Name: elementary steps in reaction mechanisms
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Chemistry

Matter and its Interactions

PS.Chm.5.1

Reaction Mechanisms

A chemical reaction is often represented by an overall balanced chemical equation indicating the reactants and products.
However, the actual reaction is often more complex and transpires in multiple steps. For instance, this reaction of nitric oxide with hydrogen forming nitrogen gas and water takes place in three distinct, successive steps. These steps are called the reaction mechanism.
Each step in the reaction mechanism is called an elementary reaction and represents the interaction, such as bond breakage or formation, between the reacting species.
Specific molecules, like dinitrogen dioxide and nitrous oxide, are formed during one elementary step and consumed during another. Such species are called reaction intermediates.
Reaction intermediates are low-energy products of an elementary reaction. They are often short-lived, which explains their absence in the product mixture. Reaction intermediates are not the same as activated complexes. Activated complexes are high-energy transition states existing only during the transformation of reactants to products. 
Combining the elementary steps gives the equation for the overall chemical reaction. Here, the reaction intermediates are eliminated and hence do not appear in the overall chemical equation.
The different elementary reactions may progress at varying speeds. The slowest elementary step determines the overall reaction rate. Here, the reaction of dinitrogen dioxide with hydrogen gas is the rate-limiting step.
Elementary reactions can be commonly characterized as three types, depending on the number of reacting molecules or molecularity.
In a unimolecular reaction, a single reactant molecule transforms into one or more products. In a bimolecular reaction, two distinct molecules react. A termolecular reaction, though very rare, involves three individual molecules reacting to yield intermediates or products.
Unlike the rate law for an overall chemical reaction, which is determined experimentally, rate laws for elementary reactions can be predicted from stoichiometric coefficients of their reactants. In short, the molecularity of an elementary reaction corresponds to the overall reaction order of the elementary step.
Hence, unimolecular reactions are often first-order reactions, bimolecular reactions are second-order, and termolecular reactions are of the third order.
An understanding of reaction mechanisms and kinetics helps chemists in identifying and optimizing chemical reactions.

reaction mechanisms

Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates the reacting species and the product species, but it reveals no details about how the reaction occurs at the molecular level. The reaction mechanism (or reaction path) provides details regarding the precise, step-by-step process by which a reaction occurs.
For instance, the decomposition of ozone appears to follow a mechanism with two steps:
<img alt="Static equilibrium concept with equations &#931;Fx=0, &#931;Fy=0, torque &#964;=0; diagram of forces." src="/files/ftp_upload/11381/11381_Equation_1.png" style="text-align: center; height: 70px;" data-altupdatedat="1761830439000" data-alt="Eq1" />
Each of the steps in a reaction mechanism is called an elementary reaction. These elementary reactions occur in sequence, as represented in the step equations, and they sum to yield the balanced chemical equation describing the overall reaction:
<img alt="DNA recombinant technology diagram; gene editing process; vector insertion; molecular cloning." src="/files/ftp_upload/11381/11381_Equation_2.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq2" />
Notice that the oxygen atom produced in the first step is consumed during the second and does not appear as a product in the overall reaction. Such species that are produced in one step and consumed in a subsequent one are called reaction intermediates.
While the overall reaction equation indicates that two ozone molecules react to give three oxygen molecules, the actual reaction mechanism does not involve the direct collision and reaction of two ozone molecules. Instead, one O3 decomposes to yield O2 and an oxygen atom, and a second O3 molecule subsequently reacts with the oxygen atom to yield two additional O2 molecules.
Unlike balanced equations representing an overall reaction, the equations for elementary reactions are explicit representations of the chemical change. An elementary reaction-equation depicts the actual reactant(s) undergoing bond-breaking/making, and the product(s) formed. Thus, the rate law for an elementary reaction may be derived directly from its balanced chemical equation. However, this is not the case for typical chemical reactions, for which rate laws may be reliably determined only via experimentation.
Unimolecular Elementary Reactions
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 to produce one or more molecules of product:
<img alt="Chemical equation of glucose oxidation. C6H12O6 + 6O2 &#10230; 6CO2 + 6H2O." src="/files/ftp_upload/11381/11381_Equation_3.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq3" />
The rate law for a unimolecular reaction is first order; rate = k [A].
A unimolecular reaction may be one of several elementary reactions in a complex reaction mechanism. For instance, the reaction (O3 (g) &#8594; O2 (g) + O) illustrates a unimolecular elementary reaction occurring as a part of a two-step reaction mechanism. However, some unimolecular reactions may be the only step of a single-step reaction mechanism. (In other words, an &#8220;overall&#8221; reaction may also be an elementary reaction in some cases.) For example, the gas-phase decomposition of cyclobutane, C4H8, to ethylene, C2H4, is represented by the chemical equation:
<img alt="Static equilibrium equations, &#931;Fx=0, &#931;Fy=0; diagram of forces and moments acting on a beam." src="/files/ftp_upload/11381/11381_Equation_3A.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq3A" />
This equation represents the overall reaction, describing a unimolecular elementary process. 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, showing first-order behavior:
<img alt="Chemical equilibrium diagram, reaction formula CH4 + 2O2 &#8594; CO2 + 2H2O, combustion process." src="/files/ftp_upload/11381/11381_Equation_4.png" style="text-align: center; height: 70px;" data-altupdatedat="1761830439000" data-alt="Eq4" />
This agreement between observed and predicted rate laws indicates that the proposed unimolecular, single-step process is a reasonable mechanism for the butadiene reaction.
Bimolecular Elementary Reactions
A bimolecular reaction involves two reactant species. For example:
<img alt="Helium-neon laser energy levels diagram, optical energy transitions, laser physics concept." src="/files/ftp_upload/11381/11381_Equation_5.png" style="text-align: center; height: 70px;" data-altupdatedat="1761830439000" data-alt="Eq5" />
In the first type, where the two reactant molecules are different, the rate law is first-order in A and first order in B (second-order overall)
<img alt="Chromatography system diagram, TLC process showing solvent front and sample separation paths." src="/files/ftp_upload/11381/11381_Equation_6.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq6" />
In the second type, in which two identical molecules collide and react, the rate law is second order in A:
<img alt="Enzyme catalysis mechanism, substrate-enzyme complex diagram, reaction kinetics, transition state." src="/files/ftp_upload/11381/11381_Equation_7.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq7" />
Some chemical reactions occur by mechanisms that consist of a single bimolecular elementary reaction. One example is the reaction of nitrogen dioxide with carbon monoxide:
<img alt="Force and torque equilibrium diagram; &#931;Fx=0, &#931;Fy=0, MA=0; common supports and loadings shown." src="/files/ftp_upload/11381/11381_Equation_8.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq8" />
Bimolecular elementary reactions may also be involved as steps in a multistep reaction mechanism. The reaction of atomic oxygen with ozone is the second step of a two-step ozone decomposition mechanism:
<img alt="Chemical structure diagram of 4-acetoxy-N,N-dimethyltryptamine, relevant to organic chemistry studies." src="/files/ftp_upload/11381/11381_Equation_9.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq9" />
Termolecular Elementary Reactions
An elementary termolecular reaction involves the simultaneous collision of three atoms, molecules, or ions. Termolecular elementary reactions are uncommon because the probability of three particles colliding simultaneously is very rare. There are, however, a few established termolecular elementary reactions. The reaction of nitric oxide with oxygen appears to involve termolecular steps:
<img alt="Static equilibrium; &#931;Fx=0, &#931;Fy=0 diagram; force balance on a beam with supports and loads analysis." src="/files/ftp_upload/11381/11381_Equation_10.png" style="text-align: center; height: 70px;" data-altupdatedat="1761830439000" data-alt="Eq10" />
Likewise, the reaction of nitric oxide with chlorine appears to involve termolecular steps:
<img alt="Fiber optic communication diagram, illustrating light transmission through optical fibers." src="/files/ftp_upload/11381/11381_Equation_11.png" style="text-align: center; height: 70px;" data-altupdatedat="1761830439000" data-alt="Eq11" />
Often one of the elementary steps in a multistep reaction mechanism is significantly slower than the others. Because a reaction cannot proceed faster than its slowest step, this step will limit the rate at which the overall reaction occurs. The slowest elementary step is therefore called the rate-limiting step (or rate-determining step) of the reaction.
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, Chemistry 2e, 12.6: Reaction Mechanisms.</a>

Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates ...

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Chemical Kinetics

KEY TERMS AND CONCEPTS

elementary steps in reaction mechanisms

Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps.

Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced equation for the overall reaction. In the ozone example, an oxygen atom is made in the first step and used up in the second step, so it does not appear in the final equation. A species made in one step and used up in a later step is called a reaction intermediate.

The overall equation may show two ozone molecules forming three oxygen molecules, but the mechanism shows that the two ozone molecules do not react directly with each other. Instead, one O3 molecule breaks down to form O2 and an oxygen atom. Then a second O3 molecule reacts with that oxygen atom to make two more O2 molecules. This is why elementary reaction equations give a more direct picture of bond breaking and bond making than the overall equation does.

For an elementary reaction, the rate law can often be written directly from the balanced equation for that step. For most chemical reactions, the rate law must be found by experiment. That is why elementary reactions are useful when chemists build a mechanism that matches the observed behavior of a reaction.

Elementary reactions are described by molecularity, which is the number of reactant species in the step. A unimolecular reaction has one reactant species and forms one or more products. Its rate law is first order, written as rate = k[A]. A unimolecular step may be part of a larger mechanism, such as O3(g) → O2(g) + O in ozone decomposition. Some unimolecular reactions are also the whole reaction, not just one step. The gas-phase decomposition of cyclobutane, C4H8, to ethylene, C2H4, is one example, and its first-order experimental rate law agrees with the rate law predicted for an elementary single-step process.

A bimolecular reaction involves two reactant species. If the two reactants are different, the rate law is first order in A and first order in B, for a second-order overall rate. If the two reacting molecules are the same, the rate law is second order in A. Some reactions occur by a single bimolecular elementary reaction, such as the reaction of nitrogen dioxide with carbon monoxide. Bimolecular steps can also appear in multistep mechanisms, including the reaction of atomic oxygen with ozone in ozone decomposition.

A termolecular reaction involves the simultaneous collision of three atoms, molecules, or ions. These reactions are uncommon because three particles colliding at the same time is very unlikely. Still, a few termolecular elementary reactions are known. The reaction of nitric oxide with oxygen and the reaction of nitric oxide with chlorine both appear to involve termolecular steps.

In many multistep mechanisms, one elementary step is much slower than the others. A reaction cannot proceed faster than its slowest step, so that step controls the overall rate. This slowest step is called the rate-limiting step, or rate-determining step.

Elementary Steps in Reaction Mechanisms

reaction rate

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Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced...

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Explore how chemical reactions happen in steps, including elementary reactions, reaction intermediates, molecularity, rate laws, and the rate-limiting step.

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Elementary Steps in Reaction Mechanisms

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