Intermolecular Forces and Molecular Properties Study Guide

Kinetic Molecular Theory and States of Matter

  • Kinetic molecular theory provides a model to explain the physical behavior and properties of solids, liquids, and gases.

  • Particles in a Solid:

    • Arrangement: Tightly packed, often arranged in a regular, repeating lattice pattern.

    • Movement: Particles vibrate about fixed positions; they generally do not move relative to one another.

  • Particles in a Liquid:

    • Arrangement: Particles are close together with no regular arrangement.

    • Movement: Particles move past each other while remaining in essentially constant contact.

  • Particles in a Gas:

    • Arrangement: Particles are far apart with no regular arrangement.

    • Movement: Particles move independently of one another except during collisions.

Intramolecular Forces versus Intermolecular Forces

  • Intramolecular Forces:

    • Forces of attraction that hold atoms or ions together within a compound.

    • Ionic Bond: Electrostatic attraction between oppositely charged ions.

    • Covalent Bond: Sharing of valence electrons between two atoms.

  • Intermolecular Forces:

    • Forces of attraction existing between distinct atoms or molecules within a substance.

    • Intermolecular forces (IMF) are significantly weaker than intramolecular forces.

Ionic Bond Strength and Coulomb's Law

  • Coulomb's Law: A mathematical formulation describing the electrical forces of attraction or repulsion between two charged objects.

  • Factors Determining Ionic Bond Strength:

    • Magnitude of Charge on the Ions: A higher magnitude of charge yields a stronger ionic bond (e.g., an attraction between +2+2 and −2-2 charges is stronger than an attraction between +1+1 and −1-1 charges).

    • Ionic Radius: A smaller ionic radius results in a shorter inter-nuclear distance, yielding a stronger ionic bond.

  • Physical Implications:

    • Stronger attraction between ions directly increases the overall strength of the ionic bond.

    • Greater ionic bond strength leads to higher boiling points and melting points.

London Dispersion Forces and Polarizability

  • Definition and Mechanism:

    • London dispersion forces arise from the temporary formation of instantaneous dipoles, which induce temporary dipoles in neighboring atoms or nonpolar molecules.

    • This phenomenon occurs in isolated nonpolar atoms (such as NeNe) and nonpolar covalent molecules (such as Cl2Cl_2).

  • Instantaneous Dipole and Cloud Size:

    • The magnitude of an instantaneous dipole depends directly on the physical size and spatial volume of the electron cloud surrounding an atom's nucleus or a whole molecule.

  • Polarizability:

    • Polarizability refers to the ease with which an electron cloud can be distorted by a nearby electrical charge or fluctuating dipole.

    • As the total area and volume of the electron cloud increase, the magnitude of electron cloud distortion increases.

    • Larger distortion produces a larger instantaneous dipole, which results in stronger London dispersion forces.

Dipole-Dipole Intermolecular Forces

  • Permanent Dipoles:

    • A permanent dipole moment in polar molecules causes dipole-dipole electrostatic attractions.

    • This attraction takes place between two separate, distinct molecules.

    • The interacting molecules do not need to share the same chemical identity.

  • Trend:

    • The larger the magnitude of the permanent dipole moment, the stronger the dipole-dipole intermolecular force.

Hydrogen Bonding

  • Definition:

    • Hydrogen bonding represents a particularly strong type of dipole-dipole attraction caused by very large dipole moments.

  • Structural Requirements:

    • Hydrogen bonding occurs specifically when a molecule contains a hydrogen atom directly bonded to a highly electronegative atom:

    • Fluorine (H−FH-F)

    • Oxygen (H−OH-O)

    • Nitrogen (H−NH-N)

Classification and Summary of Intermolecular Forces

  • Nonpolar Molecules:

    • Contain only London dispersion intermolecular forces.

  • Polar Molecules Without H−FH-F, H−OH-O, or H−NH-N Bonds:

    • Contain dipole-dipole intermolecular forces.

    • Contain London dispersion intermolecular forces.

  • Polar Molecules With H−FH-F, H−OH-O, or H−NH-N Bonds:

    • Contain hydrogen bonding.

    • Contain dipole-dipole intermolecular forces.

    • Contain London dispersion intermolecular forces.

  • Ionic Compounds:

    • Ionic compounds contain only ionic bonds, which are classified as intramolecular forces rather than intermolecular forces.

  • Decision Guide for Determining Primary Intermolecular Forces:

    • Step 1: Evaluate if the chemical species is polar or nonpolar.

    • If nonpolar: Dispersion forces are the primary and only intermolecular force.

    • If polar: Determine if hydrogen bonding criteria are met (H−FH-F, H−OH-O, or H−NH-N bonds present).

      • If Yes: Hydrogen bonding is the strongest intermolecular force present.

      • If No: Dipole-dipole forces are the strongest intermolecular force present.

Quantitative and Relative Force Comparisons

  • Comparing Intramolecular Forces:

    • To compare the relative strengths of ionic compounds, lattice energy values are evaluated.

    • To compare the relative strengths of covalent compounds, bond enthalpy values are evaluated.

    • Intramolecular forces are universally stronger than intermolecular forces.

  • Comparing Intermolecular Force Types:

    • The hierarchy of intermolecular strength is defined as:     \text{Hydrogen bonding} > \text{dipole-dipole} > \text{dispersion}

  • Comparing Dipole-Dipole Forces Between Polar Molecules:

    • The molecule with the larger dipole moment possesses the stronger dipole-dipole intermolecular force.

    • The overall dipole moment of a molecule depends on its molecular shape (geometry) and the electronegativity differences among its constituent atoms.

  • Comparing Dispersion Forces Between Nonpolar Molecules:

    • The molecule with the larger instantaneous dipole (due to a larger electron cloud / greater polarizability) possesses the stronger dispersion force.

  • General Scope:

    • While various exceptions exist within broader chemical systems, these fundamental rules serve as the core framework for CHEM 102 comparisons.

Pre-Class Practice and Problem Sets

  • Assignment Set 1: Binary Ionic and Nonmetal Compounds:

    • Classify as ionic or covalent, identify charges on each ion, and draw Lewis structures for:

    • LiFLiF

    • MgOMgO

    • ScNScN

    • LiILiI

    • NaINaI

    • KIKI

  • Assignment Set 2: Ionic Structure and Charge Identification:

    • Identify compound type as ionic or covalent, write Lewis structures, and determine ion charges for:

    • NaClNaCl

    • MgOMgO

    • MgF2MgF_2

  • Assignment Set 3: Molecular Geometry and Polarity of Interhalogens:

    • Identify if ionic or covalent, write Lewis structure, determine molecular geometry, and state polarity for:

    • BrF5BrF_5

  • Assignment Set 4: Haloalkane Geometry and Polarity:

    • Identify if ionic or covalent, write Lewis structure, determine molecular geometry, and state polarity for:

    • CH2F2CH_2F_2

  • Assignment Set 5: Molecular Geometry and Polarity Comparison:

    • Identify if ionic or covalent, write Lewis structures, determine molecular geometry, and evaluate polarity for:

    • HFHF

    • CO2CO_2

    • PH3PH_3

  • Assignment Set 6: Hydroxide and Alcohol Compounds:

    • Identify if ionic or covalent, write Lewis structures, determine molecular geometry, and evaluate polarity for:

    • NaOHNaOH

    • CH3OHCH_3OH