pKb and pKa

Acidity
Basicity
The Conjugate Seesaw

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Josue_Marin_3I
Posts: 16
Joined: Wed Sep 21, 2016 2:56 pm

pKb and pKa

Postby Josue_Marin_3I » Fri Dec 02, 2016 12:17 am

I understand that the lower the pKa the stronger the acid is, but why is it that the lower the pKb the stronger the base?

Anmol Dhaliwal 2C
Posts: 25
Joined: Wed Sep 21, 2016 2:57 pm

Re: pKb and pKa

Postby Anmol Dhaliwal 2C » Fri Dec 02, 2016 2:48 am

The weaker bases have a smaller Kb which makes them have a greater pKb (this is the same for Ka and pKa). That means, the smaller the pKb is, the bigger the Kb is and therefore it would be a stronger base. For instance, methylamine has a Kb of 3.6x10^-4 and the pKb is 3.44 (low) which makes it more of a strong base than ammonia which has a Kb of 1.8x10^-5 and a pKb of 4.75 (higher than methylamine's pKb and it is a little bit weaker of a base than methylamine is). Hope this helps!

DBaquero
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Joined: Wed Sep 21, 2016 2:59 pm
Been upvoted: 1 time

Re: pKb and pKa

Postby DBaquero » Fri Dec 02, 2016 7:45 pm

The equilibrium constant (in this case Kb) is larger if the concentration of product (in this case [OH-]) is larger, so it stands to reason that high concentrations of OH- means a stronger base. You take the -log of the Kb to get the pKb which is the same as the log(1/Kb) meaning the smaller the Kb (so the smaller the [OH-], the larger the pKb. So low [OH-] means high pKb and vise versa, meaning the pKb is inversely related to base strength.

Ashley Bertholf 1E
Posts: 10
Joined: Fri Jul 15, 2016 3:00 am

Re: pKb and pKa

Postby Ashley Bertholf 1E » Fri Dec 02, 2016 11:48 pm

When looking at an equation say HCOOH + NaOH =(equilibrium sign) HCOO- + Na+ +H20, how do you know if the reaction priiduces Ka or Kb?


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