CHEM II: Intermolecular Forces

How can the properties of solids, liquids, and gases be explained?

through the kinetic energy of the molecules and the attractive forces between molecules

What is kinetic energy?

gives molecules freedom of motion; depends only on the temperature

What do attractive forces do?

try to keep the molecules together

If the attraction between atoms or molecules is very strong, how is energy affected?

it will take more energy to separate them

If the intermolecular attractive forces are high…

the normal melting and boiling point will also be high

What is intermolecular attraction due to?

attractive forces between opposite charges

larger the charge…

equals stronger attraction

longer the distance…

equals weaker attraction

What are the three types of intermolecular forces?

London dispersion, dipole-dipole, hydrogen bonding

Where is london dispersion present?

All molecules and atoms

Where is dipole-dipole present?

polar molecules

Where is hydrogen bonding present?

Molecules containing H bonded to F, O, or N

Where is ion-dipole present?

mixtures of ionic compounds and polar compounds

What causes london dispersion?

Fluctuations in the electron distribution in atoms and molecules result in a temporary dipole. This dipole causes attractive forces between molecules and atoms

What does the magnitude of the induced dipole depend on?

polarizability of the electrons and shape of the molecule

If the molar mass increases, how is the strength of dispersion forces affected?

number of electrons increases, therefore the strength of the dispersion forces also increase

How does the shape of a molecule affect the size of the induced dipole?

more surface to surface contact = larger induced dipole = stronger attraction

What kind of molecules have permanent dipoles?

polar molecules due to bond polarity and shape

What’s the order of attractive forces?

london dispersion, dipole-dipole, hydrogen bonding, ion-dipole

What is electronegativity?

a measure of the ability of an atom in a molecule to attract electrons to itself

What difference in electronegativity constitutes a pure covalent bond?

0.0 to 0.3

What difference in electronegativity constitutes a polar covalent bond?

0.4 to 1.9

What difference in electronegativity constitutes an ionic bond?

>= 2.0

What makes a molecule polar?

bonds are polar and the molecule is not symmetric

What is hydrogen bonding?

When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it; hydrogen acts as a H proton and a strong center of positive charge, thus attracting all the electron clouds from neighboring molecules

What are ion-dipole forces

the result of electrical interactions between an ion and the partial charges on a polar molecule

How to figure out what intermolecular force is being acted?

• write lewis structure

• check all the bonds polarity

• predict molecular geometry

• is there cancellation of polar bonds?

• is the net result polar or nonpolar?