$$\ce{NaCl + H2SO4 -> NaHSO4 + HCl}$$

The above reaction happens, but why doesn't the following happen? Or does it but only when one of the reactants is in excess?

$$\ce{2NaCl + H2SO4 -> Na2SO4 + 2HCl}$$

The second reaction seems plausible to me. How can I predict whether a similar type of reaction will be like the first equation or the second equation?


Sodium bisulphate (pKa ~ 2 - https://en.wikipedia.org/wiki/Sulfuric_acid) is a much weaker acid that hydrochloric acid (pKa = ~ -7), which means that chloride is not a strong enough base to deprotonate sodium bisulphate.

However all acid pbased reactions are at equilibrium. Roughly one part in 10^9 of the species involve sulphate anion, which for all practical purposes is negligible.

Table of pKa charts: http://www.lauher.com/che142/acidbase.pdf


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