Chemical Equilibria: Law of Mass Action & Beyond

[sadhu]

chemical equilibrium law is made using the fundamental rule

rate of forward reaction=rate of backward reaction

and then applied law of mass action to it

but law of mass action is not always correct in giving relation for the rate .
so why it is used there?

secondly if equilibrium depend upon the stoichiometric representation of reaction , then how can one tell which one will give correct relation ,or is it that all are correct?/

thanks in advance!

 

[chemisttree]

chemical equilibrium law is made using the fundamental rule

rate of forward reaction=rate of backward reaction

and then applied law of mass action to it

but law of mass action is not always correct in giving relation for the rate .
so why it is used there?

secondly if equilibrium depend upon the stoichiometric representation of reaction , then how can one tell which one will give correct relation ,or is it that all are correct?/

thanks in advance!

The rate of reaction is usually a function of concentration of one or more of the reagents. You see this also applied in equilibrium discussion... of course the explanation is that the rate of forward and reverse reactions are equal in those explanations.
Are you sure you understand the law of mass action?

 

[sadhu]

I think i am
well is it not this ,that rate of a chemical reaction is directly proportional to the active masses of the reactant raised to the power of their stoichiometric coefficients "

or else I slept whole year... :-(

 

[chemisttree]

I think i am
well is it not this ,that rate of a chemical reaction is directly proportional to the active masses of the reactant raised to the power of their stoichiometric coefficients "

or else I slept whole year... :-(

Substitute 'concentration' for 'active masses' and I think you are close.
Do you think that is not the case that the rate of a chemical reaction is directly proportional to the concentration of one or more of the reactants raised to its stoichiometric coefficient?

 

[sadhu]

right
but
what about the rate of reaction H2 +Cl2=>2HCl

it should be

rate=k

[Cl2]

but somewhere i studied that it is a zero order reaction i.e rate is constant irrespective of concentration of reactants

 

[pkleinod]

Complex reactions

right but
what about the rate of reaction H2 +Cl2=>2HCl? It should be
rate=k

[Cl2]

No. The reaction does not go as written in the equation, which is merely a summary of the overall reaction. The reactions of H2 with halogens are
complex chain reactions involving several elementary steps:

Br2 + M \leftarrow \rightarrow M + 2Br

Br + H2 \leftarrow \rightarrow HBr + H

H + Br2 \rightarrow HBr + Br.

In such cases, you cannot expect the law of mass action based on the overall reaction to work. For a discussion of this type of reaction, look in "The Foundations of Chemical Kinetics" by Sidney W. Benson (1960) or in a more
modern kinetics book.

 

[sadhu]

that means i need to know the mechanism before applying the law of equilibrium
none of the books i have got mention anything regarding finding mechanism before applying law of equilibrium to reactions they only tell that in kinetics part.

 

[mysqlpress]

that means i need to know the mechanism before applying the law of equilibrium
none of the books i have got mention anything regarding finding mechanism before applying law of equilibrium to reactions they only tell that in kinetics part.

unless a single stage reaction
because all reactants are "reacted" within one step: that step is called rate-determining step.

for most of reactions, you cannot expect that the rate of reaction has any relationship with the stoichoimetric coefficients of all reactants or products...and the order of reaction with respect to the reactants/products are as well...