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Acids are (in one definition) molecules that can donate a proton (H⁺ ) and bases are (in one definition) molecules that can accept a proton. We talk about “acids” and “bases” but these are just temporary roles that molecules take on – it’s their “job” not their “identity” and they go back and forth between their “conjugate acid” & “conjugate base” forms because once an acid gives one up it can take one (act as a base) and vice versa. But some really prefer to do one or the other and the more they like to do one (take or give), the more they hate to do the other because it’s not the *process* of giving up or taking a proton they like, they’re just happier having or not having it. 

Different molecules have different proton greedinesses which we can describe using a value called pKa. pH is a measurement of H⁺ concentration (where lower pH means more H⁺) and pKa is the pH at which 1/2 the copies of an acid will be in the deprotonated (conjugate base) form. It’s a measure of “how extreme” (i.e. basic/alkaline) conditions must be in order for an acid to give up its H⁺ 

  • at any pH above an acid’s pKa, any particular molecule of that acid is more likely to be deprotonated than protonated (cookie jar’s running low so cookie monsters donate or at least don’t steal)
  • at any pH below an acid’s pKa, any particular molecule of that acid is more likely to be protonated than deprotonated (cookies galore!)

The chances of cookie donation increase the further above pKa you are (e.g. 1% deprotonated @ 2 pH units below pKa & 99% deprotonated @ 2pH units above pKa 

Different acids have different “willingness thresholds” 

  • STRONGER acids are more generous & have LOWER pKas – even a slight deficit of H⁺ in their surroundings & they’ll give one up
  • WEAKER acids are “greedier” –  they have HIGHER pKas meaning they won’t give up an H⁺ until there’s a big deficit