Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Kryptid on 10/12/2003 19:03:04

Title: Is pKa logarithmic?
Post by: Kryptid on 10/12/2003 19:03:04

pKa is the measurement of how acidic a molecule is (how easily it gives up a proton). The lower the pKa of a molecule, the more acidic it is.

But I have a question; is the pKa scale logarthmic? If it is, then Hydrogen Iodide (pKa = -10) should be about 10 times more acidic than Hydrogen Bromide (pKa = -9). If the scale is not logarithmic, then HI should only be about 1.1 times more acidic than HBr. So is the scale logarithmic or not?

Oh yeah, and another question; what is the pKa of Hydrogen (H2)? My college textbook just says that it is "very large".

Title: Re: Is pKa logarithmic?
Post by: Ylide on 10/12/2003 19:17:30

Yes, pKa is logarithmic.  It's mathematically identical to pH, where one whole integer of difference is a factor of 10.  The Ka (acid dissociate constant) is calculated by 10^-pKa.  So, if the pKa is a negative number, the dissociation constant will be quite large, therefore the H+ ion concentration quite large when the acid is dissolved in water, thus making it a stronger acid.  Conversely, a very large pKa means a very small Ka and very little H+ dissociation.  

I don't have an exact number for the pKa of H2, but now you know what a "very large" pKa means.  Neutral organic compounds have pKa's on the order of 40-60, I would imagine hydrogen being in that ballpark...in other words, billions of times less acidic than real acids.




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