Problem 1 - General Chemistry: Principles and Modern Applications

Question

Which of the following do you expect to be most water soluble, and why? \(\mathrm{C}_{10} \mathrm{H}_{8}(\mathrm{s}), \mathrm{NH}_{2} \mathrm{OH}(\mathrm{s})\) \(\mathrm{C}_{6} \mathrm{H}_{6}(\mathrm{l}), \mathrm{CaCO}_{3}(\mathrm{s})\).

Step-by-Step Solution

Answer

NH2OH is the most water soluble compound as it can form hydrogen bonds with water due to its polarity.

Step 1: Identification of Compound Type

First identify whether each compound is polar or non-polar. C10H8 and C6H6 are nonpolar hydrocarbons. NH2OH is a polar covalent compound due to the presence of hydrogen bonds. Lastly, CaCO3 is an ionic compound.

Step 2: Determination of Solubility

Using the 'like dissolves like' concept, hydrocarbons(e.g C10H8 and C6H6) with nonpolar characteristics will not be easily dissolved in water. CaCO3, though ionic, is generally insoluble in water as the strong ionic bonds between the calcium and carbonate ions are too difficult for water molecules to break. The polar compound, NH2OH, would be expected to dissolve most readily in water as it can form hydrogen bonds with water.

Key Concepts

Polar vs Nonpolar Compounds

In chemistry, understanding the difference between polar and nonpolar compounds is key to predicting solubility behavior. Polar compounds have a tendency to dissolve well in polar solvents, like water, because of their unequal distribution of electrons. This results in a molecular dipole where one side holds a partial negative charge and the other a partial positive charge. A classic example is water (H₂O).

On the other hand, nonpolar compounds have electrons distributed more symmetrically, and as a result, have no dipole moment. Nonpolar compounds frequently include hydrocarbons, such as \(\mathrm{C}_{10} \mathrm{H}_{8}\) and \(\mathrm{C}_{6} \mathrm{H}_{6}\), which consist mainly of carbon and hydrogen atoms. Nonpolar molecules mainly interact through London dispersion forces, which are weaker than the hydrogen bonds present in polar compounds.

Thus, in the context of solubility, it’s evident that polar compounds will interact better with polar solvents, whereas nonpolar compounds are better suited to dissolve in nonpolar solvents.

Like Dissolves Like Principle

The "like dissolves like" principle is a time-tested guideline that helps predict the solubility of substances. Essentially, this principle states that chemical substances will dissolve in solvents that share a similar polarity.

This principle is based on the fact that polar solvents, like water, can overcome the forces holding together polar solutes because of their own similar polar interactions. For instance, \(\mathrm{NH}_{2} \mathrm{OH}\) is polar, which means it can form attractions, specifically hydrogen bonds, with water molecules, leading to dissolution.

• Polar substance + Polar solvent: High solubility
• Nonpolar substance + Nonpolar solvent: High solubility
• Polar substance + Nonpolar solvent or vice versa: Low solubility

Nonpolar solutes, such as \(\mathrm{C}_{10} \mathrm{H}_{8}\) and \(\mathrm{C}_{6} \mathrm{H}_{6}\), lack this capability when interacting with water, thereby resulting in low solubility.

Ionic Compounds Solubility

Ionic compounds dissolve based on the balance of ionic bonds and their interaction with the solvent. Although many ionic compounds dissolve well in water as it disrupts the ionic lattice, not all do. This depends significantly on the lattice energy of the compound, which is the strength of the attractions between the ions.

Take \(\mathrm{CaCO}_{3}\), for example. Despite being ionic, \(\mathrm{CaCO}_{3}\) does not dissolve well in water. This is because its lattice energy is quite high due to strong attractions between \(\mathrm{Ca}^{2+}\) and \(\mathrm{CO}_{3}^{2-}\) ions, which water molecules have a hard time breaking.

Therefore, when assessing an ionic compound's solubility, consider both the nature of the ionic bonds and the potential interactions with the solvent.