Question

Use intermolecular forces to explain why nonpolar solutes are insoluble in polar solvents.

Short answer

Nonpolar solutes are insoluble in polar solvents because they have different types of intermolecular forces. According to the 'like dissolves like' principle, nonpolar solutes dissolve in nonpolar solvents and vice versa.

Step-by-step solution

Understand Polar and Nonpolar molecules

Polar molecules have a net dipole due to the presence of polar bonds that are arranged asymmetrically. This is characterized by a significant difference in electronegativity between two covalently bonded atoms. Non-Polar molecules occur when electrons are shared equally between atoms of a diatomic molecule or when polar bonds in a larger molecule cancel each other out.

Examine Intermolecular forces

These are forces of attraction between different molecules, affecting properties like boiling point, melting point, and solubility. They can be London dispersion forces, dipole-dipole interactions, and hydrogen bonds. Nonpolar molecules mainly experience London Dispersion Forces.

Discuss 'Like dissolves like'

This principle states that solutes will dissolve best in solvents that have similar types of intermolecular forces. Polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes.

Conclusion

Nonpolar solutes are insoluble in polar solvents because nonpolar substances don't have charges and therefore aren't attracted to the opposite charges of the polar solvent molecules. This is guided by the 'like dissolves like' principle. So polar and nonpolar substances tend not to mix.

Key concepts

Polar and Nonpolar Molecules

Molecules can either be polar or nonpolar based on how their electrons are shared among atoms. In polar molecules, there is an unequal sharing of electrons because of a significant difference in electronegativity between atoms. This creates a molecule with a net dipole moment, where one end has a partial positive charge and the other a partial negative charge.
Think of water ( H_2O ) as an example, where the oxygen atom is more electronegative than the hydrogen atoms. As a result, it pulls the shared electrons closer, resulting in a polar molecule.

• Polar molecules are characterized by an uneven distribution of charges.
• Examples include water, ammonia, and hydrogen chloride.

On the other hand, nonpolar molecules have an even distribution of electrical charge. This usually happens in two ways: when the atoms in a molecule share electrons equally or when any dipoles present are symmetrically arranged and cancel each other out. This means there's no net dipole moment.

• Nonpolar molecules include gases such as oxygen, nitrogen, and carbon dioxide.

The nature of a molecule as polar or nonpolar drastically influences its interactions and behavior with other substances.

Solubility

Solubility refers to the ability of a solute to dissolve in a solvent. It describes how well two substances mix to form a homogeneous solution, which depends largely on how the molecules interact with each other. This interaction is largely determined by the type of intermolecular forces present.
There are several types of intermolecular forces:

• London Dispersion Forces: Weak forces that occur in all molecules but are the only forces acting in nonpolar molecules.
• Dipole-Dipole Interactions: Occur between polar molecules due to the attraction of opposite charges.
• Hydrogen Bonds: A stronger type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine.

When thinking about solubility, it's important to realize that these forces must be similar for a solute to dissolve in a solvent.
For instance, polar solutes dissolve well in polar solvents because the dipole-dipole attractions can form effectively, while nonpolar solutes rely on London Dispersion Forces and dissolve well in nonpolar solvents.

Like Dissolves Like Principle

The 'like dissolves like' principle is a simple rule of thumb in chemistry used to predict solubility. It helps explain why certain substances mix well together while others do not.
According to this principle:

• Polar solutes (those with charged ends) dissolve well in polar solvents because their similar charge distributions allow them to interact through dipole-dipole forces or hydrogen bonding.
• Nonpolar solutes dissolve well in nonpolar solvents due to the prevalence of weaker London Dispersion Forces, which are the main intermolecular force acting in nonpolar substances.

When you put a nonpolar solute into a polar solvent, the lack of complementary intermolecular forces means that they don't attract each other sufficiently. Therefore, the solute won't dissolve well. This principle also explains common phenomena like why oil doesn't mix with water or why salt does. It's all about matching the types of intermolecular forces.