pH<pKa means acidic form, but why?

by shrew
Tags: acidic, form, means, ph<pka
shrew is offline
Dec10-12, 06:52 PM
P: 6
I've memorized the following:
pH<pka, then acidic form
pH>pka, then basic form
And I'm guessing that pH=pka means neutral.

However, my instincts tell me it would be the opposite. Take NH3, ammonia, with a pKa of around 40. And then there's NH2-, which from a google search, has an incredibly high pKa. These are both basic.

So what the above rules state, is that if you added an amount of one of the above bases into a solution with pH=0, pH=7, or pH=14... in all cases the above bases would be in their acid forms?

So they're bases yet they're going to be in their acidic forms... if practically every solvent you add them to will turn them into acids, how can they be thought of as bases?
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Ygggdrasil is offline
Dec10-12, 09:26 PM
Other Sci
Sci Advisor
P: 1,342
Compounds that are strong bases are considered strong bases because they will remove protons from other substances. For example, consider the reaction of the base NH2- with a compound like methanol:

NH2- + CH3OH --> NH3 + CH3O-

In reacting with methanol the base NH2- is converted to its conjugate acid NH3. Therefore, a base being converted to its conjugate acid means that the base has done its job: it has removed a proton from another substance and it will comfortably hold on to that proton and not let go of it. Hence, most strong bases will almost always exist as their conjugate acids once you mix that base with any other substance. Similarly, most strong acids will almost always exist as their conjugate bases once you mix the acid with any other substance.

Note that even though the conjugate acid of a strong base may be referred to as an acid, it will not be a very strong acid. These conjugate acids will be very unlikely to donate their protons to other substances. For example, NH3 is a very weak acid and it takes a lot to remove the proton from NH3 to reform NH2-.
shrew is offline
Dec11-12, 01:46 AM
P: 6
Thanks, this was the type of confirmation I needed. :) I understand now, and this may be closed.

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