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    If,

    NaCl + H2SO4 --> NaHSO4 + HCL

    then why is,

    NaF + H2SO4 --> NaHSO4 + HF

    not right, why is it,

    2NaF + H2SO4 ---> Na2SO4 + 2HF

    ???
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    For this you need to think about the relative bond enthalpies of HF and HCl. Sulfuric acid has two protons it can donate, however these two protons have different pKa's. In the top example, the first proton is donated, reacts with the chlorine ion in solution, forming HCl. The formation of the H-Cl bond releases enough energy to outweigh the energy needed to remove the proton from sulfuric acid. However, now you have HSO4-, meaning that it's going to be harder to remove the second proton as an already negative species isn't going to want to lose another positive charge. As a result, the formation of a second H-Cl bond does not release enough energy to outweigh the energy required to remove the second proton from HSO4-.

    For HF, the story is different. The H-F bond is much stronger than the H-Cl bond, so the energy released from the formation of the second HF is enough to outweigh the energy required to remove the second proton from HSO4-.

    Hope this helps.
 
 
 
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