Question

Replacing an atom by a heavier isotope reduces the rate constant of this reaction. How would you extend this idea to rationalize isotope effects from a thermodynamic point of view, focusing on the dissociation of a weake acid?

​HA (aq) <=> H⁺ (aq) + A^-        pK_​a(HA)=-logK​_a
​DA (aq) <=> D⁺ (aq) + A^-        pK_​a(DA)=-logK_​a
​​Which acid (HA or HD) would you expect will be weaker, having a larger pK_a​ value? explain in detail

Answer 1

Relacing an atom with an heavier isotope makes the bond stronger and hence the rate of reaction reduces by this isotopic displacement also known as the isotopic effect.

Thus, of the two acids, the one with the heavier isotope in it would dissoiate less readily than the other acid with normal atom in it.

HA would have a lower pKa so is a stronger acid when compared to DH.

DH would have a higher pKa, weaker acid than HA due to lesser dissoiation or ionization.