# Zero order reactions

I have come across many reactions that are zero order reactions but at one glance I'm unable to tell if they are zero order or not. Is there any criteria that can be used to identify these reactions or are they purely experimentally determined?

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To your question of whether zero-order reactions can be recognized “at one glance” and from an non-experimental criterion, the answer is clearly no. The only theoretical way of firmly establishing the order of a reaction (if it has one) is to fully calculate its kinetic law (or rate equation), which can only be done if you know the scheme of the reaction, i.e. the list of all elementary reactions involved.

There is one case where you can take a shortcut, namely if you know (or strongly suspect) that the reaction in question is an elementary reaction… for example, if it involves no catalyser, proceeds in a single step with only one transition state and no intermediate. If that is the case, then the order of the reaction is determined by the stoechiometry (A –> X is first order, 2 A -> X or A + B -> X are second order). However, this does not apply in your case, because elementary reactions cannot have zero order!

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 My doubt was due to $\ce{H2 + Cl2->2HCl }$ in the presence of light .this is a zero order reaction but isnt this also a bimolecular elementary rxn?? if it is then it does not go according to your ans – Ashu May 6 '12 at 20:29 $\ce{H2 + Cl2 -> 2 HCl}$ proceeds (like many photochemical reactions) by a complex mechanism… See here for an example given about bromide. Photochemical reactions are usually not elementary reactions. – F'x♦ May 6 '12 at 20:47 oh yeah now i remember that a photon splits the halogen into two free radicals and thus the reaction proceeds....thanks – Ashu May 7 '12 at 5:35

Determining the rate of the kinetics of a reaction isn't done with only one measurement, or looking at only one graph.

What zero order kinetics means, is that the rate of the reaction is independent of the concentration of chemical species. To determine the kinetics of a process, you need to take a series of measurements while altering one variable, namely the concentration of one of the chemical species under investigation.

In zero order kinetics, you should observe that as you change the concentration of reactant, for example, the rate of the reaction remains constant.

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 That is to say, explicitly, that the order for each component in the reaction mixture is experimentally determined. – Janice DelMar May 5 '12 at 18:37 Of course, and if all are 0, then the reaction follows zero order kinetics. My bad. – CHM May 5 '12 at 18:44