In
chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
, molecularity is the number of molecules that come together to react in an
elementary (single-step) reaction[Atkins, P.; de Paula, J. Physical Chemistry. Oxford University Press, 2014] and is equal to the sum of
stoichiometric coefficients
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals ...
of
reactant
In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...
s in the elementary reaction with effective collision (
sufficient energy) and correct orientation.
Depending on how many molecules come together, a reaction can be unimolecular, bimolecular or even trimolecular.
The kinetic order of any elementary reaction or reaction step is ''equal'' to its molecularity, and the
rate equation
In chemistry, the rate law or rate equation for a reaction is an equation that links the initial or forward reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reacti ...
of an elementary reaction can therefore be determined by inspection, from the molecularity.
The kinetic order of a complex (multistep) reaction, however, is not necessarily equal to the number of molecules involved. The concept of molecularity is only useful to describe elementary reactions or steps.
Unimolecular reactions
In a unimolecular reaction, a single molecule rearranges atoms, forming different molecules.
This is illustrated by the equation
:
A -> P,
where refers to chemical
product(s). The reaction or
reaction step A reaction step of a chemical reaction is defined as: ''"An elementary reaction, constituting one of the stages of a stepwise reaction in which a reaction intermediate (or, for the first step, the reactants) is converted into the next reaction int ...
is an
isomerization
In chemistry, isomerization or isomerisation is the process in which a molecule, polyatomic ion or molecular fragment is transformed into an isomer with a different chemical structure. Enolization is an example of isomerization, as is tautomeriz ...
if there is only one product molecule, or a
dissociation
Dissociation, in the wide sense of the word, is an act of disuniting or separating a complex object into parts. Dissociation may also refer to:
* Dissociation (chemistry), general process in which molecules or ionic compounds (complexes, or salts ...
if there is more than one product molecule.
In either case, the rate of the reaction or step is described by the
first order rate law
:
where is the
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', ''number concentration'', an ...
of
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
A, is time, and is the
reaction rate constant In chemical kinetics a reaction rate constant or reaction rate coefficient, ''k'', quantifies the rate and direction of a chemical reaction.
For a reaction between reactants A and B to form product C
the reaction rate is often found to have the ...
.
As can be deduced from the rate law equation, the number of A molecules that decay is proportional to the number of A molecules available. An example of a unimolecular reaction, is the
isomerization
In chemistry, isomerization or isomerisation is the process in which a molecule, polyatomic ion or molecular fragment is transformed into an isomer with a different chemical structure. Enolization is an example of isomerization, as is tautomeriz ...
of
cyclopropane
Cyclopropane is the cycloalkane with the molecular formula (CH2)3, consisting of three methylene groups (CH2) linked to each other to form a ring. The small size of the ring creates substantial ring strain in the structure. Cyclopropane itself ...
to propene:
![Izomerization of cyclopropane](https://upload.wikimedia.org/wikipedia/commons/c/c6/Izomerization_of_cyclopropane.jpg)
Unimolecular reactions can be explained by the
Lindemann-Hinshelwood mechanism.
Bimolecular reactions
In a bimolecular reaction, two molecules collide and exchange energy, atoms or groups of atoms.
This can be described by the equation
A + B -> P
which corresponds to the second order rate law:
.
Here, the rate of the reaction is proportional to the rate at which the reactants come together. An example of a bimolecular
reaction is the
SN2-type
nucleophilic substitution
In chemistry, a nucleophilic substitution is a class of chemical reactions in which an electron-rich chemical species (known as a nucleophile) replaces a functional group within another electron-deficient molecule (known as the electrophile). The ...
of
methyl bromide by
hydroxide ion:
CH3Br + OH^- -> CH3OH + Br^-
Termolecular reactions
A termolecular
[ (or trimolecular) reaction in ]solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Soluti ...
s or gas mixtures involves three reactants simultaneously colliding, with appropriate orientation and sufficient energy.[J.I. Steinfeld, J.S. Francisco and W.L. Hase ''Chemical Kinetics and Dynamics'' (2nd ed., Prentice Hall 1999) p.5, ] However the term ''trimolecular'' is also used to refer to three body association reactions of the type:
A + B -> ceC
Where the M over the arrow denotes that to conserve energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
and momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
a second reaction with a third body is required. After the initial bimolecular collision of A and B an energetically excited reaction intermediate
In chemistry, a reaction intermediate or an intermediate is a molecular entity that is formed from the reactants (or preceding intermediates) but is consumed in further reactions in stepwise chemical reactions that contain multiple elementary st ...
is formed, then, it collides with a M body, in a second bimolecular reaction, transferring the excess energy to it.
