Problem 5 - General Chemistry: Principles and Modern Applications

Question

Arrange the following compounds in the expected order of increasing solubility in water, and give the basis for your arrangement: \(\mathrm{Li}_{2} \mathrm{CO}_{3}, \mathrm{Na}_{2} \mathrm{CO}_{3}\) \(\mathrm{MgCO}_{3}.\)

Step-by-Step Solution

Answer

The compounds in increasing order of solubility are: \(\mathrm{MgCO}_{3}<\mathrm{Li}_{2}\mathrm{CO}_{3}<\mathrm{Na}_{2}\mathrm{CO}_{3}\)

Step 1: Understand solubility rules

Solubility rules state that all alkali metal salts are soluble. Alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs). Carbonates are usually insoluble except those of alkali metals and ammonium ion (NH4+). Among alkali metals, lithium salts are less soluble than sodium salts. Thus, \(\mathrm{Li}_{2}\mathrm{CO}_{3}\) should be less soluble than \(\mathrm{Na}_{2}\mathrm{CO}_{3}\). Magnesium carbonate (\(\mathrm{MgCO}_{3}\)) is largely insoluble in water.

Step 2: Arrange the compounds

Based on the solubility rules, the given compounds should be arranged in increasing order of their solubility in water as follows: \(\mathrm{MgCO}_{3}<\mathrm{Li}_{2}\mathrm{CO}_{3}<\mathrm{Na}_{2}\mathrm{CO}_{3}\)

Key Concepts

Alkali Metal Salts

Alkali metal salts are generally known for their excellent solubility in water. This group includes compounds formed with alkali metals such as lithium (Li), sodium (Na), and potassium (K), among others. The reason behind their high solubility lies in their atomic structure and the nature of their ionic bonds. Alkali metals have only one electron in their outer shell, which they easily lose to form positively charged ions.

• These ions readily dissolve in water, making water a great medium to separate these ions from their corresponding salts.
• Lithium carbonate (\(\mathrm{Li}_{2}\mathrm{CO}_{3}\)) and sodium carbonate (\(\mathrm{Na}_{2}\mathrm{CO}_{3}\)) are examples of alkali metal carbonates.
• Among them, sodium carbonate is more water-soluble than lithium carbonate due to the lighter charge density of the sodium ion.

This is why when arranging these carbonates by their solubility, sodium carbonate is expected to dissolve better than lithium carbonate in water.

Insoluble Carbonates

While alkali metal carbonates are soluble, most other carbonates are not. The term "insoluble carbonates" refers to compounds like magnesium carbonate (\(\mathrm{MgCO}_{3}\)) that don't easily dissolve in water. This characteristic depends largely on the ionic interactions in these compounds.

Magnesium carbonate does not dissolve well because the magnesium ion \(\mathrm{Mg}^{2+}\) forms stronger bonds with carbonate ions \(\mathrm{CO}_{3}^{2-}\) than water can break.

• This makes the crystal lattice of magnesium carbonate more stable, thereby reducing its solubility.
• Hence, in our order of increasing solubility, magnesium carbonate is the least soluble among the compounds considered.

It's important to understand that exceptions to solubility rules often relate to such unique intermolecular forces and should not override general trends without a solid reason.

Water Solubility

Water solubility is a key concept that determines how substances interact with water. It is generally categorized based on the ability of a compound to dissolve in water, dictated by specific solubility rules.

Compounds that are easily soluble tend to liberate their ions when they come in contact with water. The "like dissolves like" principle is a straightforward rule of thumb.

• Ionic compounds tend to dissolve in water because water, being a polar solvent, stabilizes the ions.
• Nonpolar compounds are less soluble due to their inability to interact favorably with water molecules.

In the provided exercise, the solubility of the compounds (\(\mathrm{Li}_{2}\mathrm{CO}_{3}, \mathrm{Na}_{2}\mathrm{CO}_{3}, \mathrm{MgCO}_{3}\)) in water reflects their differing tendencies to separate into ions.

Knowing how water solubility works will help predict behavior in various solutions.