Integrated SequencePhysics Chemistry Organic Biology

Web Resources

Chem1 Virtual Textbook - Molecules in motion: introduction to kinetic molecular theory
Careful, thorough, easy-to-understand introduction to kinetic theory with many interesting examples and illustrations.

Chem1 Virtual Textbook - More on the kinetic-molecular model
Continuation of Dr. Lower's excellent introduction to kinetic theory.

HyperPhysics - Kinetic Theory

Purdue University - The Kinetic Molecular Theory
Illustrated introduction to kinetic theory with a good discussion of the relationship between the microstate, the perspective of the particles, and the macrostate as described in the gas laws.

PY105 Notes - Kinetic Theory
Excellent introduction to kinetic theory.

HyperPhysics - Mean Free Path

HyperPhysics - Maxwell Speed Distribution

HyperPhysics - The Maxwell-Boltzmann Distribution

HyperPhysics - Molecular Constants

HyperPhysics - Equipartition of Energy

HyperPhysics - Graham's Law



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Special points of emphasis

Heat and Temperature

The Ideal Gas and Kinetic Theory

Why do some substances possess a higher molar heat capacity than other substances? The molar heat capacity of real and ideal gases depends on the available translational and rotational 'places' available for kinetic energy. Picture an imaginary merry-go-round on wheels, a vehicle which can hold kinetic energy not only in translational motion but also in its rotation and, as well, the oscillations of the horses up and down. If a certain amount of kinetic energy were imparted on this vehicle, the energy could be distributed throughout all these 'partitions', some would increase its translational motion, but there is also room for energy in the rotations and vibrations. Because there are so many places to put energy, it would take a lot of energy before any of the partitions would become vigorous.

As a simple example, picture a diatomic molecule, such as O2. In the terminology of kinetic theory, a diatomic molecule possesses more degrees of freedom than a single atom. O2 can have vibrational and rotational motions as well as translational motion. An increase in temperature will cause the average translational energy to increase, but it also causes the energy associated with vibrational and rotational motion to increase. The oxygen gas will require a higher energy input to change the temperature by a certain amount. O2 has a higher molar heat capacity than a monatomic gas such as helium.

Large, complex molecules possess many different modes of rotation and vibration, so it takes a large amount of heat flow before the motion becomes vigorous. The energy is divided into so many partitions. The particles of the ideal gas, though, can only move translationally, in the x,y, and z directions, so the molar heat capacity of an ideal gas is the theoretical minimum.




Work, Energy, and Power

Momentum and Impulse

The Ideal Gas and Kinetic Theory

The States of Matter

Boyle's Law and Charles' Law are much more accessible if you think about them in terms of fundamental mechanical principles in play at the molecular level. For example, Boyle's Law describes how the pressure changes with the volume of an ideal gas at constant temperature. At its heart, Boyle's Law is telling you how pressure increases with an increase in gas density. Imagine the gas at the point of view of the particles. At greater density, even though the particles are not moving more vigorously (the temperature is constant), there are more particles colliding on a given area. At constant temperature, the pressure goes up as the volume goes down in inverse proportionality.

Charles' Law, on the other hand, describes how pressure increases with temperature in a constant volume. Even though the particle density is the same (the volume is the same), because the particles are moving faster at higher temperature, the collisions, which give rise to pressure, must be more vigorous. At constant volume, the pressure is directly proportional to temperature.








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