- #1

- 2

- 0

I have always been taught in dimensional analysis that units should cancel out before you log a number. The problem I have, as stated in the title of this thread, is that units don't seem to cancel out when applying bimolecular rate constants to the Eyring equation:

ln[(

where,

On a related note, if one were to calculate the enthalpy and entropy of activation for a reaction using pseudo-first-order rate constants, what kind of physical meaning do they have or are they meaningless by themselves (as above, this introduces a systematic difference in both the enthalpy and entropy of activation and would cancel out perfectly upon subtraction)?

Thank you very much in advance!

ln[(

*k*h)/(*k*_{B}T)] = -ΔH^{‡}/(RT) + ΔS^{‡}/Twhere,

*k*is the rate constant in question and the fact that lots of units are left after cancelling really bothers me. I have consulted Atkins and Google (and Wiki of course!) but to no avail. It seems that the wealth of materials available on the Internet seems to just use this equation for bimolecular reactions. This leads to a systematic error in both the enthalpy and entropy of activation (if something is indeed missing). If comparisons were to be made between enthalpies and entropies derived from rate constants with the same molecularity they would perfectly canceled out; however, if comparisons were to be made between rate constants with different molecularity then there is a problem. Am I missing something painfully obvious?On a related note, if one were to calculate the enthalpy and entropy of activation for a reaction using pseudo-first-order rate constants, what kind of physical meaning do they have or are they meaningless by themselves (as above, this introduces a systematic difference in both the enthalpy and entropy of activation and would cancel out perfectly upon subtraction)?

Thank you very much in advance!

Last edited: