1. Intermolecular Forces
Bonding Unit
Part C) Intermolecular Forces

2. INTER vs. INTRA
Intramolecular forces = attractive forces that hold particles together in bonds
Bonds between atoms in a compound
Intra - within
Intermolecular forces = attractive forces between individual molecules
Inter – between or among
Intermolecular forces are weaker than intramolecular

3. London Dispersion Forces
Weak forces that result from temporary shifts in the density of electrons in electron clouds
When 2 molecules are in close contact, electron cloud of one molecule repels the electron cloud of other
So electron density is greater in one region than other
Weak dispersion force exists b/w oppositely charged regions
Exist between all particles
Strength of force increases with mass of molecule

4. London Dispersion Forces VIDEO

5. Dipole-Dipole Forces
Attractions between oppositely charged regions of polar molecules
Negative region of one molecule is attracted to the positive region of another molecule.

6. Dipole-Dipole Forces VIDEO

7. Hydrogen Bonds Strongest force Requires
A dipole-dipole attraction that occurs between molecules containing a hydrogen atom bonded to a small, highly electronegative atom
Strongest force
Requires
Very polar bonds/molecules
Hydrogen with Nitrogen, Oxygen, or Fluorine

8. Hydrogen Bonding VIDEO

9. Strength Comparison INTRAMOLECULAR Forces INTERMOLECULAR Forces
Ionic bonds
Metallic bonds
Covalent bonds
INTERMOLECULAR Forces
Hydrogen bonding
Dipole-dipole
London dispersion forces
Increasing Strength

10. Identifying the strongest IMF in a molecule
Given a molecule
Look at atoms & Lewis dot structure
If has H bonded to O, N, or F = hydrogen bonding
If it is a polar molecule = dipole-dipole forces
Need to calculate bond polarity and look at shape
If arrows cancel out/no dipole = nonpolar
If arrows don’t cancel out/dipole present = polar
If neither = London dispersion forces

11. PRACTICE: Identify the strongest IMF present in the molecule.
For each of the following molecules, identify what types of IMFs are present.
HBr
PCl5
H2O O2

12. PRACTICE: Identify the strongest IMF present in the molecule.
For each of the following molecules, identify what types of IMFs are present.
CCl4
CH3OH HF
H2Se

13. IMFs and Physical Properties
IMFs control how well molecules stick together
If the IMFs between molecules are strong, it will take more energy to pull those molecules apart
IMFs impact many physical properties of molecules
Melting point/boiling point
Surface tension
Viscosity
Evaporation/Vapor pressure
Solubility

14. Melting Point & Boiling Point
Melting point: temperature at which a solid becomes a liquid
Boiling point: temperature at which a liquid becomes a gas
If molecules stick together more, they'll be tougher to break apart
Stronger intermolecular forces → higher MP and BP

15. Surface tension
Surface tension: The property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules.
If the IMFs between the liquid particles are strong then the particles will be more attracted to each other.
The forces at the surface pull inward, leading to a reduction in the liquid’s surface area and thus a more spherical shape
Stronger intermolecular forces → higher surface tension.

16. Vapor Pressure Evaporation: conversion of a liquid to a gas
Vapor Pressure:  pressure of the vapor resulting from evaporation of a liquid into a gas above a sample of the liquid in a closed container. (Pressure of a vapor above its liquid)
Substances with relatively weak forces:
results in the liquid evaporating more rapidly and higher vapor pressure
Substances with stronger intermolecular forces → Lower vapor pressure