|The orientation aspect of collision theory, for the reaction NO3 + CO ⇌ NO2 + CO2, the idea that colliding molecules have to be aligned in a certain favorable way, during the instance of impact, in order for a reaction to occur or for the activation energy barrier to be surmounted. |
“You can’t react if you don’t collide.”
In gas phase reactions, where particles, such as those found in the air of a room, move about a speeds in excess of a 1,000 miles per hour, at room temperature, collision theory becomes visually intuitive (particularly when viewed in the context of the Boltzmann chaos assumption), whereas in the liquid phase, and for surface catalyzed reactions, where particles move about at slower speeds, in the neighborhood of one mile per hour for human chemical reactions, collision theory becomes a bit more of a gray area.
Collision theory was proposed in 1916 by German chemist Max Trautz and British scientist William Lewis, in order to qualitatively explain how chemical reactions occur and why reaction rates differ for different reactions. 
Collision theory, in simple terms, is based on the assumption that for a chemical reaction to occur it is necessary for the reacting species, i.e. atoms or molecules, to come together or collide with one another. 
According to collision theory, in order for a chemical reaction to occur, the chemical entities have to collide. Not all collisions, however, bring about chemical change. A collision will be effective in producing chemical change only if the species brought together possess a certain minimum value of internal energy U, equal to the activation energy EA of the reaction.  These are called effective collisions and result in the transformation of reactant molecules into products.  In human terms, a pair that collides, dates, and then transforms into a married couple, would be an example of an effective collision.
Effective collisions occur as a consequence of the fact that only a fraction of the molecules have sufficient energy and the right orientation at the moment of impact to break the existing bonds and form new bonds. In other words, not only must the colliding species posses certain energies, but they must also be oriented in a manner favorable to the necessary rearrangement of bonds, atoms, and electrons involved. Thus, according to collision theory, the rate at which a chemical reaction proceeds is equal to the frequency of effective collisions.
|Promo slogan (Ѻ) for the 2004 film Crash by Paul Haggis, giving an detailed look at "collisions" in the social sphere (see: social collision theory).|
Social collision theory
See main: Social collision theoryIn human reaction terms, collision theory applies absolutely; however, the visual conception of collisions between human molecules, moving over substrate, requires a bit of discussion, as well as the development of new terminologies. In other words, collision theory was developed originally for reactions between atoms and molecules in the gas or liquid phase. Subsequently, applying it to human reactions occurring over substrate will require further development. In particular, a theory of human molecular reaction orbitals, i.e. probabilistic activity orbitals, describes collisions such that from a time-accelerated point of view, collisions of human probability orbitals, oriented in various unique energetic manners, will play a significant role in determining the outcome of basic human molecular collisions.
Hence, collision theory supposes that molecular collisions must always precede a chemical reaction.  To give examples of frequency, in normal gas phase systems each molecule hits other molecules at a rate of 10E9 times per second in bimolecular collisions, whereas trimolecular collisions occur at a rate of 10E5 per second. In liquid phase systems, the collision rates are less frequent than this. In human molecular systems, i.e. air-vapor phase, substrate-attached systems, collision rates are more difficult to estimate; moreover there are social collisions as well as intimate collisions to consider, among others.
The 2004 Award-winning film Crash gives an excellent overview of the intricacies involved social collisions. In the film, several characters, living in Los Angeles, collide during an eventful 36-hour period in which car accidents, shootings, and carjackings bring them together. Most of the characters depicted in the film are racially prejudiced in some way and become involved in conflicts which force them to examine their own prejudices. Through these characters’ interactions, the film attempts to depict and examine not only racial tension but the distance between strangers in America. 
