The molecularity of the rate-determining step determines the overall reaction order
the verdict
INSUFFICIENT LEANING
refutedsupported
the weight of evidence
3 sources for · 0 against
AS REPORTEDno primary record reached; this is what the reporting says
Reference sources indicate that the molecularity of the rate-determining step relates to the kinetic rate law and reaction order in specific mechanisms, but the retrieved evidence only partially covers the general universal claim.
molecularity. The sum of the elementary steps gives the net reaction. When determining the overall rate law for a reaction, the slowest step is the step
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs.
A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage of an overall chemical reaction. The detailed steps of a reaction are not observable in most cases. The conjectured mechanism is chosen because it is thermo
2 NO2 → NO3 + NO (slow)
NO3 + CO → NO2 + CO2 (fast)
Each step is called an elementary step, and each has its own rate law and molecularity. The sum of the elementary steps gives the net reaction.
When determining the overall rate law for a reaction, the slowest step is the step that determines the reaction rate. Because the first step (in the above reaction) is the slowest step, it is the rate-determining step. Because it involves the collision of two NO2 molecules, it is a bimolecular reaction with a rate
r
{\displaystyle r}
which obeys the rate law
r
=
k
[
N
O
2
(
t
)
]
2
{\displaystyle r=k[NO_{2}(t)]^{2}}
.
Other reactions may have mechanisms of several consecutive steps. In organic chemistry, the reaction mechanism for the benzoin condensation, put forward in 1903 by A. J. Lapworth, was one of the first proposed reaction mechanisms.
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs.
A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage of an overall chemical reaction. The detailed steps of a reaction are not observable in most cases. The conjectured mechanism is chosen because it is thermodynamically feasible and has experimental support in isolated intermediates (see next section) or other quantitative and qualitative characteristics of the reaction. It also describes each reactive intermediate, activated complex, and transition state, which bonds are broken (and in what order), and which bonds are formed (and in what order). A complete mechanism must also explain the reason for the reactants and catalyst used, the stereochemistry observed in reactants and products, all products formed and the amount of each.
Reaction intermediates are chemical species, often unstable and short-lived. They can, however, sometimes be isolated. They are neither reactants nor products of the overall chemical reaction, but temporary products and/or reactants in the mechanism's reaction steps. Reaction intermediates are often confused with the transition state. The transition states are, in contrast, fleeting, high-energy species that cannot be isolated. The kinetics (relative rates of the reaction steps and the rate equation for the overall reaction) are discussed in terms of the energy required for the conversion of the reactants to the proposed transition states (molecular states that correspond to maxima on the reaction coordinates, and to saddle points on the potential energy surface for the reaction).
This form shows that the rate-determining step does not involve CO. Instead, the slow step involves two molecules of NO2. A possible mechanism for the overall reaction that explains the rate law is:
2 NO2 → NO3 + NO (slow)
NO3 + CO → NO2 + CO2 (fast)
Each step is called an elementary step, and each has its own rate law and molecularity. The sum of the elementary steps gives the net reaction.
When determining the overall rate law for a reaction, the slowest step is the step that determines the reaction rate. Because the first step (in the above reaction) is the slowest step, it is the rate-determining step. Because it involves the collision of two NO2 molecules, it is a bimolecular reaction with a rate
r
{\displaystyle r}
which obeys the rate law
r
=
k
[
N
O
2
(
t
)
]
2
{\displaystyle r=k[NO_{2}(t)]^{2}}
.
Other reactions may have mechanisms of several consecutive steps. In organic chemistry, the reaction mechanism for the benzoin condensation, put forward in 1903 by A. J. Lapworth, was one of the first proposed reaction mechanisms.
Chain initiation: this can be by thermolysis (heating the molecules) or photolysis (absorption of light) leading to the breakage of a bond.
Propagation: a chain carrier makes another carrier.
Branching: one carrier makes more than one carrier.
Retardation: a chain carrier may react with a product reducing the rate of formation of the product. It makes another chain carrier, but the product concentration is reduced.
Chain termination: radicals combine and the chain carriers are lost.
Inhibition: chain carriers are removed by processes other than termination, such as by forming radicals.
Even though all these steps can appear in one chain reaction, the minimum necessary ones are Initiation, propagation, and termination.
An example of a simple chain reaction is the thermal decomposition of acetaldehyde (CH3CHO) to methane (CH4) and carbon monoxide (CO). The experimental reaction order is 3/2, which can be explained by a Rice-Herzfeld mechanism.
This reaction mechanism for acetaldehyde has 4 steps with rate equations for each step :
measurement of the effect of temperature (Arrhenius equation) to determine the activation energy
spectroscopic observation of reaction intermediates
determination of the stereochemistry of products, for example in nucleophilic substitution reactions
measurement of the effect of isotopic substitution on the reaction rate
for
Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step.
This "bottleneck concept" has analogies in everyday life. For example, if a crowd is leaving a theater through a single exit door, the time it takes to empty the building is a function of the number of people who can move through the door per second. Once a group gathers at the door, the speed at which other people leave their seats and move along the aisles has no influence on the overall exit rate. When we describe the mechanism of a chemical reaction, it is important to identify the rate-determining step and to determine its "molecularity". The molecularity of a reaction is defined as the number of molecules or ions that participate in the rate determining step. A mechanism in which two reacting species combine in the transition state of the rate-determining step is called bimolecular. If a single species makes up the transition state, the reaction would be called unimolecular. The relatively improbable case of three independent species coming together in the transition state would be called termolecular.
