A potential energy barrier similar to an activation energy is modeled by a rubber device cut from half of a racquet ball. When the rubber device is turned inside out and dropped from a small distance onto a hard surface, the device snaps into its preferred shape and pops considerably higher than the point from where it was dropped. A Ping-Pong ball can be "launched" from the device with considerable velocity. This suggests an exergonic reaction in which some activation energy must be added before reaction can proceed.
Very often, a flame or spark of energy is needed to initiate a reaction. This spark is known as the "activation energy," and it helps give the reactant particles the extra energy they need to collide effectively and initiate the reaction. The pilot light in a furnace or stove, the spark plug in a car's engine, and the flint and steel striker on a cigarette lighter all serve as reaction activators. The balls in this demonstration illustrate a physical potential energy barrier similar to the molecular energy barrier called the activation energy.
Energy is conserved.
Potential energy may be converted into kinetic energy.
The activation energy of a process is the extra energy required to change from one stable state to another. Extra energy is frequently required to initiate a change even when the change is exothermic.
The energy of reaction is the difference in potential energy stored in chemical bonds before and after reaction.
Wear goggles. The flying rubber device and Ping-Pong balls can cause eye injury.
Invert a suitable piece of rubber device trimmed from half of a racquet ball. (See Lab Hints.) Hold it 3-5 cm from the surface of a table. Release. Note what happens. Repeat the drop from progressively higher and higher starting points. Note what happens.
Repeat dropping the device with the concave side down (flat rim down). Note what happens.
Invert the device, and place a Ping-Pong ball inside the depression. Release convex side down. Note what happens.
Construction of the the device, while simple, is nevertheless tricky. Obtain a new racquet ball. Squeeze the ball, and rotate it to locate the very thin equatorial seam. Mark it with a pen.
Insert the point of a scissors (or a utility knife -- either way use EXTREME care) into the seam, and cut the ball into two halves along the seam.
Use a scissors to trim along the edge of one of the halves. Trim it down a little bit at a time, and after each trimming, try inverting and dropping the piece from 30-40 cm onto a hard flat surface.* Continue trimming and testing until the device pops from the inverted (flexed) configuration to the normal (relaxed) configuration upon landing. If too much of the edge is trimmed away, the inverted configuration cannot be maintained, and often the difference between a device that works and one that does not is only a very thin sliver of device. There is no simple rule as to how much material to trim away; an empirical approach is required!
* To invert the device, push on the outer surface until it snaps into an inside-out configuration. When drop testing it hold the inverted device horizontally, and drop it straight down with its concave (inside) surface facing downward. If it rotates while falling and hits on an edge, test it again; it may be ready, but just did not land correctly.
When dropped from a height of only 3-5 cm, the device tends to bounce back normally with no change in its configuration. When dropped, however, from a great enough height (30-40 cm), the device "un-inverts" itself by popping into the relaxed configuration and springing upward to a height of 60 cm or more.
When held a small distance above the table top, the device has increased potential energy relative to what it would have at the surface. When it is dropped, the potential is converted into kinetic energy (downward motion). When it strikes the table, it pushes downward on the surface, and the surface pushes upward on the device, stopping it momentarily and causing a small deformation in the deviceÕs shape. This stretches the device slightly; thus the kinetic energy is momentarily converted back into the potential energy of the stretched device. Just like a bouncing ball, when the device unstretches to the shape it had while falling, it pushes downward on the surface and the surface pushes upward on the device. This accelerates the device upward and the potential energy gets converted once again into kinetic energy (this time, as upward motion). As the device climbs back to original height, the kinetic energy is converted again back to potential, and we are theoretically back where we started. (Actually, given air resistance and the less than perfect elasticity of the collision, some of the energy does get lost to the surroundings in the form of heat and sound -- after all, we do hear it bounce!
When it is dropped from a greater distance, all of the same energy transitions mentioned above occur again, with one important addition. Since it has a greater potential energy to begin with, it will strike the surface with a greater kinetic energy and the deformation of the device will be more pronounced. In fact, it is so pronounced that it pushes the device past the inversion point, and the unstretching of the device this time is not one that restores it back to the shape it had while falling; it is the much greater unstretching of the device back to its original unflexed configuration. This pushes much harder on the table, and the table pushes much harder back, accelerating it upward to a height of 3-5 times its original height! Whereas this event appears to defy the laws of physics, for it seems that energy is being created, what is essentially happening is that a potential energy is being tapped that was not being tapped with the smaller drop.
