Unimolecular & Bimolecular Reactions

  1. An elementary reaction is a single step reaction with a single transition state and no intermediates.
  2. The molecularity of a reaction refers to the number of molecules involved in an elementary reaction step. It is the number of molecules coming together to react in an elementary reaction. Molecularity is defined only for “simple/one-step/elementary reactions”.
  3. Because there can only be discrete numbers of particles, the molecularity must take an integer value(non-zero). Also, There are no known elementary reactions involving four or more molecules.
  4. On the basis of Molecularity, single-step reactions can be described as unimolecular, bimolecular, or termolecular.
  5. A unimolecular reaction is an elementary reaction in which the rearrangement of a single molecule produces one or more molecules of product. In a unimolecular reaction, a single molecule shakes itself apart or its atoms into a new arrangement, as in the isomerization of cyclopropane to propene. e.g Decomposition of NH4NO2 to N2 and 2H2O
  • An example of unimolecular reaction is radioactive decay, in which particles are emitted from an atom.
  • Other examples include
    1. cis-trans isomerization,
    2. thermal decomposition,
    3. ring opening (or ring-expansion or ring-contraction),
    4. carbocation-rearrangements and
    5. self-racemization.
  • Unimolecular reactions are often first-order reactions.
  • Unimolecular Reaction Steps
  • The elementary reaction step,
  • is unimolecular because there is only one molecule reacting, that is, molecule “A” is reacting. This unimolecular reaction step implies the rate law,
  • or, equivalently,

In words, these elementary reaction steps say that the molecule, A, spontaneously transforms into B at some rate k1. The algebraic sign in front of k1 tells whether you are gaining product or losing reactant depending on whether the concentration in the derivative is increasing or decreasing.

An elementary reaction step may be reversible or irreversible.

This reversible unimolecular step implies the following rate laws,


(Either one of these may be used, depending on whether we are trying to account for the disappearance of reactant, A, or the appearance of product, B, in our mechanism for a particular reaction.)

A unimolecular reaction step can have more than one product, for example,

  1. A bimolecular reaction involves the collision of two particles. In a bimolecular reaction, a pair of molecules collide and exchange energy, atoms, or groups of atoms, or undergo some other kind of change. e.g, 2Br. -> Br2
  2. Bimolecular reactions are common in organic reactions such as nucleophilic substitution. The rate of reaction depends on the product of the concentrations of both species involved, which makes bimolecular reactions second-order reactions. Note that the converse of this rule does not follow, that is, for example second-order rate law does not imply that the reaction is bimolecular – the reaction might be complex ! Also, care must be taken to ensure that the reaction /step that we are considering is really elementary. For example, the reaction H2(g) + I2(l) -> 2HI(g) may look simple, but it is not elementary, and in fact, it has a very reaction mechanism, and hence a complex rate law.
  • NO + O3 = NO2 +O2, Rate = k [NO] [O3]
  • Cl + CH4 = HCl + CH3, Rate = k [Cl] [CH4]
  • Ar + O3 = Ar + O3*, Rate = k [Ar] [O3]
  • A + A = B + C, Rate = k [A]2
  • A + B = X + Y, Rate = k [A] [B]
  • There are several varieties of bimolecular steps. For example,

implies the rate law,


and so on. In Equations 8, 9, and 10 we have given only one product, “C.” We would get the same rate laws if there had been two or more products, for example as in,

Reversible Bimolecular Steps

The bimolecular reaction

implies the rate law

or it could be written as a rate of loss of A or B as we have seen above. The reversible bimolecular reaction,


and its variants.

  • Bimolecular elementary reactions are believed to account for many homogeneous reactions.
  • For elementary reactions, rate law is same as law of mass action; hence order and molecularity are same. (The rate law for an elementary step is derived from the molecularity of that step.)The following examples are given to illustrate this point: O3 = O2 + O, Rate = k [O3] or, in general
  • A = B + C + D, Rate = k [A]
    A* = X + Y, Rate = k [A*]
  • A* represents an excited molecule.
  • Elementary reactions add up to complex reactions.

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