Not enough kinetic energy for reactants to leap over reaction barrier.
A "hot" reaction in progress showing reactants leaping over the activation energy barrier.
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Not enough kinetic energy for reactants to leap over reaction barrier. | The Kinetic Energy of Atoms | A "hot" reaction in progress showing reactants leaping over the activation energy barrier. |
Tying Things Together
INTRODUCTION. Every high school course in introductory chemistry seems to include units and labs on the gas laws and on heat capacity. There are so many wonderful numbers for students to crunch that there is the prevailing tendency to make these two physical phenomena ends in themselves rather than a means for exploring chemical reactions. In short, not seeing the forest for the trees.
The question that should always be asked by the intelligent sceptical scientist is: "So what?" What is the significance? What does this have to do with chemistry? What we have been presented seems to be physics and not chemistry!"
In brief, chemical reactions occur for reasons that (1) the reaction is possible, and because (2) the atoms or molecules have collided with sufficient force to "stick" (form a product). For such productive collisions to occur, the atoms must have sufficient kinetic energy.
But how can we glimpse the kinetic energy of atoms or small molecules?
Precisely! - by studying the gas laws and heat capacities. Gases have pressures because the atoms and small molecules are bouncing off the walls of the container - each collision is - with prefect elasticity - transferring its kinetic energy into the wall of that container, while heat capacity is merely storing up even more jiggle energy in the vibrations of atomic nuclei.
Force = mass x (velocity)2
Many interesting lab exercises can derive from this equation,* but they are all various renditions of the F = mv2.
Now the question devolves upon the chemistry teacher as to how to use long-standing gas law and heat capacity lab exercises to show that chemical energy can vary with the imposed conditions.
TEMPERATURE
Temperature is a necessary and intimate variable in chemistry. Temperature is, in fact, a way that we can express the kinetic energy of molecules, and similarly of the momentum of those molecules. This is important to chemistry because atoms and molecules must collide with sufficient force to overcome repulsive forces and interact with the attractive forces caused by shared electrons, etc.
Whatever substance in whichever state - solid, liquid or gas (or even plasma!) - has a temperature as nothing is as that idealized temperature of absolute zero. If one portion of a substance is warmer than another portion, it means that the momentum of the atoms in the warmer portion is greater than that of the atoms in the cooler portion. For a gas such as helium, warm He atoms are moving faster than cooler ones. What we read as helium's "temperature" is relative to what might be called the momentum of the mythically statistical average He atom in that particular container. As the compressed gas, for example, cools, the atoms slow down. And because they slow down, their collisions with the walls of the container are not as forceful, and thus the overall pressure is reduced.
A smart student might pose the question of why a gas warms up when it is compressed. Afterall, how in the world did the pushing in of a piston make the atoms move faster? The student reasons that all that was done was to lessen the space between the atoms, But that should amount to more frequently collisions with each other and the wall, but why should it be making the atoms move faster? Here it is suggested that the students take a look at "bouncing ball physics". After reading it, consider that the helium "ball" is extremely small and the "paddle" (piston) is huge. As the piston goes in, those He atoms nearest the piston are accelerated and they transfer their increased momentum to others further away from the advancing piston. Soon the increased momentum is transferred to all the atoms in the cylinder.
However, the student still wants to know how that gas can cool considering the perfect elasticity of the He atoms' bouncings. Of course, when a He atoms bounces off the surface of the piston, it is not bouncing off a flat surface - it is bouncing off the atoms or molecules that make up that surface. Thus, when the small gas molecules bounce against - say - an iron atom, a small amount of momentum is transferred back to the iron atom, which vibrates against its neighbors and so on. Heat is thus slowly removed from the gas until the momentums of both the light and fast He atoms and the sluggish but massive Fe atoms are equilized.
While there is much more behind this explanation, it ought to suffice for most students for the remainder of their lives.
Proposed Topic Outline
Discuss the classical reaction curve (symbolized on both sides of the title, above)
* Including this insight: Write the force equation for a mass that is travelling at the speed of light, "c". That is indeed what someone with the initials of A.E. did before scratching out the "F", replacing it with an "E", and then winning a Nobel Prize!
** In beginning chemistry courses, the instructor might want to have the students combine all the various constants into one term so that the important variables are conspicuous.
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