Collision model theory and applications

Collision Model: Explanation and Application

The Collision Model is the theory that chemical reactions are the result of collisions between molecules. These molecular collisions must be strong enough to break bonds in the reacting substances. Breaking the bonds result in a rearrangement of the original configuration of atoms and a new product or products is formed ("Collision Model," Answer Corporation, 2006). Not every collision between molecules will create new products. In fact, the vast majority of collisions do not because they are not strong enough to break the existing bonds, which are often quite strong between molecules. For the collision to be successful, the molecules have to be oriented in such a way that the activation energy is sufficient to generate a reaction in the substance in question. Temperature, the presence of catalysts, the concentration of the substance, and other factors affect activation energy. Also, the molecules within molecular bonds are also held together with different degrees of strength which affect the potential and kinetic energy released from collisions. "In order to break these bonds, the colliding molecules have to have a large amount of kinetic energy from the collision. If they do not have enough energy, the reaction will not occur" ("Reaction rate: Collision Model," Chemistry, 2008).

The Arrhenius Equation is an equation that measures the activation energy of a particular reaction and quantifies the collision model in a way that can be useful for scientists conducting experiments. It "represents the dependence of the rate constant k of a reaction on the absolute temperature T: k = A exp (-Ea/RT). In its original form the pre-exponential factor A and the activation energy Ea are considered to be temperature-independent" ("The Arrhenius Equation," IUPAC, 1997). Strictly speaking, the Arrhenius Equation can be applied only to gas reactions. The Arrhenius Equation is founded on the "empirical observation" that conducting a reaction at a higher temperature increases the reaction rate of the compound (Keutch 2006). However, in actual practice the equation is also used in a variety of scientific experiments where reaction rates are involved, to determine the relative impact of temperature and other catalytic agents. The equation also takes note of the fact that when reactions do occur, the rate of collisions depends on the concentrations of the reactants, since the more molecules there are in a confined space, the greater the likelihood the molecules will collide with one another (Hutchinson 2006). This is why the more concentrated a substance, the more swiftly and the more likely a reaction will occur than in a diluted solution. This is also why heat increases the rate of reaction because it increases the speed of the molecules moving around in the contained space, increasing the likelihood and rate of collision. Common sense as well as "chemical intuition" thus suggests that the higher the temperature, the faster a given chemical reaction will occur within a confined space and the stronger the substance the more likely a reaction will occur (Hutchinson 2006). Ultimately this all 'boils down' (no pun intended) to the energy of the substance. "Quantitatively this relationship between the rate a reaction proceeds and its temperature is determined by the Arrhenius Equation. At higher temperatures, the probability that two molecules will collide is higher. This higher collision rate results in a higher kinetic energy, which has an effect on the activation energy of the reaction. The activation energy is the amount of energy required to ensure that a reaction happens. For a reaction to occur, at least some bonds in the reactant molecule must be broken, so that atoms can rearrange and new bonds can be created. At the time of collision, bonds are stretched and broken as new bonds are made. Breaking these bonds and rearranging the atoms during the collision requires the input of energy" (Hutchinson 2006).

Different experimental parameters will thus impact the product of the Arrhenius Equation. For example, the Arrhenius Equation can show the effect of a change of temperature on the "rate constant" and therefore the change in the rate of the reaction (Clark 2002). If the rate constant doubles, the rate of the reaction will likewise double. Also, the equation shows how "a catalyst will provide a route for the reaction with lower activation energy" (Clark 2002). A catalyst is a substance that speeds up a reaction without changing its own substance, lowering the activation energy required for reactions and allowing collisions to be more effective, and more collisions and more reactions. A homogeneous catalyst is present in the same phase as the reacting molecules while a heterogeneous catalyst exists in a different phase then the reacting molecules, such as a solid ("Chemical Kinetics: Chapter 12," AP Chemistry Notes, 2008).