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Activity vs. Concentration

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eddieq:
why is activity preffered over concentration in electrochemistry? :biggrin2:

Borek:
In general, activity is what is important when doing any equlibrium calculations. In all possible places - dissociation constants, solubility products, complexation constants, Nernst equation and so on - it is activity that is important, not the concentration.

In diluted solutions that makes no difference, the more concentrated the solution, the worse the situation.

eddieq:
Does that mean it is more accurate if activity is used ? what scientific reason is there to back it up like ... does it have anything to do with ideal and non ideal situations???

Borek:
For the diluted solutions there is no problem, but once the solutions get more concentrated discrepancy between calculated and observed concentrations in equlibrium rises. That's why the activity concept was introduced. Look for Debye-Huckel theory, activity coefficients and ionic strength of the solution. Formulas for the calculations are presented on my web site (try info on pH calculation), but for the explanation you have to either find a book or google the web.

ssssss:
If you see your book you will find a table that tells relation between conductivity of the electrolyte with conc. you can see the sharp difference.

In case of strong electrolytes such as HCL when the conc is high the charge transfer through the ions is quite difficult because of strong electrostatic force between them but this is not equally liable with weak electrolytes such as acetic acid.In fact weak electrolytes work better with lower conc.[ostwald law states conductivity is inversely prprtional to square root of conc. for weak electrolytes].

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