3.10. Solubility | College Board | AP Chemistry

Summary

This video explains the solubility of polar and non-polar compounds in various solvents. It covers key concepts such as the polarity of molecules, types of intermolecular forces (dipole-dipole, London dispersion), and the principle 'like dissolves like'. The video details how ionic compounds like NaCl dissolve in water, while covalent compounds like sugar also interact with water molecules through partial charges. It also explores why oil and water don't mix, due to their differing polarities, and highlights the role of intermolecular forces in determining solubility.

Takeaways

• 😀 Polar and non-polar molecules are classified based on their dipole moment and molecular geometry.
• 😀 Non-polar molecules include homonuclear diatomic molecules, hydrocarbons (alkanes, alkenes, alkynes), and certain polyatomic molecules like CO2 and CCl4.
• 😀 Polar molecules include heteronuclear diatomic molecules (e.g., HF, HBr) and polyatomic molecules with asymmetric shapes (e.g., water, SF4).
• 😀 Organic compounds like ethers, aldehydes, ketones, alcohols, carboxylic acids, and amines are polar due to their functional groups.
• 😀 Ionic compounds, such as NaCl, dissociate in water due to electrostatic interactions between ions and water molecules.
• 😀 Water molecules have partial negative charges on oxygen and partial positive charges on hydrogen, allowing them to interact with ions like Na+ and Cl-.
• 😀 The 'like dissolves like' rule explains that polar compounds dissolve in polar solvents, while non-polar compounds dissolve in non-polar solvents.
• 😀 Examples of solubility include methanol dissolving in water (both are polar) and hexane dissolving in toluene (both are non-polar).
• 😀 Oil (non-polar) does not mix with water (polar) because non-polar molecules cannot interact effectively with polar molecules.
• 😀 Intermolecular forces such as dipole-dipole attraction (for polar molecules) and London dispersion forces (for non-polar molecules) determine solubility.

Q & A

What determines whether a molecule is polar or nonpolar?

The polarity of a molecule is determined by the dipole moment, which arises due to the geometry of the molecule and the electronegativity difference between the atoms.

Can you give examples of nonpolar molecules?

Examples of nonpolar molecules include homonuclear diatomic molecules like H2, O2, and Cl2, as well as polyatomic molecules like CO2, BF3, and CCl4, and hydrocarbons such as alkanes, alkenes, alkynes, benzene, and its derivatives.

What types of molecules are polar?

Polar molecules include heteronuclear diatomic molecules like HF and HBr, as well as polyatomic molecules with asymmetric shapes such as water (H2O) and SF4. Organic compounds like alcohols, aldehydes, ketones, carboxylic acids, and amines are also polar.

How does NaCl dissolve in water?

When NaCl dissolves in water, the sodium (Na+) and chloride (Cl-) ions dissociate and are surrounded by water molecules. The water molecules interact with the ions through dipole-dipole interactions, with the oxygen atom of water attracting Na+ and the hydrogen atoms attracting Cl-.

What is the significance of the 'like dissolves like' rule?

'Like dissolves like' means that polar compounds are soluble in polar solvents, and nonpolar compounds are soluble in nonpolar solvents. For example, methanol is soluble in water because both are polar, while hexane, a nonpolar compound, is insoluble in water.

Why do oil and water not mix?

Oil and water do not mix because water is a polar molecule and oil consists of nonpolar molecules. According to the 'like dissolves like' rule, polar molecules cannot dissolve nonpolar molecules, preventing oil and water from mixing.

What intermolecular forces are involved in the solubility of polar and nonpolar compounds?

In polar compounds, solubility is facilitated by dipole-dipole interactions between the solute and solvent molecules. In nonpolar compounds, solubility relies on London dispersion forces, which are present in both the solute and solvent.

Why is NaCl able to dissolve in water while sugar, another covalent compound, also dissolves?

NaCl dissolves in water due to the ion-dipole interactions between the Na+ and Cl- ions and the water molecules. Sugar, although covalent, dissolves in water due to the hydrogen bonding between the polar regions of the sugar molecule and the water molecules.

How can we predict if a compound is polar or nonpolar?

We can predict the polarity of a compound based on its molecular structure. Polar molecules often have asymmetrical shapes and significant electronegativity differences between atoms, while nonpolar molecules have symmetrical shapes or consist of atoms with similar electronegativities.

What happens when a nonpolar molecule is dissolved in a polar solvent?

When a nonpolar molecule is dissolved in a polar solvent, there is no significant attraction between the solute and solvent molecules, which usually leads to poor or no solubility. This is because the intermolecular forces (like dipole-dipole in polar solvents) do not match with those in nonpolar solutes.