Weak acids/bases + salts

[Jepstein30]

Reviewing the Acid/Base chapter in TBR and noticed they don't really cover this topic.

From the official AAMC topic list:

5a. dissociation of weak acids and bases with or without added salt
5b. hydrolysis of salts of weak acids or bases

Can someone please give me a quick rundown of this topic or point me towards the right resource? Been awhile since Gen Chem and am not sure how a salt would impact dissociation (adding NaCl would cause Na+ and Cl+ to dissociate into solution which would reduce the concentration of both H+ and OH-.. but wouldn't those effects cancel eachother out? so unless you have a salt that is comprised of an ion that will react with H+ and OH- and one that won't.. not sure where to go with this.)

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[Jepstein30]

Bump!

Anyone??

 

[asf503]

Reviewing the Acid/Base chapter in TBR and noticed they don't really cover this topic.

From the official AAMC topic list:

5a. dissociation of weak acids and bases with or without added salt
5b. hydrolysis of salts of weak acids or bases

Can someone please give me a quick rundown of this topic or point me towards the right resource? Been awhile since Gen Chem and am not sure how a salt would impact dissociation (adding NaCl would cause Na+ and Cl+ to dissociate into solution which would reduce the concentration of both H+ and OH-.. but wouldn't those effects cancel eachother out? so unless you have a salt that is comprised of an ion that will react with H+ and OH- and one that won't.. not sure where to go with this.)

Hint 1: Studying buffer solutions and Le Chatlier's will make the topic clear.

Hint 2: Observe the spontaneous dissociation of H2CO3: H2CO3 --> H+ + HCO3-
You add NaHCO3 to this solution. How would you predict the direction equilibrium would shift?

 

[Jepstein30]

Hint 1: Studying buffer solutions and Le Chatlier's will make the topic clear.

Hint 2: Observe the spontaneous dissociation of HCl in water: HCl + H2O --> H30+ + Cl-
You add NaCl to this solution. How would you predict the direction equilibrium would shift? I know HCl is not a weak acid, but I'm sure you get the point.

Ah okay, so really its just an application of Le Chatlier's principle.

Cl increases so reverse rxc is favored leading to less H+ in solution so pH increases. Wouldn't that also be balanced out, however, with Na picking up OH- in equivalent quantities?

So when we are told about adding a salt to a solution and asked what the effect on the pH would be, we need to find the complex equilibrium?

 

[asf503]

Ah okay, so really its just an application of Le Chatlier's principle.

Cl increases so reverse rxc is favored leading to less H+ in solution so pH increases. Wouldn't that also be balanced out, however, with Na picking up OH- in equivalent quantities?

So when we are told about adding a salt to a solution and asked what the effect on the pH would be, we need to find the complex equilibrium?

No no no no no. I think we need to focus on the what solubilization is:

So:

NaCl has high solubility in water. Solubilizing a compound does not break up the solvent- that is, Na+ does NOT break up H20 to form NaOH and H+ in this reaction

THIS DOES NOT HAPPEN
NaCl+ H2O --> NaOH + HCl

What Solubilizing a compound does is break that compound up and solvate it- that is, surround it- with the solvent molecules. So NaCl --> Na+ + Cl- and...

The Na+ is surrounded by the oxygen side (the more electorneg. side) of water. The Cl- is surrounded by the hydrogen side (the less electroneg. side) of water. The covalent bonds of water are not broken in this process

So when you add NaHCO3 to a solution containing H2CO3, the pH SHOULD go up as Le Chatlier's dictates. The NaHCO3 dissolves and HCO3- is released into the solution, driving the reaction backwards.

Does that help? Or did I just confuse you?

Again, you should really study buffers. 100% guarantee you that you'll get at least 1 question on the MCAT about them.

 

[blesss]

So when you add NaCl to a solution containing HCl, the pH SHOULD go up as Le Chatlier's dictates. The NaCl dissolves and Cl- is released into the solution, driving the reaction backwards.

Does that help? Or did I just confuse you?

Again, you should really study buffers. 100% guarantee you that you'll get at least 1 question on the MCAT about them.

.

Edit - Disregard my 2am ramblings.

