Chapter 10 Notes

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Intramolecular vs. Intermolecular Forces

Intramolecular forces: the combination of attractive and repulsive electrical forces between pairs of electrons within a molecule

Intermolecular forces: the forces that act between molecules and ions

We know that the molecules of a liquid or solid are composed of atoms held together by "intramolecular forces" called bonds

But individual molecules are aggregated together in a large sample by attractive forces called "intermolecular forces"

Intermolecular forces have considerable influence on the physical properties of all substances

Intermolecular forces (IMFs):

The preferred phase at a given set of conditions is a balance between the energy of the particles and intermolecular forces between the particles

If the forces between particles are strong enough, the substance is a liquid or, if stronger, a solid

If the forces are weak and sufficient energy is present, the particles separate from each other, so the gas phase is the preferred phase

The energy of the particles is mostly determined by temperature, so temperature is the main variable that determines what phase is stable at any given point

The attractions between molecules, due to IMFs, are not nearly as strong as the intramolecular attractions that hold compounds together

What forces define intermolecular interactions? There are two types of IMFs:

Dispersion forces: occur between all types of molecules, whether they are polar or nonpolar, but these are the only forces holding nonpolar molecules and noble-gas atoms together

Dipole-dipole forces: only occur between polar molecules

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States of Matter:

The fundamental difference between states of matter is the distance between particles

Gas: total disorder; a lot of empty space; particles have complete freedom of motion

Liquid: disorder; particles or cluster of particles are free to move relative to each other

Solid: ordered arrangement; particles essentially in fixed positions (close together)

The physical state of matter in a particular temperature and pressure is determined by the relative magnitude of two forces:

The kinetic energy (proportional to temperature) of the particles

The strength of the attractions between the particles

The stronger the interactive forces, the greater the amount of kinetic energy is needed to overcome those forces and allow the substances to melt or vaporize

In other words, higher temperatures are needed to achieve melting or vaporization

Dispersion (London) Forces:

A force present in all substances with electrons is the dispersion force

These forces occur in most nonpolar molecules and noble gas atoms

Are especially important because they're the only intermolecular force

LDF's are caused by an instantaneous pole (polarization) that can occur randomly at any given instant within a nonpolar molecule or atom

Induced dipole: a momentary shift in electron density = dispersion

While the electrons in the 1s orbital of helium would repel each other, it does happen that they occasionally wind up on the same side of the atom

At that instant, the helium atom is polar, with an excess of electrons on the left side and a shortage on the right

Another helium atom nearby would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1

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Intermolecular Forces (cont.):

The boiling point increases as the atomic number increases

Separating liquid-phase particles that are attracted to each other requires energy, and the stronger the particles' attractions for each other, the greater the amount of energy needed to separate them

The greater the energy required to separate these particles, the higher the boiling points

The larger the molecule, the greater the LDF's. Why?

Answer: large molecules have many electrons and it's easier for an uneven electron distribution to occur