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I have been recently studying chemical kinetics. I've understood what the rate law and the rate constant mean, and also that, kinetics depends completely on experimental observations, due to which we have zero order reactions as well as fractional order reactions.

Take for example, this: $$\text{Rate of a certain chemical reaction} = k [A]^p [B]^q [C]^r $$

Here, $[A]$, $[B]$ and $[C]$ are the molar concentrations of the three reactants involved, while $p$, $q$ and $r$ are the number of molecules of each respectively taking part in the reaction.

So, $\text{the order of the reaction} =p+q+r $.

Now, this $p$, $q$ and $r$ are completely determined experimentally, and cannot be determined theoretically.

How are these values determined experimentally? How does one find out how many molecules of each reactant are taking part in the reaction? Is there any special instrument to do this? If not, then how do scientists find the order of a reaction?

As an attempt to shorten out the answering area, consider this reaction: $$\ce {NO2 +CO -> NO +CO2} $$ Now, experimentally it has been determined that $\text {the rate of this reaction} = k[\ce{NO2}]^2$. I want to know how have scientists determined that the rate depends on the square of the concentration of $\ce {NO2} $ and not on the other reactant.

N.B.: My question is how it can be experimentally determined, and not why it has to be experimentally determined.

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    $\begingroup$ One important thing to note is that p, q, and r are only the number of molecules of each involved if this the rate law for an elementary reaction (something which takes place in one step). Otherwise, they are as you said just empirical parameters that can take on negative and fractional values and don't directly relate to how many molecules are involved in the reaction. $\endgroup$ – Tyberius Sep 2 '17 at 15:40
  • $\begingroup$ @Tyberius OK, I get that. I just called them number of molecules to express myself. You can edit to express them better. $\endgroup$ – BuddingPhysicist Sep 2 '17 at 15:42
  • $\begingroup$ Chemguide has a decent intro: chemguide.co.uk/physical/basicrates/experimental.html $\endgroup$ – orthocresol Sep 2 '17 at 19:29
  • $\begingroup$ If you are looking more in terms of the experimental setup, I would look at orthocresol's link. If you are more interested in the generic methodology, the Wikipedia article on order of reaction gives a good description. $\endgroup$ – Tyberius Sep 2 '17 at 20:20
  • $\begingroup$ The comment to this question answers some of your question chemistry.stackexchange.com/questions/117236/… $\endgroup$ – porphyrin Jul 1 at 18:33
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Experimetally :- one reactant is kept constant and concentration of other is changed and accordingly rate of reaction is measured...then dependence with thta reactant is measured.....and then other one is kept constant and ist one is changed then similarly rate of reaction and thus dependence is measured....... We cant determine the actual dependence of rate of reaction on the concentration of reactants therotically ......we have tk calculate it and then we can formulate the rate law or order of reaction

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