The reaction can be explained as two consecutive reactions:
These reactions frequently have a pressure and temperature dependence region of transition between second and third order kinetics.IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
definition of ''Troe expression'', a semiempirical expression for the rate constant of termolecular reaction
/ref>
Catalytic reactions are often three-component, but in practice a complex of the starting materials is first formed and the rate-determining step is the reaction of this complex into products, not an adventitious collision between the two species and the catalyst. For example, in hydrogenation with a metal catalyst, molecular dihydrogen first dissociates onto the metal surface into hydrogen atoms bound to the surface, and it is these monatomic hydrogens that react with the starting material, also previously adsorbed onto the surface.
Reactions of higher molecularity are not observed due to very small probability of simultaneous interaction between 4 or more molecules.[Carr, R. W. Chemical Kinetics. In Encyclopedia of Applied Physics. WILEY-VCH Verlag GmbH & Co KGaA, 2003][
]
Difference between molecularity and order of reaction
It is important to distinguish molecularity from order of reaction
In chemistry, the rate law or rate equation for a reaction is an equation that links the initial or forward reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reacti ...
. The order of reaction is an empirical quantity determined by experiment from the rate law of the reaction. It is the sum of the exponents in the rate law equation.[Rogers, D. W. Chemical Kinetics. In Concise Physical Chemistry, John Wiley and Sons, Inc. 2010.] Molecularity, on the other hand, is deduced from the mechanism of an elementary reaction, and is used only in context of an elementary reaction. It is the number of molecules taking part in this reaction.
This difference can be illustrated on the reaction between nitric oxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
and hydrogen:Keith J. Laidler
Keith James Laidler (January 3, 1916 – August 26, 2003), born in England, was notable as a pioneer in chemical kinetics and authority on the physical chemistry of enzymes.
Education
Laidler received his early education at Liverpool College. H ...
, ''Chemical Kinetics'' (3rd ed., Harper & Row 1987), p.277
2NO + 2H2 -> N2 + 2H2O,
where the observed rate law is , so that the reaction is ''third order''. Since the order does ''not'' equal the sum of reactant stoichiometric coefficients, the reaction must involve more than one step. The proposed two-step mechanism has a rate-limiting first step whose molecularity corresponds to the overall order of 3:
Slow: 2 NO + H2 -> N2 + H2O2
Fast: H2O2 + H2 -> 2H2O
On the other hand, the molecularity of this reaction is undefined, because it involves a mechanism of more than one step. However, we can consider the molecularity of the individual elementary reactions that make up this mechanism: the first step is termolecular because it involves three reactant molecules, while the second step is bimolecular because it involves two reactant molecules.
See also
* Reaction rate
* Dissociation (chemistry)
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an acid ...
* Lindemann mechanism In chemical kinetics, the Lindemann mechanism (also called the Lindemann–Christiansen mechanism or the Lindemann–Hinshelwood mechanism) is a schematic reaction mechanism for unimolecular reactions. Frederick Lindemann and J.A. Christiansen pro ...
* Crossed molecular beam Crossed molecular beam experiments are chemical experiments where two beams of atoms or molecules are collided together to study the dynamics of the chemical reaction, and can detect individual reactive collisions.
Technique
In a crossed molec ...
* Cage effect
In chemistry, the cage effect (also known as geminate recombination) describes how the properties of a molecule are affected by its surroundings. First introduced by Franck and Rabinowitch in 1934, the cage effect suggests that instead of actin ...
* Reaction progress kinetic analysis
In chemistry, reaction progress kinetic analysis (RPKA) is a subset of a broad range of kinetic techniques utilized to determine the rate laws of chemical reactions and to aid in elucidation of reaction mechanisms. While the concepts guiding reacti ...
References
{{Reaction mechanisms
Chemical kinetics