chloride salts are generally soluble in water. CuCl₂ is soluble. - **FeCO₃ (iron(II) carbonate)**: Carbonates, except those of alkali metals and ammonium, are typically insoluble. FeCO₃ is insoluble.

Step 3: Analyzing each compound group (b)

In group (b), - **AgI (silver iodide)**: Although iodides are usually soluble, silver iodide is an exception and is insoluble. - **Ag₃PO₄ (silver phosphate)**: Phosphates are typically insoluble except those of alkali metals and ammonium. Ag₃PO₄ is insoluble. - **AgNO₃ (silver nitrate)**: Nitrate salts are generally soluble. AgNO₃ is soluble.

Step 4: Analyzing each compound group (c)

In group (c), - **K₂CO₃ (potassium carbonate)**: Carbonates are mostly insoluble, but those of alkali metals like potassium are soluble. K₂CO₃ is soluble. - **KI (potassium iodide)**: As an alkali metal iodide, KI is soluble. - **KMnO₄ (potassium permanganate)**: Permanganates of alkali metals are soluble. KMnO₄ is soluble.

Key Concepts

Water Solubility

Water solubility involves understanding how certain compounds dissolve in water. This process depends on forces like ion-dipole interactions and the nature of the compound. Soluble compounds dissociate in water releasing their ions, while insoluble compounds do not mix well.
Most compounds containing nitrates ( O₃⁻ ), acetates ( CH₃COO⁻ ), and perchlorates ( ClO₄⁻ ) dissolve easily in water. This is because they form strong ion-dipole interactions with water. For salts of alkali metals (like potassium and sodium), dissolution is also common due to their stable solvation with water molecules.

• Water molecules can surround ions, separating them and allowing them to move freely. This results in a solution.
• Water as a polar solvent interacts well with ionic compounds, meaning ion-dipole interactions are crucial in water solubility.

Recognizing which compounds will or won't dissolve requires memorization and understanding of these solubility rules.

Insoluble Compounds

Insoluble compounds generally do not dissolve in water, often remaining as solids. Common types include many oxides, carbonates, and phosphates. For instance, metal oxides and carbonates, except for those of alkali metals, tend not to dissolve.
This insolubility occurs because the forces holding the compound together are stronger than those created when these compounds interact with water molecules.
Certain exceptions exist based on specific rules; for example, while most iodides dissolve, silver iodide ( AgI ) does not.

• These compounds might form suspensions rather than true solutions.
• Understanding these exceptions helps predict behavior in various chemical reactions and extraction processes.

Learning about these rules is essential for predicting how substances behave in aqueous environments.

Soluble Compounds

Soluble compounds readily dissolve in water. Their ability to dissociate makes them play crucial roles in biological and chemical processes. Compounds like K₂CO₃ (potassium carbonate) and KI (potassium iodide) are good examples of soluble compounds.
As they dissolve, these compounds release ions, allowing for conductive solutions essential in many applications:

• Soluble substances facilitate biological processes by providing needed ions.
• In chemistry, they aid in reactions and help in the synthesis of products.

Solubility is often linked with compounds like nitrates and acetates, which naturally dissolve due to their ion-dipole interactions with water. Recognizing these patterns allows chemists to predict how mixtures behave in solutions, optimizing the design and use of materials across industries.