Molecularity
Encyclopedia
Molecularity in chemistry
is the number of colliding molecular entities
that are involved in a single reaction step
. While the order of a reaction is derived experimentally, the molecularity is a theoretical concept and can only be applied to elementary reaction
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
and momentum
a second reaction with a third body is required. After the initial bimolecular collision of A and B an energetically excited reaction intermediate
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.
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.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
is the number of colliding molecular entities
Molecular entity
According to the 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 solutionSolutionIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
s or gas mixtures are very rare, because of the improbability of three molecular entities simultaneously collidingCollision frequencyCollision 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 an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
and momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
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. Most chemical reactions are stepwise, that is they take more than one elementary step to complete...
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.
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.
See also
- Reaction rateReaction rateThe reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
- Order of reactionOrder of reactionIn chemical kinetics, the order of reaction with respect to certain reactant, is defined as the power to which its concentration term in the rate equation is raised .For example, given a chemical reaction 2A + B → C with a rate equation...