 

[Jepstein30]

No no no no no. I think we need to focus on the what solubilization is:

So:

NaCl has high solubility in water. Solubilizing a compound does not break up the solvent- that is, Na+ does NOT break up H20 to form NaOH and H+ in this reaction

THIS DOES NOT HAPPEN
NaCl+ H2O --> NaOH + HCl

What Solubilizing a compound does is break that compound up and solvate it- that is, surround it- with the solvent molecules. So NaCl --> Na+ + Cl- and...

The Na+ is surrounded by the oxygen side (the more electorneg. side) of water. The Cl- is surrounded by the hydrogen side (the less electroneg. side) of water. The covalent bonds of water are not broken in this process

So when you add NaCl to a solution containing HCl, the pH SHOULD go up as Le Chatlier's dictates. The NaCl dissolves and Cl- is released into the solution, driving the reaction backwards.

Does that help? Or did I just confuse you?

Again, you should really study buffers. 100% guarantee you that you'll get at least 1 question on the MCAT about them.

Didn't get to buffers yet, but just found a few paragraphs on salts and acidity in EK.

According to the book (at least how I understand it) adding NaCl wouldn't change the pH of the solution at all. So now I'm a little confused... i'll think out loud here and hopefully you can correct me if I make a mistake!

Na and Cl are the conjugate pairs for strong base NaOH and strong acid HCl. Therefore, since strong acids and bases completely dissociate in water, you would not expect addition of Na OR Cl to drive the formation of NaOH and HCl.. because the products would dissociate regardless. = no change in pH.

Better example would be NH4NO3.
NO3- conjugate pair of HNO3 = strong acid so no effect on pH
NH4+ conjugate pair of NH3 = weak base, doesn't completely dissociate
NH4+ <-> NH3 + H+
Addition of NH4+ would drive reaction to the right leading to decrease in pH, salt overall would be weakly acidic

Is that not all correct?

 

[Jepstein30]

So when you add NaCl to a solution containing HCl, the pH SHOULD go up as Le Chatlier's dictates. The NaCl dissolves and Cl- is released into the solution, driving the reaction backwards.

Does that help? Or did I just confuse you?

Again, you should really study buffers. 100% guarantee you that you'll get at least 1 question on the MCAT about them.

Wait.. wouldn't it all dissociate into H+ and Cl-? So kind of irrelevant?

Also don't get how you get the pH to increase according to Le Chatlier's.. the only way NaCl would increase the pH is if it reacts with H+ to form HCl.. but HCl would dissociate since it is a strong acid so back to square one and just increased concentration of Cl-.

 

[asf503]

According to the book (at least how I understand it) adding NaCl wouldn't change the pH of the solution at all. So now I'm a little confused... i'll think out loud here and hopefully you can correct me if I make a mistake!

No you're totally right, it was my mistake to continue using my poorly written example. As it is a strong acid, adding NaCl really wouldn't have an affect on increasing the pH. I apologize for using that example and I can see how it's confusing...I think I mentioned up there that I knew that HCl was NOT a weak acid but it was just to prove a point. I originally introduced that example on the fly and it was not a very good one.

For a strong acid, like HCl or H2SO4, introducing their conjugate base would do nothing since it would just dissociate again.

Let's do something MUCH more straight forward, which is Carbonic Acid, H2CO3. It is a weak acid, so not all of it dissociates.



H2CO3 --> H+ + HCO3-

When you add it's conjugate base as a salt- say you add NaHCO3 - that salt will dissociate completely (Alkali earth metals dissociate very well in water) and drive the reaction in the reverse.


Again, I'm sorry for continuing to use that poor example that I brought up in my first post.

Are things a little more clear? Or am I really screwing this up?


Note: I fixed my original post in case any other readers get confused

 

[Jepstein30]

No you're totally right, it was my mistake to continue using my poorly written example. As it is a strong acid, adding NaCl really wouldn't have an affect on increasing the pH. I apologize for using that example and I can see how it's confusing...I think I mentioned up there that I knew that HCl was NOT a weak acid but it was just to prove a point. I originally introduced that example on the fly and it was not a very good one.

For a strong acid, like HCl or H2SO4, introducing their conjugate base would do nothing since it would just dissociate again.

Let's do something MUCH more straight forward, which is Carbonic Acid, H2CO3. It is a weak acid, so not all of it dissociates.

H2CO3 --> H+ + HCO3-

When you add it's conjugate base as a salt- say you add NaCO - that salt will dissociate completely (Alkali earth metals dissociate very well in water) and drive the reaction in the reverse.

