Editing Rate-determining step (section)

==Composition of the transition state==
A useful rule in the determination of mechanism is that the concentration factors in the rate law indicate the composition and charge of the [[activated complex]] or [[transition state]].<ref name=Espenson>{{cite book |last=Espenson |first=J. H. |title=Chemical Kinetics and Reaction Mechanisms |publisher=McGraw-Hill |year=2002 |isbn=0072883626 |pages=127–132 |edition=2nd}}</ref> For the {{NOx|x=2}}–CO reaction above, the rate depends on [{{NOx|x=2}}]<sup>2</sup>, so that the activated complex has composition {{chem|N|2|O|4}}, with 2 {{NOx|x=2}} entering the reaction before the transition state, and CO reacting after the transition state.

A multistep example is the reaction between [[oxalic acid]] and chlorine in aqueous solution: {{chem|H|2|C|2|O|4}} + {{chem|Cl|2}} → 2 {{CO2}} + 2 {{chem|H|+}} + 2 {{chem|Cl|-}}.<ref name=Espenson/>
The observed rate law is 
:<math chem>v = k \frac{\ce{[Cl2][H2C2O4]}}{[\ce{H+}]^2[\ce{Cl^-}]},</math>
which implies an activated complex in which the reactants lose 2{{chem|H|+}} + {{chem|Cl|-}} before the rate-determining step. The formula of the activated complex is {{chem|Cl|2}} + {{chem|H|2|C|2|O|4}} − 2 {{chem|H|+}} − {{chem|Cl|-}} + {{mvar|x}}&thinsp;{{H2O}}, or {{chem|C|2|O|4|Cl|(|H|2|O|)|{{mvar|x}}|–}} (an unknown number of water molecules are added because the possible dependence of the reaction rate on {{H2O}} was not studied, since the data were obtained in water solvent at a large and essentially unvarying concentration).

One possible mechanism in which the preliminary steps are assumed to be rapid pre-equilibria occurring prior to the transition state is<ref name=Espenson/>
:{{chem|Cl|2}} + {{H2O}} {{eqm}} HOCl + {{chem|Cl|-}} + {{chem|H|+}}   
:{{chem|H|2|C|2|O|4}} {{eqm}} {{chem|H|+}} + {{chem|HC|2|O|4|-}}
:HOCl + {{chem|HC|2|O|4|-}} → {{H2O}} + {{chem|Cl|-}} + 2 {{CO2}}