Reasoning

[Graphite]

Why is the the pKa much higher for the 2nd ionisation of H2SO4? I am just trying to find a reason for it...

[Mao]

Why is the the pKa much higher for the 2nd ionisation of H2SO4? I am just trying to find a reason for it...

From a kinetics viewpoint, the rate of proton capture by is much faster than rate of deprotonation by . This means there's a big increase in enthalpy associated with the second ionization.

The source of this enthalpy increase is most probably due to the increased charge (more negative charges distributed over a smaller space --> more electronic repulsion within the molecule)

[Graphite]

But aren't both hydrogens in the same chemical environment? Why does a negative charge matter?

[Mao]

But aren't both hydrogens in the same chemical environment? Why does a negative charge matter?

The two Hs in H2SO4 are indeed equivalent, but the H in HSO4- is not the same as H in H2SO4. It's easy to rip one H, but much harder to rip the second H.

Imagine you are a neutral H2SO4 molecule. You donate a H+ because as H2SO4, you only loosely hold on to your H+, and so water is able to capture it. However, now that you have an overall negative charge, this significantly increases how strongly you are holding on to the remaining hydrogen. It is much more difficult to rip the second H+ away.

[Graphite]

ooooo makes sense. Thanks!