The collision theory in solutions

In solutions, the molecules do not move as freely as in the gas phase. Instead, they move in the potential field of neighboring molecules. If two molecules come into contact by random displacements in solution, they will collide a number of times before they separate. This is called an encounter. If the colliding particles do not react, their encounters are determined by the Brownian movement they are random and diffusion controlled by Pick s laws. If a bimolecular reaction occurs in every collision, it is said that the reaction is diffusion controlled. The rate of such reactions is the maximum rate attainable in a liquid medium. Most reactions have large energy barriers and steric requirements therefore, very many collisions are needed before a reaction will take place. An example of a reaction that probably occurs in every collision is the recombination of two methyl radicals to ethane in aqueous solution  

The rate constant of the disappearance of CH3 (aq) radicals is 3.2x10 dm mor s at 25 °C. The activation energy is 16 kJmor. The rate constant in the gas phase is 2.0x 10 dm mol s and the activation energy is zero. The theory predicts the Arrhenius frequency factors for bimolecular reactions to be somewhat larger in solutions than in the gas phase, but there is not much evidence to support this prediction. The average value of the frequency factor for the second order reactions in solutions is 10 dm mol s , the same as in the gas phase.  

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