Intermolecular Forces Bonding Unit Part C) Intermolecular Forces

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

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

London Dispersion Forces VIDEO

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.

Dipole-Dipole Forces VIDEO

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

Hydrogen Bonding VIDEO

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

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

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

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

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

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

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.

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