CHEM 1311 Lecture Notes - Lecture 11: Electrostatics, Hydrogen Bond, Electronegativity
Chapter 12: Intermolecular Forces and the
Physical Properties of Liquids and Solids
Focus of this section: Intermolecular forces
and resulting forces
Intermolecular Forces:
• gases—rapidly moving particles separated by large distance
• liquids and solids—condensed phases
= particles are significantly closer, so interactions are
stronger
= no “solid” or “liquid” laws
• Intermolecular forces (IM forces)-- attractive forces holding
particles together in the condensed phases of
matter (pertain to molecules, atoms or ions)
= control most physical properties
= determine if substance will be a solid, liquid or gas
= Result from Coulombic attractions (electrostatic
interactions)
-- Depend on the magnitude of the charge and
distance between charges
= most are weaker than ionic interactions (involve partial
charges)
intermolecular are MUCH intramolecular
forces weaker than ones
(bonds)
physical properties chemical properties
Types of IM forces.
van der Waals forces = attractive forces between atoms and
molecules of pure substances
1. Dipole-dipole interactions – attractive forces between
polar molecules (i.e. those with a dipole)
= much weaker than covalent bonds (~ 1%)
= distance dependence:
EIM α d–3
= weakness due to:
-- only partial charges, not full charges
-- collisions cause the dipoles to not be perfectly
aligned
2. Hydrogen bonding – attractive force in polar molecules
= special kind of dipole-dipole interaction
= H atom bonded to a small, highly electronegative
element (N, O and F)
R–X–H •••• Y–R (X = F, O, N)
δ– δ+ δ–
= very strong: -- bond very polar (large diff in χ)
-- large partial charges
-- partial charges highly concentrated
on small atoms
-- H very small, so H can approach Y
closely
= 5 to 10x stronger than other dipole-dipole interactions
= strong enough to be seen in vapor phase:
HF
Document Summary
Intermolecular forces: gases rapidly moving particles separated by large distance, liquids and solids condensed phases. = particles are significantly closer, so interactions are and resulting forces stronger. = no solid or liquid laws: intermolecular forces (im forces)-- attractive forces holding particles together in the condensed phases of matter (pertain to molecules, atoms or ions) = determine if substance will be a solid, liquid or gas. - depend on the magnitude of the charge and distance between charges. = most are weaker than ionic interactions (involve partial charges) forces are much weaker than intramolecular (bonds) ones intermolecular physical properties. Types of im forces. van der waals forces = attractive forces between atoms and chemical properties molecules of pure substances: dipole-dipole interactions attractive forces between polar molecules (i. e. those with a dipole) = much weaker than covalent bonds (~ 1%) - collisions cause the dipoles to not be perfectly aligned: hydrogen bonding attractive force in polar molecules.
