General Chemistry for Engineers - Lecture Notes - 35

General Chemistry for Engineers

Course Details

  • Course Name: General Chemistry for Engineers (CHEN 1201)

  • Instructor: David Saeb

  • Department: Chemical and Biological Engineering

Week 13 Lecture Overview

  • Lecture 35 covering Chapters 12.1 - 12.3

  • Previous Lecture: Focus on orbital hybridization.

  • Today's Topics:

    • Continuing discussion on orbital hybridization.

    • Intermolecular forces.

Sigma and Pi Bonds in Benzene (C6H6)

  • Question: Identify the number of sigma (σ) and pi (π) bonds in benzene, C6H6.

  • Options:

    • A. 3 σ bonds, 6 π bonds

    • B. 6 σ bonds, 6 π bonds

    • C. 6 σ bonds, 3 π bonds

    • D. 12 σ bonds, 3 π bonds

    • E. 15 σ bonds, 3 π bonds

Hybridization of Central Atom in NO2−

  • Electron Groups: 3

  • Hybridization Options:

    • A. sp

    • B. sp2

    • C. sp3

  • Question: Determine the hybridization of the central atom in the nitrite ion (NO2−).

Distinguishing Properties of Phases

  • States of Matter: Gas, Liquid, Solid

Behavior:

  • Gas: Fluid, compressible.

  • Liquid: Fluid, incompressible (mostly).

  • Solid: Rigid, incompressible.

Structure:

  • Gas: Random, dilute.

  • Liquid: Random, dense.

  • Solid: Ordered, dense.

Molecular Interaction:

  • Gas: Molecules are in motion with weak interactions.

  • Liquid: Molecules are in motion with strong interactions.

  • Solid: Molecules fixed on a lattice with strong interactions.

Intermolecular Forces

  • Definition: Forces existing between molecules.

  • Origin: Based on electrostatic interactions.

  • Bonding Forces (strong): Involve large charges at small distances.

  • Intermolecular Forces (weak): Involve small charges at larger distances.

Types of Intermolecular Forces:

  1. London Dispersion Forces

  2. Dipole-Dipole Interactions

  3. Hydrogen Bonds

  4. Ion-Dipole Forces

Effects of Intermolecular Forces

  • Stronger intermolecular forces result in:

    • More difficulty pulling molecules apart in condensed phases (liquids and solids).

    • Liquids:

    • Higher boiling point (Tbp).

    • Larger enthalpy of vaporization (ΔHvap).

    • Greater surface tension and viscosity.

    • Solids:

    • Higher melting point (Tm).

    • Larger enthalpy of fusion (ΔHfus).

London Dispersion Forces

  • Occur due to instantaneous dipoles.

  • Characteristics:

    • More electrons in a molecule lead to greater dispersion forces.

Examples:

  • Larger Alkanes: Exhibit larger dispersion due to the larger number of electrons.

    • Example Molecule: Diesel (alkanes)

  • Size and Geometry: The magnitude of London Dispersion Forces is affected by the size and shape of the molecules.

    • For instance:

    • n-Pentane:

      • Molar mass = 72.15 g/mol

      • Boiling point = 36.1 °C

    • Neopentane:

      • Molar mass = 72.15 g/mol

      • Boiling point = 9.5 °C

Comparison of Boiling Points of Noble Gases

  • Question: Which noble gas has the highest boiling point?

  • Options:

    • (A) He

    • (B) Ne

    • (C) Ar

    • (D) Kr

    • (E) Xe

Boiling Points Summary:

  • Noble Gas Data:

    • He:

    • Molar Mass = 4.00 g/mol

    • Boiling Point = 4.2 K

    • Ne:

    • Molar Mass = 20.18 g/mol

    • Boiling Point = 27 K

    • Ar:

    • Molar Mass = 39.95 g/mol

    • Boiling Point = 87 K

    • Kr:

    • Molar Mass = 83.80 g/mol

    • Boiling Point = 120 K

    • Xe:

    • Molar Mass = 131.30 g/mol

    • Boiling Point = 165 K

Dipoles and Polar Bonds

  • Dipole Definition: A dipole is a molecular structure that has separated positive and negative charges.

    • Example: NH4+ with dipole characteristics shown in molecular diagrams.

Polar Molecules and Their Dipoles

  • Question: Which of the following molecules has a dipole?

    • (A) CH4

    • (B) CO2

    • (C) SO2

    • (D) CCl4

    • (E) C2H2 (acetylene, with a triple bond between the carbon atoms)

Comparing Boiling Points of Organic Molecules

  • Molecules to Compare: Methane, Chloroethane, Ethane.

  • Options for Increasing Boiling Point:

    • A. CH3CH2Cl < CH3CH3 < CH4

    • B. CH4 < CH3CH2Cl < CH3CH3

    • C. CH4 < CH3CH3 < CH3CH2Cl

    • D. CH3CH2Cl < CH4 < CH3CH3

Hydrogen Bonding

  • Definition: A specific interaction that occurs between a hydrogen atom covalently bonded to a highly electronegative atom (commonly nitrogen, oxygen, or fluorine) and a lone pair of electrons on another electronegative atom.

  • Characteristics: Hydrogen bonds are stronger than dipole-dipole interactions and represent one of the most significant types of intermolecular forces in chemistry.

Ranking Molecular Boiling Points

  • Substances to Rank:

    • Given structures that relate to boiling point comparisons among molecular structures.

  • Options for Ranking:

    • (A) 1 > 2 > 3

    • (B) 2 > 1 > 3

    • (C) 3 > 1 > 2

    • (D) 2 > 3 > 1

    • (E) 3 > 2 > 1

  • Note: (1 > 2) indicates that substance 1 has a higher boiling point than substance 2.

Ion-Dipole Forces

  • Explanation of ion-dipole forces relates to the interaction between ionic compounds and polar compounds.

Summary of Intermolecular Forces (Table 11.4)

  • Types of Forces:

    1. Dispersion: Present in all molecules and atoms.

    • Strength: 0.05 - 20+ kJ/mol

    1. Dipole-Dipole: Present in polar molecules.

    • Strength: 3 - 20+ kJ/mol

    1. Hydrogen Bonding: Occurs when H is bonded to F, O, or N.

    • Strength: 10 - 40 kJ/mol

    1. Ion-Dipole: Found in mixtures of ionic and polar compounds.

    • Strength: 30 - 100+ kJ/mol

Real-World Examples of Intermolecular Forces

  • Protein Folding and Dynamics: Role of intermolecular forces such as dispersion and hydrogen bonds in stabilizing biomolecular structures.

  • Diagram illustrating the contributions of various bonds and forces in stabilizing protein structures.