Intermolecular Forces and Physical Properties (5B) - MCAT Chemical and Physical Foundations of Biological Systems

A polar solute ("like dissolves like") is more soluble in water. Polar solutes form favorable dipole or hydrogen bond interactions with water, enhancing solubility per "like dissolves like".

$\Pi=iMRT$. Osmotic pressure follows a relation analogous to the ideal gas law, based on effective molarity and temperature.

$\Delta T_b=iK_b m$. Elevation is a colligative effect proportional to particle molality and the solvent's boiling constant $K_b$.

$\Delta T_f=iK_f m$. Colligative depression depends on effective particle concentration ($i m$) and the solvent-specific constant $K_f$.

$P_A=\chi_A P_A^\circ$. In ideal solutions, solvent vapor pressure scales with its mole fraction times the pure solvent's vapor pressure.

$\chi_A=\frac{n_A}{\sum n_i}$. Mole fraction represents the proportional contribution of a component to the total moles in a mixture.

$m=\frac{\text{mol solute}}{\text{kg solvent}}$. Molality uses solvent mass, providing a temperature-independent concentration measure for colligative properties.

$M=\frac{\text{mol solute}}{\text{L solution}}$. Molarity quantifies solute concentration based on total solution volume, useful for volumetric calculations.

Normal boiling point is when vapor pressure equals $1\ \text{atm}$. Boiling occurs when vapor pressure equals atmospheric pressure, standardized at $1\ \text{atm}$ for normal conditions.

$\Delta H_{sub}=\Delta H_{fus}+\Delta H_{vap}$. Sublimation combines the processes of fusion and vaporization in a single solid-to-gas transition.

Stronger intermolecular forces increase $\Delta H_{vap}$. More energy is needed to overcome stronger attractions when transitioning from liquid to gas.

Stronger intermolecular forces increase surface tension. Stronger cohesive forces pull surface molecules inward, minimizing surface area and increasing tension.

Stronger intermolecular forces increase viscosity. Increased attractions hinder molecular flow, making the liquid more resistant to deformation.

Stronger intermolecular forces decrease vapor pressure. Stronger forces retain more molecules in the liquid, reducing evaporation rate at a fixed temperature.

Stronger intermolecular forces increase boiling point. Greater attractions require more thermal energy to separate molecules into the gas phase.

Ion-dipole interactions between ions and water dipoles. Water molecules orient their dipoles around ions, stabilizing them through electrostatic attractions in aqueous solutions.

Attraction between an ion and the partial charges of a polar molecule. Full ionic charges attract the oppositely charged portions of polar solvent molecules like water.

London dispersion < dipole-dipole < hydrogen bonding < ion-dipole < ionic. This ranking reflects increasing interaction strength from temporary induced dipoles to permanent charge attractions and full ionic bonds.

Attractive forces from instantaneous and induced dipoles. Fluctuating electron distributions create temporary dipoles that induce similar dipoles in nearby molecules, leading to weak attractions.

Greater polarizability (larger electron cloud; higher molar mass). Larger, more distortable electron clouds enhance temporary dipole formation and strength of induced attractions.

Less branching (greater surface area) increases dispersion forces. Straighter chains provide more surface area for contact, strengthening induced dipole interactions.

Attraction between permanent dipoles of polar molecules. Permanent partial charges in polar molecules align to create attractive forces between opposite ends.

H bonded to N, O, or F; interacts with N, O, or F lone pair. High electronegativity creates a strongly partial positive H, enabling attraction to lone pairs on another electronegative atom.

N, O, or F. These highly electronegative atoms polarize the H-X bond sufficiently for strong hydrogen bond formation.