Average kinetic energy = Here the bar over the KE indicates average kinetic energy. The "m" is the mass in kilograms and the "v" is the velocity in meters / second.

Pressure is a commonly used word. It is applied to psychological stress and mechanical conditions. Pressure has a very specific meaning in chemistry. It is defined as force divided by area. It may be worth while to recall that a force is a "push" or a "pull". Pounds per square inch is a pressure unit seen on inflated objects like basket balls, inflated boats and car tires.

This unit has the qualities of force (weight is a measure of force)in the numerator and area, inches x inches = inches², in the denominator.

When you stand on the floor, you exert a force against the floor. The pressure you exert is your body weight in pounds divided by the surface area under your feet or shoes. In snow country you can see people wear snow shoes. They do this to avoid sinking in the snow. This trick spreads a person's body weight over a larger area. The constant body weight and larger area produces a lower pressure on the snow surface. This is another reason why large vehicles have such big tires. The larger mass must be distributed over more area or else the higher pressure on the road surface would mechanically break down the road.

Pressure units are varied. Commonly used pressure units are listed here.

Unit	symbol
Millimeters of mercury		mm
Pounds per square inch		PSI
torr		torr
Pascal	Pa
atmospheres	atm

The normal reference for pressure is the earth's atmosphere. One atmosphere is related to these units.

	76 cm
1 atmosphere	760 mm Hg
	101,325 Pa
		14.7 psi
		29.

Pressure measurements

A list of pressure units is good as a reference but how is pressure measured.

The barometer is the simplest instrument for measuring atmospheric pressure. The earth's atmosphere has a weight of 14.7 pounds over a square inch of surface. This is the weight of a column of air that extends from the earth's surface to the edge of the atmosphere. This weight changes as the temperature and composition of the air mass changes. A barometer uses a substitute column of mercury fluid in place of the air. A cheaper barometer can be mae using a column of water.

Mercury is the most dense fluid available. It has a density of 13.6 g/mL. This is 13.6 times the density for water. Original barometers where made using water. This was a nuisance because the length of the mercury column is 76 cm fluid

The manometer is the simplest tool for measuring gas pressure differences. A manometer is a u-tube. One side of the "U" is open to the aair and the other side is connected to a closed container. The "U" is filled with a fluid. If both sides of the "U" have the same liquid levels then the pressure inside and the pressure outside are he same. The difference between the liquid levels equals the pressure difference between inside and outside. The liquid level will be lower on the side with greater pressure.

Gas Laws

There are two types of gas equations. There is the class of equations describing a single state and the class of equations that compare two states.

The single state equations are

Single state

Single state equations
Dalton's law	Ptotal = P1 + P2 +		Boyle's law		P1V1 = P2V2
Universal gas law	PV = nRT		Charle's law		V1/T1 = V2 /T2
			Gay-Lussac's law		P1/T1 = P2 /T2
		Combined gas law		P1V1/T1 = P2V2 /T2
		Avogadro's law		n1/V1 = n2/V2
All these equations are follow assumptions of the kinetic molecular theory for gases.

Gas properties change with temperature and pressure. A reference standard temperature and pressure, are set at STP, 273.16^o Kelvin and 1 atmosphere pressure.

All gases at STP have the same molar volume, 22.4 Liters. This naturally comes about when the Ideal / Perfect Gas law is used to calculate volume for 1 mole at STP.

Molecular attractions

Atractions between molecules are classified as

Dipole-dipole interactions

London forces also known as van der Waals forces

Hydrogen bonding

The relative size of these interactions is important so the relative effects are understood.

Relative strength of forces

Covalent bonds > Hydrogen bonding > Dipole-dipole interactions > London forces van derWaals force

400 kcal > 12-16 kcal > 2-0.5 kcal > less than 1 kcal

Dipole-dipole interactions exist between molecules that are polar. This requires the presence of polar bonds and a unsymmetric molecule.

London forces exist in nonpolar molecules. These forces result from temporary charge imbalances. These temporary charges in one molecule attract opposite charges in nearby molecules. The temporary charges exist because the electrons in a molecule or ion move randomly in the structure. A local positve charge in one molecule will be attracted to a temporary negative charge in a neighboring molecule.

Example:

appear to be

Hydrogen bonding is a unique type of intermolecular molecular attraction. There are two requirements.

The first is a covalent bond between a H atom and either F, O, or N. (These are the three most electronegative elements.)

The second is an interaction of the H atom in this kind of polar bond with a lone pair of electrons on a nearby atom of F,O, or N.

Summary:

Possible combinations where hydrogen bond can exist. The first entry shows the covalent bond to the O or N atom. These atoms form two and three covalent bonds. The single covalent bond between O,N,F is shown and the dashed line shows the hydrogen bond. NOTICE the H atom is attracted to a lone pair on the nearby N,O,F atom.

A covalent bond between -O-H ----- :O-

A covalent bond between -N-H ----- :O-

A covalent bond between F-H ----- :O-

A covalent bond between -O-H ----- :N-

A covalent bond between -N-H ----- :N-

A covalent bond between F-H ----- :N-

A covalent bond between -O-H ----- :F-

A covalent bond between -N-H ----- :F-

A covalent bond between F-H ----- :F-

Hydrogen bonding is resposible for the expansion of water when it freezes. The water molecules in the solid have tetrahedral spacial arrangement for the two lone pairs and two single bonds radiating out from the oxygen. The lone pairs on the "O" atoms are attracted to nearby water molecules through hydrogen bonds. A cage like structure results. The cage has an hexagon shaped openning.

Exercise: Which of the following molecules show display hydrogen bonding?

Methane, CH4	methyl ether, CH3OCH3
Hydrogen peroxide, H2O2	methyl alcohol, CH3OH

Intermolecular forces and properties of liquids

Boiling point an liquid volatility are two physical properties that depend on intermolecular forces. The stronger the intermolecular forces the less volatile the substance. High intermolecular attractions produce a higher normal boiling point. You probably want to ask, WHAT IS A NORMAL BOILING POINT? Is there an ABNORMAL BOILING POINT?

Terms to know