Change in concentration vs. reaction rate

  1. Maylis

    Maylis 852
    Gold Member


    I am wondering, why is it that

    ##\frac {d[C]}{dt} \ne k[C]## in general, where ##C## is a chemical species, and the product ##k[C]## is the reaction rate, ##r##. ##r_{c} = k[C]##

    My thoughts is that because the units aren't necessarily the same, therefore they can't be the same. But I was wondering about a more physical explanation.
  2. jcsd
  3. What you're describing is a unimolecular reaction. If you have a reaction that is more than just one thing changing on its own, you have at least a bimolecular reaction and you can't talk about it just in terms of a single concentration.

    The units of the rate constant are whatever they have to be for whatever type of reaction you have. You'll learn all about this in P-chem.
  4. DrDu

    DrDu 4,461
    Science Advisor

    What do the square brackets stand for, exactly?
  5. Maylis

    Maylis 852
    Gold Member

    Concentration of the species
  6. DrDu

    DrDu 4,461
    Science Advisor

    You have to be careful, here. The velocity of the reaction may in deed be formulated as the change of the concentration of a species with time. However, even in unimolecular reactions, the expression on the right hand side depends rather on the chemical activity than on concentration.
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