A balanced equation indicates what is reacting and what is produced, but it reveals no details about how the reaction actually takes place. The reaction mechanism or reaction path provides details regarding the precise, step-by-step process by which a reaction occurs. The decomposition of ozone, for example, appears to follow a mechanism with two steps:. Each of the steps in a reaction mechanism is an elementary reaction. These elementary reactions occur precisely as represented in the step equations, and they must sum to yield the balanced chemical equation representing the overall reaction:.
Notice that the oxygen atom produced in the first step of this mechanism is consumed in the second step and therefore does not appear as a product in the overall reaction. Species that are produced in one step and consumed in a subsequent step are called intermediates. While the overall reaction equation for the decomposition of ozone indicates that two molecules of ozone react to give three molecules of oxygen, the mechanism of the reaction does not involve the direct collision and reaction of two ozone molecules.
Instead, one O 3 decomposes to yield O 2 and an oxygen atom, and a second O 3 molecule subsequently reacts with the oxygen atom to yield two additional O 2 molecules.
Unlike balanced equations representing an overall reaction, the equations for elementary reactions are explicit representations of the chemical change taking place. For this reason, the rate law for an elementary reaction may be derived directly from the balanced chemical equation describing the reaction. This is not the case for typical chemical reactions, for which rate laws may be reliably determined only via experimentation.
The molecularity of an elementary reaction is the number of reactant species atoms, molecules, or ions. For example, a unimolecular reaction involves the reaction of a single reactant species to produce one or more molecules of product:. A unimolecular reaction may be one of several elementary reactions in a complex mechanism.
For example, the reaction:. However, some unimolecular reactions may be the only step of a single-step reaction mechanism. For example, the gas-phase decomposition of cyclobutane, C 4 H 8 , to ethylene, C 2 H 4 , is represented by the following chemical equation:.
This equation represents the overall reaction observed, and it might also represent a legitimate unimolecular elementary reaction. The rate law predicted from this equation, assuming it is an elementary reaction, turns out to be the same as the rate law derived experimentally for the overall reaction, namely, one showing first-order behavior:. This agreement between observed and predicted rate laws is interpreted to mean that the proposed unimolecular, single-step process is a reasonable mechanism for the butadiene reaction.
A bimolecular reaction involves two reactant species, for example:. For the first type, in which the two reactant molecules are different, the rate law is first-order in A and first order in B second-order overall :.
For the second type, in which two identical molecules collide and react, the rate law is second order in A :. Intramolecular Energy Transfer 1. Theory of Slater 2. Theory of Rice, Ramsperger, and Kassel 3. Intramolecular Energy Transfer 4. Randomization of Energy and Experimental Evidence 5. Theoretical Treatment of Energy Randomization 6.
Potential Energy Surfaces in Unimolecular Reactions 1. Unimolecular Process in Classical Phase Space 2. An Alternative Derivation 4.
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Pertinent Degrees of Freedom 1. Degrees of Freedom Excluded by Conservation Requirements 2.
Internal Degrees of Freedom 3. External Degrees of Freedom 4. Model for Particle References Chapter 6. Calculation of Energy-Level Densities 1.
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Terminology and Basic Concepts 2. Inversion of the Partition Function 5. Conclusions Exercises References Chapter 7. Unimolecular Rate with an Effective Potential 1. The Rotational Potential 2. Type 1 Vibrational Potential 3. Effective Potential for Type 1 Reaction 4. Averaging over Equilibrium Distribution 3. Averaging over Steady-State Distribution 4. Evaluation of Centrifugal Correction Factors 5. Pressure Dependence 6. Temperature Dependence 7. Theoretical Parameters and Experimental Observables 8.
Quantum chemistry: the quantum theory of unimolecular reactions.
Credit: University of New Mexico. Explore further. More information: Marissa L. Weichman et al.
Theory of Unimolecular Reactions - 1st Edition
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