Kinetic Molecular Theory & Intermolecular Forces

ORKMT

&IMF’s

Chapter 13 – 2010 – Mr Nelson

States of MatterThe fundamental difference between states of matter is the distance between particles.

The States of Matter• The state a substance is in at a particular temperature

and pressure depends on two variables:

– the kinetic energy of the particles;– the strength of the attractions between the particles.

Solid Liquid Gas• Container

• Compress

• Flow

• Diffusion

Kinetic-Molecular Theory

A model that aids in our understanding of how gas particles behave and respond to changes.

Theory of KMT

Gases consist of large numbers of molecules that are in continuous, random motion.– Most molecules– Nobles - atoms

Theory of KMT

Total volume of all the molecules of the gas is negligible relative to the total volume in which the gas is contained.

Theory of KMT

Attractive and repulsive forces between gas molecules are negligible.

Theory of KMT

• Energy can be transferred between molecules during collisions– Elastic collisions

• the average kinetic energy of the molecules does not change with time

Theory of KMT

The average kinetic energy of the molecules is proportional to the absolute temperature.

Effusion (diffusion through a hole)

Effusion is the escape of gas molecules through a tiny hole into an evacuated space.

Effusion

Let’s compare effusion for helium and nitrogen– Helium: 4 g/mol

– N2: 28 g/mol

• With a lower mass the helium balloon would deflate faster.

Graham's LawKE1 KE2=

1/2 m1v12 1/2 m2v2

2=

=m1

m2

v22

v12

m1m2

v22

v12

=v2

v1

=

Intermolecular Forces (IMFs)

• IMF’s are found between molecules– Not between the atoms that compose the molecule

– IMF’s are weaker that single, double or triple bonds

IMF’s

IMF’s are strong enough to control physical properties such as:

• boiling and melting points• vapor pressure• viscosity

IMF’s

IMF’s as a group are referred to as van der Waals forces.

van der Waals Forces

• Dipole-dipole interactions

• Hydrogen bonding

• London dispersion forces

Dipole-Dipole Interactions• Molecules that have

permanent dipoles are attracted to each other.

Dipole-Dipole Interactions

The more polar the molecule, the higher is its boiling point.

London Dispersion Forces

• Helium has two electrons in the 1s orbital that repel each other.

• However, it does happen that they occasionally wind up on the same side of the atom.

London Dispersion Forces

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

London Dispersion Forces

Another helium nearby, then, would have a dipole induced in it:

• The electrons on the left side of helium become attracted to the dipole in He atom 1

London Dispersion Forces

London dispersion forces are attractions between an instantaneous dipole and an induced dipole.

London Dispersion Forces

• These forces are present in all molecules, whether they are polar or nonpolar.

• Only IMF’s present in Noble Gases

• The tendency of an electron cloud to distort in this way is called polarizability.

Two Main Factors Affecting London Forces

• Molecular weight– London forces increase with increased MW.

• Number of electrons– larger electron clouds are easier to polarize.

Which Have a Greater Effect?Dipole-Dipole Interactions or Dispersion Forces

• If two molecules are of comparable size and shape, dipole-dipole interactions will likely the dominating force.

• If one molecule is much larger than another, dispersion forces will likely determine its physical properties.

Boiling Points

• The nonpolar series (SnH4 to CH4) follow the expected trend.

• The polar series follows the trend from H2Te through H2S, but water is quite an anomaly.

Hydrogen Bonding

• The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong.

• We call these interactions hydrogen bonds.

Hydrogen Bonding• Hydrogen bonding arises

in part from the high electronegativity of nitrogen, oxygen, and fluorine.

Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.