[earthnation112]

The following is a past paper question:

1. Write equations illustrating successive acid-base equilibria and the dissociation constants, K for sulphuric acid.
2. Draw the structure and hence predict pKa₁.
3. Using Pauling’s rules predict the successive pKa's of H₂SO₄.


For the first question my answer was:

[tex]H_{2}SO_{4} \leftrightharpoons H^{+} + HSO_{4}^-[/tex]
[tex]K= \frac{H^{+} + HSO_4^- } {H_{2}SO_{4}}[/tex]

For question 2, I made the structure and used pauling's first rule to predict the pKa₁ which was:

[tex]pK_{a} \approx 8 - 5p[/tex] (where p is the number of oxoacids attached to central element)

So my anwer was= [tex] pK_{a} \approx 8 - (5 \times 2)= -2[/tex]

How do I predict for successive pKa's? I remember reading that to solve this you use paulings second rule which is "successive pka values for acids increase by five units for each successive proton transfer". What does that exactly mean?

Would I be right to assume that this means each successive pKa value increases by five approximately?  I know how to calculate pKa₁ but how do I calculate pKa₂. Do I simply just add five?

Lastly H₂SO₄ has two successive pKa values pKa₁ and pKa₂ and H₃AsO₄ has three successive pKa values which are pKa₁ pKa₂ pKa₃ is the amount of successive pka based of the number of hydrogen's right? Thank You

[Borek]

[tex]K= \frac{H^{+} + HSO_4^- } {H_{2}SO_{4}}[/tex]

Sum in the numerator?

[tex]pK_{a} \approx 8 - 5p[/tex] (where p is the number of oxoacids attached to central element)

Oxoacids?

How do I predict for successive pKa's? I remember reading that to solve this you use paulings second rule which is "successive pka values for acids increase by five units for each successive proton transfer". What does that exactly mean?

Would I be right to assume that this means each successive pKa value increases by five approximately?

Yes, that's what it means.

Lastly H₂SO₄ has two successive pKa values pKa₁ and pKa₂ and H₃AsO₄ has three successive pKa values which are pKa₁ pKa₂ pKa₃ is the amount of successive pka based of the number of hydrogen's right?

I assume there is a period after pKa₃? Only hydrogens from X-OH groups (where X is the central atom) are acidic. There are some examples of oxoacids where the hydrogen is bonded directly to X and then it doesn't dissociate.

[earthnation112]

Sum in the numerator?

Is this better:

[tex]K = \frac{[H^{+}][H_2{}SO_4^-]}{[H_{2}{}SO_4]}[/tex]

Oxoacids?

Sorry it shoud be number of oxo groups not oxoacids.

Yes, that's what it means.

So pka₁ = -2, and pKa₂ = 5

Lastly H₂SO₄ has two successive pKa values pKa₁ and pKa₂ and H₃AsO₄ has three successive pKa values which are pKa₁ pKa₂ pKa₃ is the amount of successive pka based of the number of hydrogen's right?

I assume there is a period after pKa₃? Only hydrogens from X-OH groups (where X is the central atom) are acidic. There are some examples of oxoacids where the hydrogen is bonded directly to X and then it doesn't dissociate.

Thank You