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In 1st year chemistry questions, the concept of acid strength is often introduced to students including the 3 factors that provide a heuristic explanation for most of the common acids and conjugate bases: Electronegativity of the atom directly attached to the protic hydrogen, resonance stabilisation and inductive effects. It is also often taught that resonance stabilisation dominate inductive effects.

Now it seems there is the following curious case: $\ce{HNO3}$ and $\ce{HCl}$. Both are considered as strong acids with negative pKa and experimental measurements showed that the pKa of $\ce{HCl}$ (and all halide acids except HF) are lower than $\ce{HNO3}$ and hence $\ce{HCl}$ is stronger.

The common explanation is often given as because $\ce{Cl}$ is more electronegative than $\ce{N}$. However such explanation is quite unsatisfactory as it ignores the fact that the $\ce{NO_3^-}$ is not only larger in size than $\ce{Cl^-}$, but has its negative charge being more delocalised among the oxygens than $\ce{Cl^-}$ (argued from its 3 resonance contributors where each oxygen has a formal charge of -2/3 and also that in MO theory, none of the antibonding orbitals of $\ce{NO_3^-}$ are occupied. Thus based on these heuristic, it seems there is every reason that the electron delocalisation of $\ce{NO_3^-}$ could have dominate the electronegativity of $\ce{Cl^-}$ as their values are only -0.12 apart (meanwhile $\ce{O}$ is roughly 0.24 apart from $\ce{Cl^-}$)

It is also hinted in this comment, that $\ce{Cl^-}$ is more solvated than $\ce{NO_3^-}$ and hence may play a role in the easier dissociation of $\ce{HCl}$. However, such concepts will not be covered in great detail for first years, and hence such a complication can be potentially confusing to students.

How can we check whether delocalisation or electronegativity effect dominates in the stability of a conjugate base, and how should we rationalise to students that electronegativity often beats delocalisation?

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  • $\begingroup$ But in nitric acid, hydrogen is bonded to O, not N. And H-X bond strength is also an important consideration, as far as I’m aware. $\endgroup$ – orthocresol Sep 22 '19 at 15:03
  • $\begingroup$ Well, that adds to the confusion, O is 0.24 more electronegative than Cl, and yet Cl wins, so it means the usual 3 heuristics have not been able to give a satisfactory explanation, and yet such an answer is often given as an explanation for this HCl vs HNO3 case. Bond strengths are sometimes discussed, but often in the context of haloacids. $\endgroup$ – Secret Sep 22 '19 at 15:07

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