32

GENERAL CHEMISTRY FOR ENGINEERS

Course Information

  • CHEN 1201

  • Instructor: Carolyn Kohlmeier

  • Department: Chemical and Biological Engineering

Week 12 Lecture 32

  • Topics Covered:

    1. Bond Energies

    2. Bond Length

    3. VSEPR Theory

VSEPR Theory

Definition

  • VSEPR (Valence Shell Electron Pair Repulsion) theory explains how the repulsions between electron groups around a central atom dictate molecular geometry.

  • Key Terms:

    • Electron Groups: Can include lone pairs and bonds.

    • Molecular Geometry: The spatial arrangement of bonded atoms in a molecule.

Geometry and Electron Groups

  • As the electron groups repel each other, they maximize their separation, leading to specific geometric structures:

    • 2 electron groups: Linear

    • 3 electron groups: Trigonal planar

    • 4 electron groups: Tetrahedral

    • 5 electron groups: Trigonal bipyramidal

    • 6 electron groups: Octahedral

  • Greater electron density leads to greater repulsion between groups.

Preferred Geometries Based on Electron Repulsion

  • The following geometries are preferred based on the number of regions of electron density:

    1. Linear (2 regions): 180°

    2. Trigonal Planar (3 regions): 120°

    3. Tetrahedral (4 regions): 109.5°

    4. Trigonal Bipyramidal (5 regions): 90° and 120°

    5. Octahedral (6 regions): 90°

Impact of Lone Pairs

Repulsion Order

  • Lone pairs exert more repulsion compared to bonding pairs. The order of repulsion strength is:

    • LP-LP > LP-BP > BP-BP

  • LP: Lone Pair

  • BP: Bonding Pair

Examples

  • Ammonia (NH3):

    • Electron Geometry: Tetrahedral

    • Molecular Geometry: Trigonal Pyramidal

Molecular Geometries

  • Categories of Shapes Based on Regions of Electron Density:

    • 2 Regions: Linear

    • 3 Regions:

    • Trigonal Planar (e.g., H2CO)

    • Bent (e.g., SO2)

    • 4 Regions:

    • Tetrahedral (e.g., CH4)

    • Bent (e.g., H2O)

    • Trigonal Pyramidal (e.g., NH3)

    • 5 Regions: Trigonal Bipyramidal, T-shaped, See-saw, Linear

    • 6 Regions: Octahedral, Square Pyramidal, Square Planar

Steps to Determine Molecular Shape Using VSEPR

  1. Determine the Lewis Structure of the molecule.

  2. Count the Number of Electron Groups around the atom of interest.

  3. Arrange Electron Groups to maximize their separation from each other.

  4. Account for Lone Pairs, as they take up more space than bonding pairs.

Examples of Molecular Geometry Determination

Linear Example

  • Carbon Dioxide (CO2):

    • Molecular Geometry: Linear

    • Bond Angle: 180°

Trigonal Planar Example

  • Formaldehyde (H2CO):

    • Molecular Geometry: Trigonal Planar

    • Bond Angle: Approximately 120°

  • Sulfur Dioxide (SO2):

    • Bonding: 2 regions of density, 1 lone pair

    • Molecular Shape: Bent

    • Bond Angle: Approximately 120°, with slight variations (e.g., 117° - 122°)

Tetrahedral Example

  • Methane (CH4):

    • Molecular Geometry: Tetrahedral

    • Bond Angle: 109.5°

  • Water (H2O):

    • Molecular Geometry: Bent

    • Bond Angle: 104.5°

  • Ammonia (NH3):

    • Molecular Geometry: Trigonal Pyramidal

    • Bond Angle: Approximately 107°

Trigonal Bipyramidal Example

  • Molecular Structures:

    • All 5 atoms bonded to the central atom consist of a trigonal bipyramidal shape; however, variations occur with less than five bonded atoms.

    • Observations of T-shaped and see-saw geometries based on bond count.

Octahedral Example

  • Shapes Involving Six Regions of Density:

    • Octahedral: All 6 bonded atoms contributing to the shape

    • Square Pyramidal: 5 bonded atoms affecting shape

    • Square Planar: When only 4 bonded atoms contribute to molecular shape

Practice Questions

Question 1:

  • What is the molecular geometry of CO3²-? Options:

    • (A) Bent

    • (B) T-shaped

    • (C) Linear

    • (D) Trigonal pyramidal

    • (E) Trigonal planar

    • Total Electrons: 24

    • Electron Groups: 3

Question 2:

  • Determine the electron geometry (eg) and molecular geometry (mg) of XeF2. Options:

    • A. eg = trigonal bipyramidal, mg = trigonal planar

    • B. eg = linear, mg = linear

    • C. eg = tetrahedral, mg = tetrahedral

    • D. eg = trigonal bipyramidal, mg = linear

    • E. eg = tetrahedral, mg = bent

Question 3:

  • Analyze the molecular geometry at each of the two labeled carbons in a specific molecule. Options:

    • A. C1 = tetrahedral, C2 = linear

    • B. C1 = trigonal planar, C2 = bent

    • C. C1 = bent, C2 = trigonal planar

    • D. C1 = trigonal planar, C2 = tetrahedral

    • E. C1 = trigonal pyramidal, C2 = see-saw