Opposing reactions The hydrogen-iodine reaction

Similarly, the number of collisions per second between unlike molecules is proportional to the product of the concentrations of the two kinds of molecules hence for the bimolecular elementary reaction of the type 

A termolecular reaction is an elementary reaction that involves the simultaneous collision of three molecules  

The frequency of occurrence of three-body collisions is very much smaller than that of two-body collisions. Consequently, if a termolecular step is essential to the progress of the reaction, the reaction is very slow. 

Examination of Eqs. (32.51), (32.52), and (32.53) shows that for elementary reactions the order of the reaction can be inferred from the stoichiometric coefficients. This is true only for elementary reactions. 

The gas phase reaction of hydrogen with iodine, investigated extensively by Bodenstein in the 1890s, provides a classic example of opposing second-order reactions. Between 300 and 500 °C the reaction proceeds at conveniently measurable rates. If we assume that the mechanism is simple, consisting of one elementary reaction and its reverse,  

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