text: In a bimolecular reaction, two molecules collide and exchange energy, atoms or groups of atoms.
  This can be described by the equation
  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 S2-type nucleophilic substitution of methyl bromide by hydroxide ion: