Intermolecular Forces, Phase Changes, and Solutions: General Chemistry Study Guide

Intermolecular Forces and Molecular Properties

Dipole Moment and Molecular Polarity

The dipole moment is a measure of the separation of positive and negative charges in a molecule. Molecules with symmetrical charge distribution tend to have low or zero dipole moments.

• Nonpolar molecules have no net dipole moment due to symmetry (e.g., CH3CH2CH3).

• Polar covalent bonds occur when atoms with different electronegativities share electrons unequally.

• Example: CF4 contains polar bonds (C-F) but is nonpolar overall due to its tetrahedral symmetry.

Surface Tension and Molecular Structure

Surface tension is the energy required to increase the surface area of a liquid. It is influenced by intermolecular forces:

• Substances with weak intermolecular forces (e.g., nonpolar hydrocarbons like C6H6) have lower surface tension.

Intermolecular Forces

Intermolecular forces (IMFs) determine physical properties such as boiling point, melting point, and solubility.

• Dipole-dipole forces occur between polar molecules (e.g., SO2).

• Dispersion (London) forces are present in all molecules, especially nonpolar ones.

• Hydrogen bonding is a strong IMF present when H is bonded to N, O, or F (e.g., H2O, ethanol).

• Ion-dipole forces occur between ions and polar molecules.

Phase Changes and Thermochemistry

Enthalpy of Vaporization ($riangle H_{vap}$)

The enthalpy of vaporization is the energy required to convert a liquid to a gas at constant pressure.

• Substances with strong IMFs (e.g., H2O) have higher $riangle H_{vap}$.

Melting Point

The melting point is the temperature at which a solid becomes a liquid. It is higher for substances with strong IMFs or ionic bonds.

• Example: Ag (silver) has a high melting point due to metallic bonding.

Heating and Phase Change Equations

When heating a substance, temperature changes and phase changes are described by different equations:

• For temperature change: $q = mC riangle T$

• For phase change: $q = m riangle H$

Solutions and Their Properties

Solution Formation

A solution forms when solute-solvent interactions are strong enough to overcome solute-solute and solvent-solvent interactions.

• Solute: The substance being dissolved.

• Solvent: The substance doing the dissolving (usually present in greater amount).

• Miscible: Liquids that mix in all proportions (e.g., ethanol and water).

• Immiscible: Liquids that do not mix (e.g., oil and water).

Colligative Properties: Boiling Point Elevation

The boiling point of a solution increases with the addition of a nonvolatile solute. The change is given by:

• $riangle T_b = K_b imes m$

• Where $K_b$ is the ebullioscopic constant and $m$ is the molality of the solution.

Vapor Pressure Lowering

Adding a solute to a solvent lowers the vapor pressure of the solvent. This is described by Raoult's Law:

• $P_{solution} = X_{solvent} imes P^0_{solvent}$

• Where $X_{solvent}$ is the mole fraction of the solvent and $P^0_{solvent}$ is the vapor pressure of the pure solvent.

Phase Diagrams

Understanding Phase Diagrams

A phase diagram shows the states of matter (solid, liquid, gas) as a function of temperature and pressure.

• Triple point: The unique set of conditions where all three phases coexist.

• Critical point: The end point of the liquid-gas boundary.

• Phase boundaries: Lines separating different states (e.g., melting, boiling, sublimation).

Organic and Biological Chemistry

Vitamin A1 (Retinol)

Retinol is a fat-soluble vitamin (Vitamin A1) important for vision and cellular growth.

• Structure: Contains a long hydrocarbon chain and an alcohol group.

• Solubility: More soluble in oil than in water due to its nonpolar hydrocarbon structure.

Key Definitions

• Solution: Homogeneous mixture of two or more substances.

• Solvent: The component present in the largest amount in a solution.

• Solute: The component present in a smaller amount, dissolved in the solvent.

• Miscible: Two liquids that mix completely in all proportions.

• Immiscible: Two liquids that do not mix, forming separate layers.

Sample Table: Types of Intermolecular Forces

Type of Force	Example	Relative Strength
Dispersion (London)	CH4, I2	Weak
Dipole-Dipole	SO2, HCN	Moderate
Hydrogen Bonding	H2O, NH3	Strong
Ion-Dipole	Na+ in H2O	Very Strong

Example Calculations

Boiling Point Elevation

For a 3.5% (by weight) NaCl solution in water:

• Calculate molality ($m$) and use $K_b$ to find $riangle T_b$.

• $riangle T_b = K_b imes m$

Vapor Pressure Lowering

• Use Raoult's Law to calculate new vapor pressure after adding solute.

Additional info:

• Questions cover topics from Ch.10 (Liquids, Solids & Phase Changes), Ch.11 (Solutions & Their Properties), and Ch.8 (Thermochemistry: Chemical Energy).

• Includes application of colligative properties, phase diagrams, and intermolecular forces.