Chemical reaction rates, collision molecules

An understanding of reaction rates can be explained by adopting a collision model for chemical reactions. The collision theory assumes chemical reactions are a result of molecules colliding, and the rate of the reaction is dictated by several characteristics of these collisions. An important factor that affects the reaction rate is the frequency of collisions. The reaction rate is directly dependent on the number of collisions that take place, but several other important factors also dictate the speed of a chemical reaction. 

On a microscopic scale, atoms and molecules travel faster and, therefore, have more collisions as the temperature of a system is increased. Since molecular collisions are the driving force for chemical reactions, more collisions give a higher rate of reaction. The kinetic theory of gases suggests an exponential increase in the number of collisions with a rise in temperature. This model fits an extremely large number of chemical reactions and is called an Arrhenius temperature dependency, or Arrhenius law. The general form of this exponential relationship is... 

Chemical reaction rate depends on the collisions of molecules, per second per unit volume. Since the number of collisions of a species is proportional to its concentration, the chemical reaction rate is proportional to the product of concentrations (mass action law). Thus, for a single homogeneous elementary chemical reaction... 

Hie chemical reaction rate is usually dependent on the molar concentrations of the reactants and not on their mass fractions, because it depends on the chance of collision of molecules. However, here the definition of in terms of mass fractions is preferred, because it can readily be incorporated into mass balances. A definition in terms of moles or molar concentrations might invite the use of mole balances instead of mass balances. Since, contrary to conservation of mass, there is no such thing as conservation of moles (because one molecule might divide into several molecules, or several might condense into one), the use of mole balances is strongly dissuaded. More information concerning the definition of conversion can be found elsewhere [2]. 

The rate of molecule-molecule collisions given by Zj = /2 N/V)Trd u estimates the upper bound of gas phase chemical reaction rates and calculates the distance a molecule travels between collisions as well as the diffusion coefficient in gases The Boltzmann distribution... 

The analogy of molecules acting as if they were hard spheres lead to the Kinetic Model. It permitted the introduction of the Arrhenius pre-exponential factor and of the frequency of collisions between reacting molecules into the study of the chemical reaction rates. 

An understanding of the nature of chemical reactions requires the details of the elementary-reaction steps in which, the molecules come together, rearrange, and leave as species that differ from the reactants. There are two descriptions that deal with the rates of chemical reactions. The collision theory considers the concept that the reaction of molecules can occur only as a result of collision of the reactant molecules. The transition-state theory focuses on the species that corresponds to the maximum-energy stage in the reaction process. This species is called the activated complex or transition state. The transition state, denoted by the symbol A for reaction (1), is a short-lived species, which is converted to C. The reader is referred to [1-10] for a thorough discussion of the energetics involved in chemical reactions. 

If we now consider molecules A (in concentration colliding with molecules B (in concentration /ig), the number of collisions per second that one A molecule makes with the B molecules is TTp cn. This expression gives the maximum chemical reaction rate, assuming that each collision between A and B molecules results in a reaction. 

Shown in Figure 3.1 is the distribution of the kinetic energies of the molecules in a gas for two temperatures. The number of molecules with kinetic energy > E. is proportional to the shaded area in Figure 3.1. It can be seen that if E. is fairly large, the number of molecules with energy > E. is very sensitive to temperature. Hence, if a certain minimum value of E is required for two colliding molecules to react chemically, it is apparent why the chemical reaction rate should be both smaller and more temperature sensitive than the collision rate. 

Another important factor that determines a chemical reaction rate is the orientation of the colliding molecules. The probability of a particular collision being favorable to a reaction is generally much less than unity. 

We define the reaction cross-section, ctr, in a way suggested by the definition of the total collision cross-section (Section 2.1.5). For molecules colliding with a well-defined relative velocity v, the reaction cross-section is defined such that the chemical reaction rate constant k v) is given by... 

A gas composed of molecules of diameter 0.5 nm takes part in a chemical reaction at 300. K and 1.0 atm with another gas (present in large excess) consisting of molecules of about the same size and mass to form a gas-phase product at 300. K. The activation energy for the reaction is 25 kj-mol. Use collision theory to calculate the ratio of the reaction rate at 320. K relative to that at 300. K. 

An ultraclean environment is another major reason for generating high vacuum. At atmospheric pressure, every atom on a solid surface is bombarded with gas molecules at a rate of trillions per second. Even under a reasonably high vacuum, 10 atm, a gas molecule strikes every atom on a solid surface about once per second. If the surface is reactive, these collisions result in chemical reactions that contaminate the surface. The study of pure surfaces of metals or semiconductors requires ultrahigh vacuum, with pressures on the order of 10 atm. 

For a better understanding of the effect of changing concentrations on the rate of a chemical reaction, it helps to visualize the reaction at the molecular level. In this one-step bimolecular reaction, a collision between molecules that are in the proper orientation leads to the transfer of an oxygen atom from O3 to NO. As with the formation of N2 O4, the rate of this bimolecular reaction is proportional to the number of collisions between O3 and NO. The more such collisions there are, the faster the reaction occurs. 

In Lab 17.1, you learned about the effect of temperature and concentration on reaction rate. Another factor that affects reaction rate is the amount of surface area of the reactants. If a chemical reaction is to take place, the molecules of reactants must collide. Changing the amount of surface area modifies the rate of collision, and, thus, the rate of reaction. If surface area increases, collision frequency increases. If surface area decreases, so does the number of collisions. In this lab, you will examine the effect of surface area on rate of reaction. You will also determine how a combination of factors can affect reaction rate. 

Chemical reactions usually occur through collision of at least two molecules, very often in a solution, and the rate of the chemical reaction is proportional to the concentrations of all reacting molecules. For example, the rate of hydrolysis of an ester in an alkaline buffered solution depends upon the concentration of both the ester and hydroxide ion ... 

Approximation refers to the bringing together of the substrate molecules and reactive functionalities of the enzyme active site into the required proximity and orientation for rapid reaction. Consider the reaction of two molecules, A and B, to form a covalent product A-B. For this reaction to occur in solution, the two molecules would need to encounter each other through diffusion-controlled collisions. The rate of collision is dependent on the temperature of the solution and molar concentrations of reactants. The physiological conditions that support human life, however, do not allow for significant variations in temperature or molarity of substrates. For a collision to lead to bond formation, the two molecules would need to encounter one another in a precise orientation to effect the molecular orbitial distortions necessary for transition state attainment. The chemical reaction would also require... 

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