Pka1 and Pka2

[Koury Khalloufi 1B]

What is the threshold for when we would use Pka2 instead of Pka1? For example, when a table is given for values of Pka's, when is it that we would use Pka2?

[305961049]

When dealing with compounds that have multiple ionizable protons (multiple acidic groups), the pKa values represent the acidity of each ionizable proton. The choice between using pKa1 or pKa2 depends on the specific situation and the number of acidic groups in the compound.

Use pKa1 when you are dealing with the first ionization or deprotonation of the compound. This is typically relevant when the compound has only one acidic group.

Use pKa2 when you are considering the second ionization or deprotonation, which occurs after the first acidic group has lost a proton. This is applicable when the compound has more than one acidic group.

For example, consider a molecule with two acidic groups (two hydrogen ions that can be ionized), denoted as A-H-H. The first ionization will give you A-H and H-, and the second ionization will give you A and 2H-. In this case, you would use pKa1 for the first ionization and pKa2 for the second ionization.

[Gitanjali_Sharma_2D]

Additionally, in most cases the difference between pKa1 and pKa2 will be quite large, so it should be very clear which one to use. My TA did say that we won’t need to use pKa2 in this class, so you don’t need to worry about it too much.