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DCl V.S. HCl


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The effect of isotopic substitution on a rate constant is called


Definitely, there will be a difference for a reaction in the gas phase:

Replacing H by D would change the reduced mass for DCl molecule, and, consequently the vibration frequency of D-Cl stretching - decrease it roughly by factor sqrt(2). The bond is getting weaker and the reaction is expected to be faster.

So far for the gas phase. But the question is regarding water solution (etching). Here the things are much more complex, and, at the first sight, I would join insane_alien in that, that in the ordinary experiment with regular equipment in the student lab, we would not feel any difference.

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The difference in mass between HCl and DCl is only 2.74% so in just about all reactions any type of difference will be negligible. In reactions involving water or any other compound where hydrogen is a larger % of the mass of the molecule then things can get more interesting. (As D2O is 11.1% heavier than H2O, and fully deuterated benzene is 7.7% heavier than normal benzene). Over a short time scale, the 2.74% difference between the rate of reaction of HCl and DCl will be negligible. You'd need a long, drawn out experiment to begin to see the differences and the overall kinetics of the experiment have to be slow enough so that the 2.74% difference in mass between the hydrogen isotopes can produce a slow enough step in the reaction for it to be seen.

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