Along these lines, we see that collision theory can also be interpreted in terms of the force of the collision, namely that the likelihood of reaction upon the collision of two molecules must certainly be affected by the force of the collision. Forceful collisions are much more apt to lead to the breaking and making of chemical bonds than are less forceful collisions. The film Crash, for example, a self-described ‘passion piece’ for Canadian screen-writer Paul Haggis, was inspired by a real life incident where Haggis’ Porsche was forcefully carjacked outside a video store on Wilshire Boulevard in Los Angeles in 1991.  This is an example of a real-life social bimolecular effective collision, between two human molecules, resulting in a human chemical reaction (life transformation), resultant change (moviegoers moved by the film), and work output (script writing).
In human romantic collisions, we note that molecules with less than sufficient energy to exert this force will not react, whereas molecules with a higher energy than this minimum can react.  Along these lines, statistically it is known that 20-28 percent of couples fall in love at first site.  In these love-at-first sight collisions, out of the many collisions that didn’t activate, a situation occurred in which one or both of the reactant pairs, owing to necessary and sufficient internal energy requirements, at the moment of collision, e.g. looks, humor, class, education, occupation (substrate attachment), fitness, wealth, status, integrity, etc., exerted a force on the other, so as to create an effective collision. In other interactive situations, collisions may be more subtle; yet the chemistry can be felt. One study, for example, found that 40 percent of single adults say that they know whether or not they have chemistry either instantly or within 15 minutes of meeting someone. 
Species concentration is also a factor in collision theory. In short, an increase in concentration of reactants increases the collision frequency between the reactants; subsequently, the effective collision frequency also increases. In other words, at higher species concentrations, more collisions will occur, resulting in more reactions. In human molecular terms, for example, more effective collisions, and subsequent reactions, will occur in a packed nightclub, than will occur in a dead nightclub.
Temperature is also a factor in collision theory. Essentially, an increase in temperature increases the average speed of the reactant molecules, the number or frequency of collisions, and fraction of molecules having kinetic energy higher than the activation energy. Resultantly, the effective collision frequency increases. In human terms, as an example, one is more likely either get pregnant or get someone pregnant during spring break, i.e. a system at warmer temperatures, as compared to winter break, i.e. a system at colder temperatures. Population densities confirm this: temperate climates, on average, are more populated, i.e. have yielded more product of a diversified variety, than polar climates.  There are more varieties of species in the tropics, for example, than anywhere else on earth.
To note, temperature T, pressure P, and volume V in human system terms are intricate topics, requiring a great deal of mental thought and discussion. There are those who will adamantly argue that the inherent ‘heat’ of human attraction, the ‘pressure’ of stressed or tense situations, or the ‘volume’ of territory, for instance, are only verbal metaphors, having nothing whatsoever to do with physics; and that they are not quantifiable by the standard gauges of thermometers, i.e. devices that measure temperature or temperature gradient, barometers, i.e. instruments used to measure atmospheric pressure, or indicator diagrams, i.e. devices that measure pressure and volume changes in working systems. The inherent difficulty lies in the fact that basic human energies and pressures are smaller in comparison to atmospheric energies and pressures. As such, no one has yet built agreed upon human thermodynamic instruments, to adequately quantify pressure (the force of neighboring molecules) and temperature (the energy transfer effect of neighboring molecules) in human systems. This is a topic of further inquiry.
|The collision rate being inversely proportional to reactant concentration aspect of collision theory.|
In any event, in sum, chemical reaction rates tend to increase with both reactant concentration and with system temperature. In biological terms, it is well documented that the varieties of life increase, i.e. reaction rate is faster, in warmer climates than as compared to colder climates. Similarly, the first to note a correlation between reaction rate and biological species concentration was German zoologist Carl Semper who, in 1881, observed that the multiplication of organisms in small ponds diminished as the number of individuals increased.  Later experiments confirmed Semper’s concentration generalizations with fruit flies and fowls in other environments. 
Assuming an initial stage species A and a final stage species B, while neglecting a large number of intermediate reaction evolutions, these types of overall system evolution reactions can be approximated as follows: A → B Thus, using Semper’s terminology, as the number of individuals, or concentration of B, increases, the multiplication, or reaction rate, of organisms, in small ponds diminishes:
In other words, the reaction rate is inversely proportion to the concentration of the products, whereby the rate decreases as product concentration increases. In this direction, the earliest collision theories regarded reactant molecules as hard spheres such that a collision was considered to have occurred when the distance d between the centers of the two molecules was equal to the sum of their radii.  In the gas phase, as discussed, an increase in concentration of the moving reactant species increases the collision frequency between the reactants, thus increasing the number of effective collisions or ‘conversion encounters’, which thus results to increase reaction rate.
Collision scenarios in human reaction life are similar, only here the collisions occur over substrate and are dynamic and non-reversible.  If, for example, a twenty-five-thousand person capacity high school begins the year with a thousand students, i.e. a low concentration of students, there will be relatively fewer reactions between students than as compared to if the school was at full-capacity. If then, by chance, a new batch of one-thousand students is suddenly introduced into the existing school system, due to possibly the closing of a neighboring school, then we should expect the reaction rate of the system to increase according to collision theory.
|The activation energy aspect of collision theory: with a catalyst, reactions occur more easily, without catalyst only with more impact energy.|
In a typical chemical reaction, as described, the reactant species come together in a collisional manner. During the collision, many chemical bonds are stretched, broken, and or formed in the products of the reaction. In human life, as an example, during the process of courtship and marriage, previous family, friendship, occupational, and social bonds, etc., will invariably be either adjusted or broken, in a significant manner, and new ones will be formed, so to fuse the requisite marriage bonds with the new developing family. During these collisional periods, the energy of the system increases to a maximum, at a point or energy height called the ‘potential barrier’ that separates the two potential energy minimas, and then decreases to the energy of the products, as detailed adjacent.
To explain collision in biological terms, we will refer to Alfred Lotka’s famous 1922 paper ‘Natural Selection as a Physical Principle’. In this paper, Lotka defines survival of the fittest as persistence of stable forms. Furthermore, after a discussion of the first and second laws of thermodynamics, he defines evolution as a change in the distribution of matter among the components of a physical system.  Next, Lotka states that living organisms are little Carnot energy transformers, which, from a second perspective, according to Lotka, can be thought of as tiny material particles in larger statistical systems. ‘Real phenomena,’ according to Lotka, ‘are irreversible; and in particular, trigger action, which plays so important a role in the process of life, is a typical irreversible process, which releases available energy from a false equilibrium.’ The term trigger action is used, by Lotka, as an overarching label to categorize sensory inputs, and the correlative nervous system processing, that trigger a significant energetic response.
Lotka approximates that organic evolution involves two types of energy transformers: accumulators (plants) and engines (animals). Moreover, Lotka argues that the similarity of the units of individual organisms invites a statistical treatment, in which the units are not just simple material particles, from the ordinary reversible elastic collisions of kinetic theory, e.g. the pool table analogy, collisions in which action and reaction are equal; but rather, the units of biological statistical mechanics are energy transformers subject to inelastic irreversible collisions of a peculiar type, i.e. collisions in which trigger action is the dominate feature.  For example, according to Lotka, ‘when the beast of prey A sights its quarry B, the latter may be said to enter the field of influence of A, and, in that sense, to collide with A. The energy that enters the eye of A in these circumstances may be insignificant, but it is enough to work the relay, to release the energy for the fatal encounter.’
Human collisions actuate according to similar principles. Romantic collisions, of course, the most interesting collisions in human chemistry and immensely more complicated in mechanism, than atomic collisions. Stories abound, for instance, in which a spouse recalls having first had their mate enter their ‘field of vision’ and knowing right then and there that he or she was the one for them. From a day-by-day perspective, we may not be directly inclined to think of human interactions as collisions, per se; but when human life is viewed at a sped-up pace, most can easily look back and remember encounters or collisions with certain people that resultantly functioned to change one’s life forever.
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