<P> More complex rate laws have been described as being mixed order if they approximate to the laws for more than one order at different concentrations of the chemical species involved . For example, a rate law of the form r = k 1 (A) + k 2 (A) 2 (\ displaystyle r = k_ (1) (A) + k_ (2) (A) ^ (2)) represents concurrent first order and second order reactions (or more often concurrent pseudo-first order and second order) reactions, and can be described as mixed first and second order . For sufficiently large values of (A) such a reaction will approximate second order kinetics, but for smaller (A) the kinetics will approximate first order (or pseudo-first order). As the reaction progresses, the reaction can change from second order to first order as reactant is consumed . </P> <P> Another type of mixed - order rate law has a denominator of two or more terms, often because the identity of the rate - determining step depends on the values of the concentrations . An example is the oxidation of an alcohol to a ketone by hexacyanoferrate (III) ion (Fe (CN)) with ruthenate (VI) ion (RuO) as catalyst . For this reaction, the rate of disappearance of hexacyanoferrate (III) is r = (Fe (CN) 6) 2 − k α + k β (Fe (CN) 6) 2 − (\ displaystyle r = (\ frac (\ ce ((Fe (CN) _ (6)) ^ (2 -))) (k_ (\ alpha) + k_ (\ beta) ((\ ce (Fe (CN) _ (6)))) ^ (2 -)))) </P> <P> This is zero - order with respect to hexacyanoferrate (III) at the onset of the reaction (when its concentration is high and the ruthenium catalyst is quickly regenerated), but changes to first - order when its concentration decreases and the regeneration of catalyst becomes rate - determining . </P> <P> Notable mechanisms with mixed - order rate laws with two - term denominators include: </P>

Difference between first order second order and zero order reactions