2.3.1: A Mechanism is a Sequence of Elementary Reactions The mechanism of a reaction is a series of steps leading from the starting materials to the products. After each step, an intermediate is formed. The intermediate is short-lived, because it quickly undergoes another step to form the next intermediate. These simple steps are called elementary reactions. Because an overall reaction is composed of a series of elementary reaction, the overall rate of the reaction is somehow dependent on the rates of those smaller reactions. 2.3.2: The Principle of Detailed Balance The principle of detailed balance is formulated for kinetic systems which are decomposed into elementary processes (collisions, or steps, or elementary reactions): At equilibrium, each elementary process should be equilibrated by its reverse process. 2.3.3: Multiple Mechanisms are often Indistinguishable The great value of chemical kinetics is that it can give us insights into the actual reaction pathways (mechanisms) that reactants take to form the products of reactions. Analyzing a reaction mechanism to determine the type of rate law that is consistent (or not consistent) with the specific mechanism can give us significant insight. 2.3.4: The Steady-State Approximation When a reaction mechanism has several steps of comparable rates, the rate-determining step is often not obvious. However, there is an intermediate in some of the steps. An intermediate is a species that is neither one of the reactants, nor one of the products. The steady-state approximation is a method used to derive a rate law. The method is based on the assumption that one intermediate in the reaction mechanism is consumed as quickly as it is generated. Its concentration remains the same in a 2.3.5: Rate Laws Do Not Imply Unique Mechanism It is standard practice to develop a multi-step reaction mechanism from the experimentally derived rate law. The fact that a mechanism consistent with the rate law has been proposed does not preclude the possibility that the reaction proceeds according to a different mechanism. Thus, it is usual practice to carry out further experiments, such as trapping reaction intermediates, to gain enough evidence to accept one of the possible valid mechanisms. 2.3.6: The Lindemann Mechanism The Lindemann mechanism was one of the first attempts to understand unimolecular reactions. Lindemann mechanisms have been used to model gas phase decomposition reactions. Although the net formula for a decomposition may appear to be first-order (unimolecular) in the reactant, a Lindemann mechanism may show that the reaction is actually second-order (bimolecular). 2.3.7: Some Reaction Mechanisms Involve Chain Reactions Chain reactions usually consist of many repeating elementary steps, each of which has a chain carrier. Once started, chain reactions continue until the reactants are exhausted. Fire and explosions are some of the phenomena associated with chain reactions. The chain carriers are some intermediates that appear in the repeating elementary steps. These are usually free radicals. 2.3.8: A Catalyst Affects the Mechanism and Activation Energy Homogeneous catalysis refers to reactions in which the catalyst is in solution with at least one of the reactants whereas heterogeneous catalysis refers to reactions in which the catalyst is present in a different phase, usually as a solid, than the reactants. 2.3.9: The Michaelis-Menten Mechanism for Enzyme Catalysis Leonor Michaelis and Maude Menten proposed the following reaction mechanism for enzymatic reactions. In the first step, the substrate binds to the active site of the enzyme. In the second step, the substrate is converted into the product and released from the substrate. 2.3.10: Predicting Rate Laws from Proposed Mechanisms Because a proposed mechanism can only be valid if it is consistent with the rate law found experimentally, the rate law plays a central role in the investigation of chemical reaction mechanisms. The discussion above introduces the problems and methods associated with collecting rate data and with finding an empirical rate law that fits experimental concentration-versus-time data. We turn now to finding the rate laws that are consistent with a particular proposed mechanism. 2.3.E: Chemical Kinetics II- Reaction Mechanisms (Exercises)