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Molecularity
 
 
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Molecularity in chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
 is the number of colliding molecular entities
Molecular entity

According to IUPAC Gold Book a molecular entity is any constitutionally or isotopically distinct atom, molecule, ion, ion pair, Radical , radical ion, complex, conformer, etc., identifiable as a separately distinguishable entity....
 that are involved in a single reaction step
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 is converted into the next reaction intermediate in the sequence of intermediates between reactants and products"....
. While the order of a reaction is derived experimentally, the molecularity is a theoretical concept and can only be applied to elementary reaction
Elementary reaction

An elementary reaction is a chemical reaction in which one or more of the chemical species react directly to form products in a single reaction step and with a single transition state....
s. In elementary reactions, the reaction order, the molecularity and the stoichiometric coefficient are the same, although only numerically, because they are different concepts.

Where the M over the arrow denotes that to conserve energy
Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
 and momentum
Momentum

In classical mechanics, momentum is the product of the mass and velocity of an object . For more accurate measures of momentum, see the section Momentum#Modern definitions of momentum on this page....
 a second reaction with a third body is required.






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Molecularity in chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
 is the number of colliding molecular entities
Molecular entity

According to IUPAC Gold Book a molecular entity is any constitutionally or isotopically distinct atom, molecule, ion, ion pair, Radical , radical ion, complex, conformer, etc., identifiable as a separately distinguishable entity....
 that are involved in a single reaction step
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 is converted into the next reaction intermediate in the sequence of intermediates between reactants and products"....
. While the order of a reaction is derived experimentally, the molecularity is a theoretical concept and can only be applied to elementary reaction
Elementary reaction

An elementary reaction is a chemical reaction in which one or more of the chemical species react directly to form products in a single reaction step and with a single transition state....
s. In elementary reactions, the reaction order, the molecularity and the stoichiometric coefficient are the same, although only numerically, because they are different concepts.

  • A reaction involving one molecular entity is called unimolecular.
  • A reaction involving two molecular entities is called bimolecular.
  • A reaction involving three molecular entities is called termolecular. Termolecular reactions in solution
    Solution

    In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent....
    s or gas mixtures are very rare, because of the improbability of three molecular entities simultaneously colliding
    Collision frequency

    Collision frequency is defined in chemical kinetics, in the background of theoretical kinetics, as the average number of collisions between reacting molecules per unit of time....
     . However the term termolecular is also used to refer to three body association reactions of the type:
Where the M over the arrow denotes that to conserve energy
Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
 and momentum
Momentum

In classical mechanics, momentum is the product of the mass and velocity of an object . For more accurate measures of momentum, see the section Momentum#Modern definitions of momentum on this page....
 a second reaction with a third body is required. After the initial bimolecular collision of A and B an energetically excited reaction intermediate
Reaction intermediate

A reaction intermediate or an intermediate is a molecular entity that is formed from the reactants and reacts further to give the directly observed products of a chemical reaction....
 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.

See also

  • Reaction rate
    Reaction rate

    The reaction rate or rate of reaction for a reactant or product in a particular chemical reaction is intuitively defined as how fast a reaction takes place....