My chemistry textbook says that in the ionisation of a polyprotic acid with Ka1, Ka2 and so on, 'the first Ka is usually the largest.' I took the word 'usually' to mean that there are exceptions to the rule, but I haven't been able to find any cases where Ka2 is greater than Ka1.

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    $\begingroup$ That's a really odd statement. In my experience, they're always numbered in decreasing order of numerical value (increasing order of $\mathrm p K_\mathrm a$). I'm curious to see if anyone comes up with an exception to this rule. $\endgroup$ – hBy2Py Oct 17 '16 at 11:10
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    $\begingroup$ How do you know which proton is the first? No, it goes the other way around: the largest Ka is the first, because that proton is naturally the first to react. It can't be otherwise. $\endgroup$ – Ivan Neretin Oct 17 '16 at 11:17
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    $\begingroup$ @IvanNeretin What about a hypothetical molecule where the loss of the first proton facilitates the loss of the second proton? $\endgroup$ – DHMO Oct 17 '16 at 11:45
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    $\begingroup$ @DHMO I don't see how's that possible, but let's assume it is. Well, then these two protons would react together and effectively have the same pKa. $\endgroup$ – Ivan Neretin Oct 17 '16 at 11:54
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    $\begingroup$ I'd have to agree with Ivan. Ka1 and Ka2 are just labels. They don't carry any information. Which one is most acidic? The one that is larger. It just intuitively makes sense let the largest one be Ka1. It's an exam question that doesn't really require any understanding of the subject. $\endgroup$ – Brian Oct 17 '16 at 13:01

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