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Chemical equilibria

  1. Jan 26, 2008 #1
    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!!!
  2. jcsd
  3. Jan 29, 2008 #2


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    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?
  4. Jan 29, 2008 #3
    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......... :-(
  5. Jan 29, 2008 #4


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    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?
  6. Jan 30, 2008 #5
    what about the rate of reaction H2 +Cl2=>2HCl

    it should be


    but somewhere i studied that it is a zero order reaction i.e rate is constant irrespective of concentration of reactants
  7. Feb 8, 2008 #6
    Complex reactions

    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 [tex] \leftarrow \rightarrow [/tex] M + 2Br

    Br + H2 [tex] \leftarrow \rightarrow [/tex] HBr + H

    H + Br2 [tex] \rightarrow [/tex] 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.
  8. Feb 14, 2008 #7
    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.
  9. Mar 1, 2008 #8
    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...
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