Polyprotic acid Exception
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Polyprotic acid Exception
I know in the outline it explains an exception to when using the first Ka compared to the second and which in negligible. Could someone please expand ln the idea I’m still a tad confused?
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Re: Polyprotic acid Exception
Hi, from what we went over in section, our TA said that for weak polyprotonic acids, we use the first Ka for the first deprotonation to find the pH, and that further deprotonation is insignificant. For strong polyprotonic acids, the first deprotonation is complete (ie polyprotonic acid H2A will fully dissociate into HA- and H3O+) and that the second deprotonation (HA- into A2- and more H3O+) will be weak, so you'll need to use that Ka value to find the pH. Hope this helps!
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Re: Polyprotic acid Exception
Hi!
From my understanding, the first deprotonation (H2A + H20 --> HA^- + H3O^+) causes a significant increase in H3O^+ concentration which causes the solution to become acidic, so we have to use an ICE table (unless its a strong acid and we can assume complete dissociation) to find the [H3O^+] and plug it into pH = -log[H3O^+]. If the second deprotonation equilibrium constant K_a2 is significantly less than the first deprotonation equilibrium constant K_a1 (i.e. K_a2 < K_a1/1000), then the increase in [H3O^+] caused by the second deprotonation (HA^- + H2O --> HA^2- + H3O^+) is so small compared to the amount after the first deprotonation that there is not a significant change in the pH.
From my understanding, the first deprotonation (H2A + H20 --> HA^- + H3O^+) causes a significant increase in H3O^+ concentration which causes the solution to become acidic, so we have to use an ICE table (unless its a strong acid and we can assume complete dissociation) to find the [H3O^+] and plug it into pH = -log[H3O^+]. If the second deprotonation equilibrium constant K_a2 is significantly less than the first deprotonation equilibrium constant K_a1 (i.e. K_a2 < K_a1/1000), then the increase in [H3O^+] caused by the second deprotonation (HA^- + H2O --> HA^2- + H3O^+) is so small compared to the amount after the first deprotonation that there is not a significant change in the pH.
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