Intermolecular Forces, Physical Properties, and Solutions – CHEM 122 Study Guide

Chapter 11: Intermolecular Forces and Physical Properties

Intermolecular Forces/Attractions

Intermolecular forces (IMFs) are the forces of attraction between molecules, which play a crucial role in determining the physical properties of substances. Understanding the types and effects of IMFs is essential for predicting behavior in chemical systems.

• Dispersion (London) Forces: Present in all molecules and atoms due to temporary fluctuations in electron distribution, leading to instantaneous dipoles.

• Dipole-Dipole Attractions: Occur between polar molecules where positive and negative ends attract each other.

• Hydrogen Bonds: A strong type of dipole-dipole interaction occurring when hydrogen is bonded to highly electronegative atoms (N, O, F).

• Ion-Dipole Attractions: Occur between ions and polar molecules, especially important in solutions.

Example: Water molecules exhibit hydrogen bonding, which accounts for water's high boiling point compared to similar-sized molecules.

IMF Effects on Physical Properties

IMFs influence several key physical properties of substances:

• Surface Tension: The energy required to increase the surface area of a liquid due to cohesive forces.

• Viscosity: Resistance of a liquid to flow; higher IMFs result in higher viscosity.

• Capillary Action: The ability of a liquid to flow in narrow spaces due to adhesive and cohesive forces.

Vaporization and Vapor Pressure

Vaporization is the process by which molecules escape from the liquid phase into the gas phase. The vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid.

• Enthalpy (Heat) of Vaporization: The energy required to vaporize one mole of a liquid at its boiling point.

• Clausius-Clapeyron Equation: Relates vapor pressure and temperature, allowing calculation of enthalpy of vaporization.

Equation:

$\ln P = -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T} \right) + C$

Where $P$ is vapor pressure, $\Delta H_{vap}$ is enthalpy of vaporization, $R$ is the gas constant, $T$ is temperature in Kelvin, and $C$ is a constant.

Sublimation and Fusion

Sublimation is the transition from solid to gas without passing through the liquid phase. Fusion (melting) is the transition from solid to liquid.

• Enthalpy (Heat) of Fusion: The energy required to melt one mole of a solid at its melting point.

• Phase Diagram: A graphical representation of the states of matter as a function of temperature and pressure.

Example: Dry ice (solid CO2) sublimes directly to gas at atmospheric pressure.

Chapter 12: Solutions and Solubility

IMF Effect on Solution Formation

The formation of solutions depends on the relative strength of IMFs between solute and solvent compared to those within each component. "Like dissolves like" is a guiding principle: polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes.

• Comparable or Stronger IMFs: Solution forms when solute-solvent attractions are comparable to or stronger than solute-solute and solvent-solvent attractions.

Factors Affecting Solubility

Solubility is influenced by several factors:

• Type of Solution: Polar or nonpolar nature of solute and solvent.

• Temperature: Generally, solubility of solids increases with temperature, while solubility of gases decreases.

• Pressure: Mainly affects the solubility of gases; higher pressure increases gas solubility.

Solution Concentration Calculations

Concentration expresses the amount of solute in a given amount of solvent or solution. Common units include molarity (M), molality (m), and percent composition.

• Molarity (M): $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Molality (m): $m = \frac{\text{moles of solute}}{\text{kilograms of solvent}}$

Example: To prepare a 1.0 M NaCl solution, dissolve 1 mole of NaCl in enough water to make 1 liter of solution.

Additional info:

• Phase diagrams typically include regions for solid, liquid, and gas, and lines representing phase transitions.

• Clausius-Clapeyron equation can be used to estimate vapor pressure at different temperatures.