Bimolecular Processes

Submitted by jwmoore on Thu, 02/17/2011 - 17:07

A second type of microscopic process which can result in a chemical reactionA process in which one or more substances, the reactant or reactants, change into one or more different substances, the products; chemical change involves rearrangement, combination, or separation of atoms. Also called chemical change. involves collision of two particles. Such a process is called a bimolecular processIn a reaction mechanism, an elementary step in which two atoms, molecules, or ions must collide in order for a reaction to occur.. A typical example of a bimolecular process is the reaction between nitrogen dioxide and carbon monoxide:

NO2 + CO → NO + CO2      (1)

Here an O atomThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. is transferred from NO2 to CO when the two molecules collide.

Several factors affect the rate of a bimolecular reaction. The first of these is the frequencyThe rate at which a periodic event occurs; specifically, the rate at which the waves of electromagnetic radiation pass a point. of collisions between the two reactantA substance consumed by a chemical reaction. molecules. Suppose we have a single molecule of type A (shown in black in Fig. 1a, or in blue in the animation) moving around in a gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. which otherwise consists entirely of molecules of kind B (indicated in white in the figure, light blue in the animation). If we double the concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. of B molecules (Fig. 1b), the number of collisions during the same time periodThose elements from a single row of the periodic table. doubles, because there are now twice as many B molecules to get in the way. Similarly, if we put twice as many A molecules into the original container, each of them collides with B molecules the same number of times, again giving twice as many A-B collisions.

The argument in the previous paragraph applies to any bimolecular process. The reaction rate must always be directly proportional to the concentration of each of the two reacting species. Thus for the general bimolecular process, A + B → products, the rate equationAn equation that describes the rate of a reaction as a function of the rate constant and the concentrations of reactants (and any other substances that affect the rate, such as products or catalysts); also called rate law. must be first order in A and first order in B:

Rate = k(cA)(cB)

The animations above serve as another way of modeling what is occurring in the still images. Notice that, as in the still image, roughly twice as many collisions occur when there are two larger blue particles or twice as many small light blue particles, than in the first initial condition. Exact count shows this is not perfectly true though, as animation (a) has 6 collisions, while animation (b) and (c) have 14 and 15 respectively. This yields a factor of 2.3 and 2.5 respectively. This highlights an important point. When we say that doubling the amount of either substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. should double the collisions, we mean collisions will, on average be doubled. Notice that the exact ratio between the animations at a specific time point fluctuates. Also, it is important to note that there are never more than 30 particles of either type in any of the simulations. When dealing with the scale of moles in an actual reaction, such a difference as 2 or 3 more collisions than predicted is vanishingly small.

Collision of two molecules is a necessary but not a sufficient condition for a bimolecular process to occur. Returning to the reaction of NO2 with CO, in which an O atom is transferred from the N atom to the C atom, we can see that the orientation of the two molecules as they collide is important. This introduces what is called a steric factor. None of the collisions depicted in Fig. 2 would result in reaction, for example, because none of them involve close contact between the C atom in CO and one of the O’s in NO2. For the reaction of CO with NO2, the steric factor is estimated to be about one-sixth, meaning that only one collision in six involves an appropriate orientation. For more complexA central metal and the ligands surrounding it; also called coordination complex. molecules, the steric factor is often much smaller. In the reaction

Image:SN2 displacement.jpg

the fraction of favorable collisions is extremely small, because OH– ions almost always hit the hydrocarbonA compound containing only the elements carbon and hydrogen. chain at a point too far from the Br atom to cause a reaction.

The existence of a steric factor implies that there is a fairly well-defined pathway along which the reactant molecules must travel in order to produce an activated complexIn the mechanism of a reaction, a species that lies at an energy peak and that can change either into products or into reactants; also called a transition state. and then the reaction products. This pathway is called the reaction coordinate, and it applies to unimolecular as well as bimolecular reactions. In the case of cisDescribes the relationship between two atoms or groups of atoms, each attached to one of two doubly bonded carbon atoms and located on the same side of the double bond. Also refers to groups located adjacent to each other in an octahedral or square planar coordination complex.-2-butene, for example, the reaction coordinate is the angle of rotation about the double bondAttraction between two atoms (nuclei and core electrons) that results from sharing two pairs of electrons between the atoms; a bond with bond order = 2., and proceeding along that pathway requires an increase in energyA system's capacity to do work..

Figure 2 Because they are incorrectly oriented, none of the above collisions will result in the transfer of an O atom from an NO2 to a CO molecule, even if the collision is sufficiently energetic.

The same is true of our example of a bimolecular reaction. As the C end of a CO molecule approaches one of the O’s in NO2, the electronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass. clouds surrounding the molecules begin to repel each other and the energy of the system rises. This is shown in Fig. 3. Only if the total energy of the two molecules exceeds the 116 kJ mol–1 activation energyThe energy barrier over which a reaction must progress in order for reactants to form products; the minimum energy that reactants must have if they are to be converted to products. can they squeeze close enough together for a C—C bond to start to form. As this occurs the N—O bond begins to break, so that the activated complex has the structure


The dashed lines indicate bonds which are just beginning to form or are in the process of breaking. As the reaction occurs, the N—O bond lengthens and the O—C distance continually gets shorter. Consequently the difference between the N—O bond length and the C—O bond length (rN—OrC—O) becomes larger and larger during the reaction. This difference, then, makes a convenient reaction coordinate.

Figure 3 Energy profile for the reaction between NO2 and CO. As the two molecules approach closely, energy is needed to squeeze them together against the mutual repulsion of their outer electrons until the C atom and an O atom are close enough to a bond. If the two colliding molecules do not have at least 116 kJ mol–1 between them, the O atom cannot be forced close enough to the C atom to transfer its allegiance from the N atom. The progress of a reaction like this is usually measured in terms of a convenient reaction coordinate, which in this case could be the N—O distance minus the C—O distance.