Chapter_5_Heating_and_Changes_of_State_A_Story_of_Polarity_and_Intermolecular_Forces_released

Chapter 5: Heating and Changes of State

Chapter Objectives

By the end of this chapter, you should be able to:

• 5.1: Use VSEPR theory to predict the geometries of molecules and polyatomic ions. (Section 5.1)

• 5.2: Use electronegativities to classify the covalent bonds in molecules. (Section 5.2)

• 5.3: Determine whether covalent molecules are polar or nonpolar. (Section 5.2)

• 5.4: Identify the major attractive force between molecules in a given substance. (Section 5.3)

• 5.5: Distinguish phenomena that demonstrate kinetic energy from those that demonstrate potential energy. (Section 5.4)

• 5.6: Identify the states of matter using five properties: density, shape, compressibility, particle interaction, and molecular movement. (Section 5.4)

• 5.7: Classify changes of state as exothermic or endothermic. (Section 5.5)

• 5.8: Use the factors that affect evaporation and condensation to rank substances in order of increasing vapor pressure. (Section 5.5)

• 5.9: Use the factors that affect boiling and melting to rank substances in order of increasing boiling point and melting point. (Section 5.5)

• 5.10: Calculate energy changes that accompany heating, cooling, or changing the state of a substance. (Section 5.6)

Section 5.1: Geometries of Molecules and Polyatomic Ions

VSEPR Theory

• VSEPR Theory: Valence-shell electron-pair repulsion theory; predicts molecular shapes based on electron pair repulsion.

• Electron Domain: Includes lone pairs and shared electrons from single, double, or triple bonds.

Electron Domains

• Electron Domains Count:

  1. Count all valence-shell electron pairs around the central atom equally.

  2. Single, double, or triple bonds count as one electron domain.

• Bonding domains (due to bonds) vs. nonbonding domains (lone pairs).

Molecular Geometry

• Electron-domain Geometry: Shape assumed when electron repulsions are minimized.

• Molecular Geometry:

  • Two Electron Domains: Linear shape.

  • Three Electron Domains: Trigonal planar.

    • Ranges:

      • Trigonal Planar (3 bonding, 0 lone pairs)

      • Bent (2 bonding, 1 lone pair)

  • Four Electron Domains: Tetrahedral shape.

    • Ranges:

      • Tetrahedral (4 bonding, 0 lone pairs)

      • Trigonal Pyramidal (3 bonding, 1 lone pair)

      • Bent (2 bonding, 2 lone pairs)

Section 5.2: The Polarity of Covalent Molecules

What is Polarity?

• Polarity: Distribution of electrical charge between atoms in a bond.

• Electronegativity: Tendency of an atom to attract electrons; increases from left to right across a period and bottom to top within a group.

Bond Polarization

• Nonpolar Covalent Bond: Electrons shared equally in a bond.

• Bond Polarization: Unequal sharing of electrons leads to polarization; more electronegative atoms gain a partial negative charge.

• Polar Covalent Bond: Electrons are unequally shared, leading to partial positive and negative charges.

Classification of Bonds

• Electronegativity Differences:

  • ∆EN = 0: Pure covalent bond

  • ∆EN < 0.4: Nonpolar covalent bond

  • 0.4 < ∆EN < 1.8: Polar covalent bond

  • ∆EN > 1.8: Ionic bond

Predicting Molecule Polarity

• Polar Molecule: Contains polarized bonds with non-symmetrical charge distribution.

• Nonpolar Molecule: No polarized bonds or symmetrical distribution of charge.

Section 5.3: Intermolecular Forces

Attractive Forces

• Intramolecular Forces: Forces within molecules (ionic/covalent bonds).

• Intermolecular Forces: Forces between molecules, including:

  • Ion-Dipole Interactions

  • Hydrogen Bonds

  • Dipole-Dipole Attractons

  • London Dispersion Forces

Interaction Types

• Ion-Dipole Interaction: Attraction between a charged ion and a polar molecule.

• Hydrogen Bonding: Attraction involving H bonded to F, O, or N, interacting with F, O, or N in neighboring molecules.

• Dipole-Dipole Interaction: Attraction between positive end of one polar molecule and negative end of another.

• London Dispersion Forces: Weak attractive forces from momentary nonsymmetric electron distributions.

Section 5.4: Energy and Properties of Matter

Kinetic vs. Potential Energy

• Kinetic Energy: Energy of particles in motion, associated with disruptive forces.

• Potential Energy: Stored energy due to position or arrangement, associated with cohesive forces.

Physical Properties of States of Matter

• Solids: Strong attractive forces, high density, definite shape, low compressibility.

• Liquids: Intermediate attractive forces, high density, indefinite shape, substantial molecule movement.

• Gases: Weak attractive forces, low density, indefinite shape, extreme molecule movement.

Section 5.5: Changes of State

Classifications

• Endothermic Processes: Absorb heat

• Exothermic Processes: Liberate heat

• State Changes Examples:

  • Melting (endothermic)

  • Freezing (exothermic)

  • Vaporization (endothermic)

  • Condensation (exothermic)

  • Sublimation (endothermic): solid to gas

  • Deposition (exothermic): gas to solid

Key Points

• Boiling Point: Temperature at which liquid's vapor pressure equals atmospheric pressure.

• Heat of Fusion: Energy required to melt 1 g of solid at constant temperature (e.g., water: 80 cal/g).

• Heat of Vaporization: Energy required to vaporize 1 g of liquid at constant temperature (e.g., water: 540 cal/g).

Chapter Exam Preparation

• Practice with VSEPR shapes and identify electron domain geometries.

• Understand bond polarities and master electronegativity trends.

• Differentiate intermolecular forces and their implications on physical properties.

• Use concepts from energy dynamics to classify transitions between states of matter.