Molecular Motions CHEM Study

Molecular Motions CHEM Study


This is a production of the Chemical
Education Material Study. It may be hard to believe that the
average speed of the molecules in air, which brings sound to our ears, is 1,000
miles per hour. When you see the lightning strike a mile away,
it is this speed of molecular motion that brings the Thunder sound some five
seconds later. When most people look at substances, they normally notice only the
bulk properties, such as shininess, shape or hardness. However when a chemist looks at substances, he often visualizes them in terms of atomic and molecular models,
which interpret the bulk properties in terms of atoms and molecules and the way these pack together. For example, this is crystalline mercury and its model. Here
is ice and it’s model. And here is a model of liquid water. This is a model of
solid carbon dioxide, or dry ice. Such models indicate a shape and orientation
for the atoms or molecules, but no motion is shown. Yet we know that solids melt to
liquids as, does this crystal and mercury, and we know most liquids are quite fluid.
Other crystals like carbon dioxide change readily from solids directly to
gases. The cold, gaseous carbon dioxide condenses water from the air to give a
white fog. Properties such as volatility and fluidity suggests that molecules
have motion. Consequently static, space-filling models do not fully
represent the properties of substances. Let’s return to the mercury sample and
look for evidence of molecular motion in it. Heating converts the apparently
stationary solid into a freely flowing liquid. Upon further heating, the mercury
boils. A condensate of liquid mercury appears. Apparently gaseous molecules
were formed, moved away from the liquid surface, and condensed to liquid at the
top of the tube. The rigid model shows structure, but it is inadequate to
account for properties involving motion. But here is a model in which motion is
apparent. It consists of about three thousand small glass beads, sandwiched
between two pieces of glass. To simulate the effect of temperature, we shake the
model with this agitated. The beads jostle about, bumping into one another
and occasionally moving past one another. There is no apparent order to the
structure. What you are seeing proves to be a rather good model of atomic motion
in a substance like liquid mercury. We may simulate cooling by decreasing
the mechanical agitation. A remarkable change occurs: The beads assume highly
ordered positions. The model now illustrates the regularly-packed
structure of crystalline mercury. Close inspection reveals that the glass beads
are still oscillating, just as do the atoms in real crystals. The motion
decreases with decreasing temperature, but slight oscillation continues, even at
absolute zero. Let’s increase the agitation, simulating a rise in
temperature. Motion increases, large blocks of atoms move with respect to one
another, but atoms in a solid seldom exchange places or intermix. Further rise
in temperature increases the motion, until finally, at a sharply defined level
of energy or temperature, the solid structure disintegrates, just as did the
solid mercury, into the random orientation and disordered motion
characteristic of liquids. You can see the larger amount of open space and the
smaller degree of structure, which characterize liquids.
Now let’s consider the molecular motions in the transition from liquid to gas. The
mercury tube has been modified slightly by introducing bits of blue glass, which
float on the mercury. As the temperature rises, the increased motions of the
mercury atoms jiggle the pieces of glass. But look what’s happening now! The pieces of glass rise and undergo a vigorous agitation. why? Mercury atoms are leaving the liquid and condensing at the top of the tube. Such motion from place to place
is called translation. The moving mercury atoms bombard the glass particles,
causing the bombarded glass to bounce about. Let’s look at translational motion
at the molecular level. Here in very slow motion, our models of several kinds of
gaseous molecules undergoing translation. That is, motion from place to place. In
addition, most of the polyatomic molecules have a further kind of motion:
rotation. And close examination shows still another kind of motion: vibration. Rotation and translation involve only
molecular tumbling and simple motions through space, but vibrational motions
can appear complicated as in this model. Here is a model of water undergoing one
possible vibrational pattern, but every actual vibration can be considered as a
combination of simpler vibrations, in which the atomic motions repeat over and over again. In one motion, the bonds stretch in an unsymmetrical fashion. Here
we have a symmetrical stretching vibration. A third vibration is mostly
bond bending. The frequency of each vibration is determined by the strength
of the bonds and the masses of the nuclei. In a water molecule containing
hydrogens of mass two, the vibrations slow down.
The simple vibrational patterns combined to give the more complicated actual
vibrations. The total molecular motion consists of various combinations of
vibrations, rotations, and translations, which can be changed by a collision. Note
the increased amplitude of vibration, indicating greater energy in the
molecule, where the amplitude of vibration could decrease when the
molecule loses energy. The energy content can also increase by absorption of light
or decrease by emission of light. Not only the total energy of a molecule but
also the distribution of its energy may be affected by a collision. Simple
motions may result, or more complicated ones. Here is a mechanical system in
which polyatomic models are being jostled about. The lack of order and the
close contact make this approximate a liquid model, rather than gas or solid. In
a liquid system, the molecules are much closer together than in a gas, hence the
molecules collide more often. But the resultant motion for complicated
molecules is again a combination of molecular translation, rotation, and
vibration. The effect of the jostling on molecular motions can be seen more
clearly if we introduce a marked model. The marked molecule can move from one
place to another only very slowly because there is so little open space
between the molecules. Thus, the net translational motion in unstirred
liquids may be very slow, even though vigorous molecular motions
are occurring. Consider this experiment: To aqueous iron-two ions we will add
aqueous purple permanganate ions. Pouring causes efficient stirring, and a rapid
reaction occurs, as shown by the color change. But what happens if we use a crystal of
potassium permanganate? The crystal drops to the bottom of the iron-two solution,
almost unchanged. Once on the bottom, the permanganate
begins to dissolve, but moves away from the solid very slowly. Diffusion in
unstirred liquids is slow, often about one meter per month. Molecular motion in a
liquid is restricted by the close packing of the molecules. Stirring, of
course, aids the molecular motion in mixing the ingredients. More rapid
reaction can then occur. Let’s do a similar experiment: Dropping a vial of
bromine into a cylinder full of air. Diffusion is more rapid than in an unstirred liquid. The large open spaces between the gaseous molecules allow each molecule to move rather freely and to intermix quite readily. Diffusion in air
has a rate of about one meter per hour, 1,000 times that in liquids. In solids,
molecular intermixing is very slow indeed. Here is a rock whose layers have
been in close contact for many millions of years. Any intermixing must be
extremely slow. Thus, if we mix solids, for example, sulfur and zinc, we expect very
slow, if any, reaction, because of the restricted molecular motion and the very
slow intermingling of the molecules. But if we place the mixture in a
porcelain dish and heat, molecular motion increases, but intermixing still does not
occur until the sulfur melts. Vigorous reaction follows as a result of the
increased molecular motion. Now consider reaction between two gases: The red
balloon contains oxygen. The blue one, hydrogen. The gases mix easily. In the
resulting solution, the intermixed molecules are colliding freely, yet
reaction does not occur. But if we supply additional energy in the form of a flame,
we have a very fast reaction indeed. So our usual static models illustrate a
great deal about structure, but dynamic models show us the molecular motion that is always present and which increases with rising temperature. Polyatomic
dynamic models show us how to interpret liquid behavior and show how molecules
intermix slowly in liquids, even though there is vigorous translational and
rotational motion. When the molecules are widely separated, as in a gas, even more
freedom of motion is possible. Translation, rotation, and vibration go on
almost unhindered. As intermixing and molecular motions increase, the ability
of molecules to react also increases. Perfumes are much used chemicals, carried by diffusion through the air. Gas diffusion is relatively slow, about one
meter per hour, but may be aided by blown air and thermal currents. Sound is
transmitted by collisions between air molecules, each of which is moving
randomly at some 1,000 miles per hour. Both a slow diffusion of perfume through
air and the rapid movement of sound are examples of molecular motions.

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