SN1 Substitution Reactions Substitution and Elimination Reactions of Alkyl Halides { } { Molecularity_and_Kinetics : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass234_0.<PageSubPageProperty>b__1]()", Nucleophilicity_and_Solvent_Effects : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass234_0.<PageSubPageProperty>b__1]()", "Steric_Hindrance_to_Rear-side_Approach_in_Nucleophilic_Substitution" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass234_0.<PageSubPageProperty>b__1]()" } { E1_Elimination_Reactions : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass234_0.<PageSubPageProperty>b__1]()", E2_Elimination_Reactions : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass234_0.<PageSubPageProperty>b__1]()", SN1_Substitution_Reactions : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass234_0.<PageSubPageProperty>b__1]()", SN2_Substitution_Reactions_and_Alkyl_Moiety : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass234_0.<PageSubPageProperty>b__1]()" } Mon, 23 Jan 2023 07:38:29 GMT Molecularity and Kinetics 1191 1191 admin { } Anonymous Anonymous User 2 false false [ "article:topic", "authorname:wreusch", "showtoc:no", "license:ccbyncsa", "licenseversion:40" ] [ "article:topic", "authorname:wreusch", "showtoc:no", "license:ccbyncsa", "licenseversion:40" ] https://chem.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fchem.libretexts.org%2FBookshelves%2FOrganic_Chemistry%2FSupplemental_Modules_(Organic_Chemistry)%2FAlkyl_Halides%2FReactivity_of_Alkyl_Halides%2FAlkyl_Halide_Reactions%2FSubstitution_and_Elimination_Reactions_of_Alkyl_Halides%2FSN1_Substitution_Reactions%2FMolecularity_and_Kinetics Molecularity If a chemical reaction proceeds by more than one step or stage, its overall velocity or rate is limited by the slowest step, the rate-determining step .
This "bottleneck concept" has analogies in everyday life. For example, if a crowd is leaving a theater through a single exit door, the time it takes to empty the building is a function of the number of people who can move through the door per second. Once a group gathers at the door, the speed at which other people leave their seats and move along the aisles has no influence on the overall exit rate. When we describe the mechanism of a chemical reaction, it is important to identify the rate-determining step and to determine its "molecularity". The molecularity of a reaction is defined as the number of molecules or ions that participate in the rate determining step.
A mechanism in which two reacting species combine in the transition state of the rate-determining step is called bimolecular . If a single species makes up the transition state, the reaction would be called unimolecular . The relatively improbable case of three independent species coming together in the transition state would be called termolecular . Kinetics One way of investigating the molecularity of a given reaction is to measure changes in the rate at which products are formed or reactants are lost, as reactant concentrations are varied in a systematic fashion. This sort of study is called kinetics , and the goal is to write an equation that correlates the observed results.
Such an equation is termed a kinetic expression, and for a general reaction of the type: \[A + B \rightarrow C + D \] it takes the form: \[
In a simple bimolecular reaction n & m would both be 1, and the reaction would be termed second order , supporting a mechanism in which a molecule of reactant A and one of B are incorporated in the transition state of the rate-determining step. A bimolecular reaction in which two molecules of reactant A (and no B) are present in the transition state would be expected to give a kinetic equation in which n=2 and m=0 (also second order). The kinetic expressions found for the reactions shown at the beginning of this section are written in blue in the following equations. Each different reaction has its own distinct rate constant, k # .
All the reactions save 7 display second order kinetics, reaction 7 is first order. It should be recognized and remembered that the molecularity of a reaction is a theoretical term referring to a specific mechanism. On the other hand, the kinetic order of a reaction is an experimentally derived number. In ideal situations these two should be the same, and in most of the above reactions this is so. Reaction 7 above is clearly different from the other cases reported here.
second order overall due to the reaction being bimolecular (i.e. there are two molecular species involved in the rate-determining step). The reaction does
In chemistry, a concerted reaction is a chemical reaction in which all bond breaking and bond making occurs in a single step. Reactive intermediates or other unstable high energy intermediates are not involved. Concerted reaction rates tend not to depend on solvent polarity ruling out large buildup of charge in the transition state. The reaction is said to progress through a concerted mechanism as
In chemistry, a concerted reaction is a chemical reaction in which all bond breaking and bond making occurs in a single step. Reactive intermediates or other unstable high energy intermediates are not involved. Concerted reaction rates tend not to depend on solvent polarity…
In chemistry, a concerted reaction is a chemical reaction in which all bond breaking and bond making occurs in a single step. Reactive intermediates or other unstable high energy intermediates are not involved. Concerted reaction rates tend not to depend on solvent polarity ruling out large buildup of charge in the transition state. The reaction is said to progress through a concerted mechanism as all bonds are formed and broken in concert. Pericyclic reactions, the SN2 reaction, and some rearrangements - such as the Claisen rearrangement - are concerted reactions. The rate of the SN2 reaction is second order overall due to the reaction being bimolecular (i.e.
there are two molecular species involved in the rate-determining step). The reaction does not have any intermediate steps, only a transition state. This means that all the bond making and bond breaking takes place in a single step. In order for the reaction to occur both molecules must be situated correctly. == References ==
Everything we examined (3) — 2 independent sources
This check searched the claim as stated. It did not run a separate search for evidence against it.