Confusion about Ph>Pka

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Ada Chung 1C
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Joined: Sat Aug 17, 2019 12:15 am

Confusion about Ph>Pka

Postby Ada Chung 1C » Tue Dec 03, 2019 9:55 am

In class, we talked about how the acid is neutral or negatively charged and I was confused as to the whole Ph>Pkka and how the solution would be more Alkaline and is thus more negatively charged. I'd like to know the reasoning behind this.

Joseph Saba
Posts: 154
Joined: Thu Jul 11, 2019 12:16 am

Re: Confusion about Ph>Pka

Postby Joseph Saba » Tue Dec 03, 2019 10:01 am

I am also confused about this. In my notes I have that "pH>Pka: the solution is more alkaline and the acid will give off H+ to form A- and is negatively charged." I'm not completely sure what the application of this means though.

Sebastian Lee 1L
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Re: Confusion about Ph>Pka

Postby Sebastian Lee 1L » Tue Dec 03, 2019 10:15 am

What Dr. Lavelle was talking about was how much an acid will dissociate depending on the acidity of the solution it is in. Remember that for Bronsted acids, the acid in solution can dissociate into hydronium ions (H3O+) and the conjugate base (A-). For example, . When we say that the acid is negatively charged in solution, that means that it dissociates into the negative anion and hydronium ions rather than staying bonded together.

If the pH of the solution is greater than the pKa of the acid, that means that the solution is more alkaline/basic than the acid. In this case, the acid will dissociate into its ions, making the acid more negatively charged. Basically, the acid will not stay HA but will become H+ and A- if the solution is more basic. This is because a more basic solution is likely to accept donated protons from the acid. For example, if you have a HF with a pKa of 3 and H2O with a pH of 7, the water is going to accept some of the H+ from HF to form H3O+ and you will then get F- once HF donates the proton.

However, if the pH is lower than the pKa (the solution is more acidic than the acid itself), then the acid will not dissociate into the ions. This happens because the solution's ions are unlikely to accept any protons from the new acid since it is already stable as a dissociated stronger acid solution. The acid isn't strong enough to donate protons in that solution.

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