Law of mass action Vs order of reaction

1. Aug 16, 2010

PrakashPhy

According to law of mass action " The rate of reaciton is directly proportional to the active mass of the reactants. (I understand active mass as molar concentration of the reactants ) "

So for a general reaction aA-------->Product

The rate of forward reaction turns out to be

R=K[A]a (R=K [A]^a)

but for a zero order reaction the rate of reaction is independent of the concentration of the reactants.

How can these two laws of chemistry exist together?

2. Aug 16, 2010

DrDu

According to law of mass action " The rate of reaciton is directly proportional to the active mass of the reactants. (I understand active mass as molar concentration of the reactants ) "

I don't know where you got that statement from, but it is definitely not a correct formulation of the law of mass action.

3. Aug 19, 2010

PrakashPhy

Then what is the statement of law of mass action? Please clarify me.

4. Aug 19, 2010

DrDu

The law of mass action makes statements about the position of the equilibrium, not about the rates of a reaction.

5. Aug 23, 2010

epenguin

However it seems there is variation in terminology also in textbooks and teaching so the student may well have encountered mass action as a kinetic concept; I think I did.

"The fact that Guldberg and Waage developed their concepts in steps from 1864 to 1867 and 1879 has resulted in much confusion in the literature as to which equation the Law of Mass Action refers. It has been a source of some textbook errors.[10] Thus, today the "law of mass action" sometimes refers to the (correct) equilibrium constant formula[11] [12] [13] [14] [15] [16] [17] [18] [19] [20], and at other times to the (usually incorrect) rf rate formula.[21] [22]" http://en.wikipedia.org/wiki/Law_of_mass_action

Better to focus on concepts and facts than their names if possible. I would not recommend the above historical article for first learning, it could confuse.

E.g. you cannot deduce R=K[A]a (R=K [A]^a) from aA-------->Product - it will be wrong more often than not.

Chemical reactions take place in a series of elementary steps (sometimes with parallel pathways of different steps). The rates of elementary steps are all of form rate = k[A], or rate = k[A]2 or rate = k[A] where [A] and are concentrations of substances reacting. But in the intermediate steps the substances reacting are not the original ones so you have to work out mathematically for any hypothetical mechanism, i.e. reaction sequence, the resulting rate laws in terms of the concentrations of initial substances which may be all sorts. It is just an area of study which I hope I have not made sound more complicated than it really is.

(On this site and no other, when I type B inside [ ] I get :surprised)