pKa and a solution's charge

[SerenaSabedra]

Hi! Can someone please explain the relationship between pKa and whether or not a solution is charged or neutral? I've never been able to fully wrap my head around the concept.

[Aditya Desai 1A]

For acids of form HA -> H+ + A-

bearing in mind when pH = pKa, protonated = deprotonated (charged = uncharged)
if the pH goes lower, think of it as adding more H+, shifting reaction left, so there is now more protonated (uncharged) form

if pH goes higher, it is like removing H+, shifting reaction right, now there is more deprotonated (charged) form

[Kimia Rategh 2A]

Hi! So there is no direct relationship between pka and whether the reaction favors the charged or uncharged particle. Rather, when comparing the pka to the pH we are indicating which direction the reaction will shift towards. If the pka is greater than the pH, then the production of the acid will be favored. Depending on whether or not the reaction is an acid (uncharged) reaction with water or if a base reacts with water to produce a conjugate acid (charged), then you would tend to the uncharged or charged particle respectively. If the pka is less than the pH, the production of the base (either conjugate or not) is favored. So if the reaction is a base reacting with water (uncharged), or an acid reacting with water to provide a conjugate base (charged), the reaction where the pka is less than the pH would result in the favoring of either the uncharged or charged particle respectively.

Hope this helps!