Again, I'm sorry for continuing to use that poor example that I brought up in my first post.

Are things a little more clear? Or am I really screwing this up?

Yea, I think I understand it now! Thanks for the additional example. Except the salt would have to be NaHCO3 (probably just a typo), right?

So again.. NaHCO3 would dissociate to Na+ and HCO3-.

Na would form NaOH = strong base, dissociates, no effect
HCO3- conjugate pair is H2CO3 = weak acid, only partially dissociates.. therefore, the salt is weakly basic. More HCO3- means more H+ consumed = pH increases.

Weird that TBR doesn't even talk about this.. I understand it's not that intense of a topic and you can break it down when considering the relevant concepts but still something I would be staring blankly at if it were not for this thread.

 

[asf503]

Yea, I think I understand it now! Thanks for the additional example. Except the salt would have to be NaHCO3 (probably just a typo), right?

So again.. NaHCO3 would dissociate to Na+ and HCO3-.

Na would form NaOH = strong base, dissociates, no effect
HCO3- conjugate pair is H2CO3 = weak acid, only partially dissociates.. therefore, the salt is weakly basic. More HCO3- means more H+ consumed = pH increases.

Weird that TBR doesn't even talk about this.. I understand it's not that intense of a topic and you can break it down when considering the relevant concepts but still something I would be staring blankly at if it were not for this thread.

Yes, it was a simple typo. I'm sorry I was trying to reply to your post while a game of LoL was loading in the background =P

 

[asf503]

Better example would be NH4NO3.
NO3- conjugate pair of HNO3 = strong acid so no effect on pH
NH4+ conjugate pair of NH3 = weak base, doesn't completely dissociate
NH4+ <-> NH3 + H+
Addition of NH4+ would drive reaction to the right leading to decrease in pH, salt overall would be weakly acidic

Is that not all correct?

NH3 is a weak base due to its pair of unshared electrons. NH4+ is the conjugate acid of NH3, so technically NH4+ is acidic. Thus the ammonium (NH4+) salt is indeed weakly acidic.

 

[asf503]

Yea, I think I understand it now! Thanks for the additional example. Except the salt would have to be NaHCO3 (probably just a typo), right?

So again.. NaHCO3 would dissociate to Na+ and HCO3-.

Na would form NaOH = strong base, dissociates, no effect
HCO3- conjugate pair is H2CO3 = weak acid, only partially dissociates.. therefore, the salt is weakly basic. More HCO3- means more H+ consumed = pH increases.

Weird that TBR doesn't even talk about this.. I understand it's not that intense of a topic and you can break it down when considering the relevant concepts but still something I would be staring blankly at if it were not for this thread.

Na+ does not form NaOH and then dissociate out, Na+ simply becomes surrounded by water molecules and chills out. It's important to know that solvating a salt does not break the solvent bonds- that's what the picture up there is representing.

I don't have much experience with TBR I used EK and Kaplan, but maybe you'll run into some practice problems down the line about this. Anyways, I'm glad to have been able to help you understand this, even if halfway through I confused the hell outta ya first. Let me know if you're still confused.

 

[Jepstein30]

NH3 is a weak base due to its pair of unshared electrons. NH4+ is the conjugate acid of NH3, so technically NH4+ is acidic. Thus the ammonium (NH4+) salt is indeed weakly acidic.

Great! And yea I meant NH3 is the weak base, not NH4+.

Na+ does not form NaOH and then dissociate out, Na+ simply becomes surrounded by water molecules and chills out. It's important to know that solvating a salt does not break the solvent bonds- that's what the picture up there is representing.

I don't have much experience with TBR I used EK and Kaplan, but maybe you'll run into some practice problems down the line about this. Anyways, I'm glad to have been able to help you understand this, even if halfway through I confused the hell outta ya first. Let me know if you're still confused.

Oh, good distinction. I had figured it wouldn't matter whether it formed or not because it would dissociate but shouldn't think of it that way.

Thanks again.. another few items knocked off the topic list!

 

[Moxess]

TBR actually does mention this kind of thing, but it's in the Gen Chem chapter on Equilibrium, not Acids and Bases.

 

[Jepstein30]

TBR actually does mention this kind of thing, but it's in the Gen Chem chapter on Equilibrium, not Acids and Bases.

Didn't notice it there, either. I do 2 chapters of each subject at at time so read those on the same day. Could have skipped or skimmed though.