(adsbygoogle = window.adsbygoogle || []).push({}); Calling kinetics experts: rate law from conductivity isnt possible!!?

Consider the usual primary halogenoalkane aqueous alkaline hydrolysis reaction

RX + OH^{-}--> ROH + X^{- }

We know the rate law is first order in RX and OH^{-}. We could separately represent the drop in OH- conductivity as an exponential decay with a constant half-life (Λ_{o}OH^{-}e^{-kt}) and the rise of X- conductivity as the inverse function of this (0.5Λ_{o}OH^{-}(1-e^{-kt}), taking the conductivity of X^{-}as 0.5x that of OH^{-}.

In practice, using excess RX, the measured (or modelled) solution conductivity during hydrolysis is obviously the sum of the ion conductivities at any point in time. The mixture conductivity drop-off appears to be an exponential-type decay, but attempts to curve fit (albeit only in Excel) show it is not, nor does it fit a recognisable integrated rate law plot. One can therefore not obtain a rate constant or order from this progress curve, which is frustrating - unless I'm mistaken!! {Its not the case for aqueous hydrolysis as this produces ions from neutral molecules rather than an exchange of ions and the graphs work fine}.

Furthermore, taking an initial rates approach and plotting initial (ΔΛ/t) vs Λ_{final}(over several initial concentrations, rather than a single Λ vs t curve as above) gives a straight line, but whose slope does not appear to be a simple multiple of the calculated k for OH^{-}decay on its own. The stoichiometry is 1:1, so the rate of [OH^{-}] decline = rate of [X^{-}] growth, and I imagined the slope would therefore be k x ratio of ion conductivities ... but it's not. Its a smaller number.

Any thoughts please?

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# Calling kinetics experts: rate law from conductivity isnt possible?

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