What does "acid dissociation decreases with acid concentration" mean?

[whyamisopretty]

Hey guys, I would be eternally grateful if someone could help me understand this--

From EK Chem p.95:

"Acid dissociation decreases with acid concentration but acid strength increases with acid concentration."

Is this saying when acid concentration increases, acid strength increase and acid dissociation increases? Is acid concentration proportional to the acid dissociation and acid strength? The phrasing of this sentence is very ambiguous and I don't understand it.

"Increasing the concentration of a weak acid by a factor of ten does not result in a tenfold increase in H+ concentration." Why is this the case?

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[amar21]

thats pretty odd!

if you related everything to the Ka it can make sense.. (at least how I see it)

Ka = ([H+][A-]/[HA])

So if we decrease H+ (acid concentration) our Ka is also going to be lower.. which means less dissociates. Also, acid strength is simply based on the concentration of [H+]... so increasing that would mean a stronger acid.

The second part - im not entirely sure. But if its a weak acid, that means it has a stronger conjugate base. So if we increase the acid tenfold we would essentially be increasing the basicity 10fold... I THINK <<-- dont take my word on that, im not very strong on this unit as I have not studied it thoroughly!

 

[whyamisopretty]

Hey, thanks Amar. I understand it a little better now, but the phrasing was just weird..

What made it even more confusing is that they had a specific example about how increasing the % dissociation does NOT mean a stronger acid. Arrrgh. I just hope I don't get a passage on this...but this is a bad way to walk into the MCAT.

thats pretty odd!

if you related everything to the Ka it can make sense.. (at least how I see it)

Ka = ([H+][A-]/[HA])

So if we decrease H+ (acid concentration) our Ka is also going to be lower.. which means less dissociates. Also, acid strength is simply based on the concentration of [H+]... so increasing that would mean a stronger acid.

The second part - im not entirely sure. But if its a weak acid, that means it has a stronger conjugate base. So if we increase the acid tenfold we would essentially be increasing the basicity 10fold... I THINK <<-- dont take my word on that, im not very strong on this unit as I have not studied it thoroughly!

 

[Quantum Chem]

Acid dissociation refers to the percentage of dissociation. Take for example strong and weak acids. Strong acids, HI, fully dissociate which means its dissociation is 100%. Weaker acids don't dissociate all the way so their percent of dissociation is less than 100%. What the book is saying is that as the concentration goes up the percent dissociation for the same amount of volume of water decreases. But at the same time the pH does decrease because the amount of H+ does increase, but it isn't linear with the increase of concentration.

 

[amar21]

do you mind posting that example?

 

[whyamisopretty]

Sure. 🙂

"Imagine the following: I have 100 acid molecules in water and 50 dissociate, so that I have 50% dissociation and 50 H+ ions. If I have 1000 acid molecules in the same amount of water, now I have 400 dissociate so that i have 40% dissociation and 400 H+ ions. More H+ in the same amount of water means more acid strength. Notice that this means that increasing the concentration of a weak acid by a factor of 10 does NOT result in a tenfold increase in H+ ion concentration."

 

[whyamisopretty]

Acid dissociation refers to the percentage of dissociation. Take for example strong and weak acids. Strong acids, HI, fully dissociate which means its dissociation is 100%. Weaker acids don't dissociate all the way so their percent of dissociation is less than 100%. What the book is saying is that as the concentration goes up the percent dissociation for the same amount of volume of water decreases. But at the same time the pH does increase because the amount of H+ does increase, but it isn't linear with the increase of concentration.

Hi Quantum, did you mean pH decreases?

 

[Quantum Chem]

Yeah, sorry. I changed it to be correct. I should really read my posts before submitting.

 

[engineeredout]

Woah this is screwy. So if I have the dissociation rxn HCl -> H+ + Cl-, even though lechatlier's principal says adding acid will shift the Equilibrium to the right, there will be a decrease in dissociation %?

 

[amar21]

Woah this is screwy. So if I have the dissociation rxn HCl -> H+ + Cl-, even though lechatlier's principal says adding acid will shift the Equilibrium to the right, there will be a decrease in dissociation %?

you are asking a different question now..

In the question asked above, we are assumming that we are reacting HCL with water and having a complete 100% dissociation. HCL --> H+ + Cl-.

Also, according to Le Chateliers principle .. if you added more [H+], would that not mean a shift to the left?

 

[Lukkie]

Acid dissociation decreases with acid concentration but acid strength increases with acid concentration.

EK is just wording the question a little vaguely but if you understand the concept you should have no problem. what they mean by 'acid concentration' is how much H+ is in the solution. thats how you tell how acidic something is - the strength of the acid (acid being the solution).

now if you put some acid in there, like H-Cl, what happens if the solutions already has lots of H+? equilibrium shifts to the left - decreased dissociation.

EK's ambiguity stems from this - what is the acid? is it the solution? or is it the stuff we put in? THEY ARE BOTH ACIDS MR. ORSAY, BE MORE SPECIFIC NEXT TIME!

 

[kehlsh]

Wow I just looked at this and went... wtf is he saying?

 

[ilovemcat]

Wow I just looked at this and went... wtf is he saying?

This doesn't make sense to me at all. The amount of Acid that dissociates is dependent on the Acid Dissociation Constant (Ka) for a given Acid. The concentration of an acid affects it's pH value (Proton Concentration) but it never affects the dissociation of that acid. A strong acid will always dissociate completely and a weak acid will only partially dissociate. I read this in EK and didn't understand it either ... and I still don't understand it, nor do I agree with it.

 

[ilovemcat]

thats pretty odd!

if you related everything to the Ka it can make sense.. (at least how I see it)

Ka = ([H+][A-]/[HA])

So if we decrease H+ (acid concentration) our Ka is also going to be lower.. which means less dissociates. Also, acid strength is simply based on the concentration of [H+]... so increasing that would mean a stronger acid.

The second part - im not entirely sure. But if its a weak acid, that means it has a stronger conjugate base. So if we increase the acid tenfold we would essentially be increasing the basicity 10fold... I THINK <<-- dont take my word on that, im not very strong on this unit as I have not studied it thoroughly!

- Weak Acids can have a Weak Conjugate Base
- As the Acid gets weaker, the conjugate Base gets stronger.

Decreasing the amount of H+ increases the pH of a solution. Ka is a constant always. However, weaker acids that dissociate partially release only a small amount of Hydrogen. The weaker the acid gets, the less hydrogen that is donated. If you compare different acids, the percent dissociation changes (ie, strong acids dissociate 100%, weaker acids dissociate less than that) But for each scenario and for each acid, Ka is a constant independent of the concentration of a solution.

 

[loveoforganic]

Consider the extreme case. If you have a liter of sulfuric acid and add a drop of water (bad idea, beside the point), is all that acid going to dissociate?