From a chemist's point of view, this ties in beautifully with the concepts of kinetics and activation energies for chemical reactions. The device is, after all, somewhat stable in the inverted configuration (this is analogous to reactants such as H2 and O2). The device is much more stable, however, in the normal configuration (analogous to a product such as H2O). There exists between the two configurations a sort of potential energy hill -- an in-between configuration which is less stable than either of the other two, but which the device must go through to transform from one configuration to the other.* This in-between state is analogous, of course, to the activated complex, and the energy required to transform the device from the inverted configuration to the unstable in-between configuration is analogous to the activation energy. When the ball is dropped a small distance, this is comparable to two reactant particles colliding with insufficient energy to form the activated complex; they simply bounce off one another, with no reaction taking place. When the ball is dropped a larger distance, giving it more kinetic energy on impact, this is comparable to heating up the particles with a spark or a flame and increasing their kinetic energy, now the collision is strong enough for the activated complex to form and the reaction (in this case spontaneous and exothermic) can occur. Also note, if the device is dropped from a sufficient height, but it lands on its side, the Òactivated complexÓ will not be formed and the reaction will not occur. This, of course, is analogous to two reactant particles colliding with sufficient energy but with an ineffective orientation.
*Trying to get the device to stay in this in between configuration proves next to impossible, like trying to balance a ball on the peak of a hill.
We usually talk about exothermic reactions being ones that "give off heat," but this might give students the misconception that heat is something existing outside of the particles, something that is somehow released into the empty space between the atoms and molecules during a reaction. This model reminds us that, for the most part, the heat is the kinetic energy of the particles, and that an exothermic reaction is one where the product particles leave the collision with a greater kinetic energy than they had going into the collision. These fast moving product particles then have the opportunity to collide with and transfer some of their energy to neighboring particles, thus raising the temperature of the surroundings. Thus, when we use a methane Bunsen burner, for example, to heat up some water, the heat is not some mysterious energy form released by the combustion and somehow absorbed between the water molecules. Instead the process involves slow moving methane and oxygen molecules colliding, as the device did with the table, with enough energy to form the activated complex, and then bounce off in the form of fast moving carbon dioxide and water molecules (although, to be sure, the actual process usually involves several discrete steps). These fast moving product molecules then collide with slow moving air molecules and both bounce off at some in-between velocity, thus the energy is transferred. These faster moving air molecules in turn collide with slow moving silicon and oxygen atoms in the outer water of the glass beaker, which in turn collide with more atoms throughout the beaker. Eventually, this chain of collisions makes its way through the trillions of atoms of the beaker's wall until the faster moving silicon and oxygen atoms collide with the slower moving water molecules, and thus the water molecules inside the beaker begin to move faster.
In the variation, the ping pong ball leaves the collision with enough velocity to carry it 10-20 times as high as the distance the device was dropped. This is a good illustration of the principle behind Graham's Law: that given the same kinetic energy, lighter particles will move faster. When the potential energy stored in the inverted device is transferred into the kinetic energy of the device bouncing higher it velocity is enough to lift the device 100 cm or so. When that same energy is transferred into the much lighter ping pong ball, its velocity carries it 300-600 cm, usually ricocheting off the ceiling. This is not to say the ping pong ball has more kinetic energy than the rebounding device, it just has a greater velocity to compensate for its lower mass.
Similarly, in a chemical reaction, the lighter product molecules will always be moving faster on average than the heavier ones. An energy level diagram is shown below.
Q1. Summarize your observations of the rubber device when inverted and dropped from 30 cm. Explain the energy changes at each step.
A1. a. The rubber device drops to the table. The potential changes to kinetic energy.
b. On collision with the table the device inverts. The kinetic energy transferred to the device causes it to move over the potential energy barrier and invert itself.
c. After inversion the device bounces much higher than the dropping point. Once over the potential energy barrier, the extra potential energy, that was stored in the rubber device, is released. This energy is transferred to kinetic energy.
Q2. Summarize your observations of the Ping-Pong ball when placed in the inverted rubber device and dropped from 30 cm. Explain any energy changes not discussed in answer 1.
A2. Step a to c all take place in the second experiment also.
In addition, part of the energy from the inversion is transferred to the kinetic energy of the Ping-Pong ball. The ball bounces wildly as the energy is dissipated.
Q3. Chemists call the energy that is required to start a reaction, the activation energy. In this model, how is the activation energy added to the model.
A3. Dropping the device adds the energy required to invert it. This step corresponds to the activation energy. The rubber device does not invert without added energy.
Q4. Can you name three ways to add the activation energy to a chemical reaction?
A4. Heat, electric spark, light, and mechanical energy (pressure or grinding) are 4 possible methods.