Data on Acid Strength (Also Diprotic Acids argh help me)

Does anyone know where I can find some legit data on acid strength in terms of pKa? Every source that I've looked at is different! I just want to know if HCl is stronger than H₂SO₄ and what their specific pKa values (some of the tables rank hydrochloric as stronger than sulphuric and vice versa). In addition, some tables list two values for sulphuric... Is this to do with the acid being diprotic? (and does being diprotic make it stronger? :confused:

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Thanks so much for any help!!!!

Reply 1

Plato's Trousers

Original post by BioIz

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Thanks so much for any help!!!!

This is the table I use most often. It rates HCl as a lot stronger than H₂SO₄ (HCl has a K_a 100,000 greater than the first K_a of H₂SO₄)

http://evans.harvard.edu/pdf/evans_pka_table.pdf

There is a Ka for each ionisable proton, so a diprotic acid will have two Ka values. In the case of sulphuric acid, they refer to the processes

K_{a1}:\;\; H_2SO_4 \rightleftharpoons H^+ + HSO_4^-

and

K_{a2}:\;\; HSO_4^- \rightleftharpoons H^+ + SO_4^{2-}

Obviously, K_a1 > K_a2

(edited 11 years ago)

Reply 2

illusionz

The conflicting opinion on the net may be because conc. sulfuric acid is the pure stuff, whereas conc. HCl is only about 40% solution, in water. Conc. sulfuric acid is about 18M (and it's dibasic), whereas conc. HCl is only about 12M (and monobasic).

Edited for clarity

(edited 11 years ago)

Reply 3

Plato's Trousers

Original post by illusionz

The conflicting data may be because conc. sulfuric acid is the pure stuff, whereas conc. HCl is only about 40% solution, in water. Conc. sulfuric acid is about 18M (and it's dibasic), whereas conc. HCl is only about 12M (and monobasic).

No. K_a is independent of concentration. It is a constant for a given acid in a given solvent

Reply 4

illusionz

Original post by Plato's Trousers

No. K_a is independent of concentration. It is a constant for a given acid in a given solvent

I am well aware of what K_a is, although perhaps I didn't phrase myself very well. What I meant was that I anticipate that a source of confusion amont the denizens of the internet could be the fact that while HCl is the stronger acid, conc. sulfuric has 3 times the concentration of H⁺ ions than conc. HCl

Reply 5

BioIz

Original post by Plato's Trousers

K_{a1}:\;\; H_2SO_4 \rightleftharpoons H^+ + HSO_4^-

and

K_{a2}:\;\; HSO_4^- \rightleftharpoons H^+ + SO_4^{2-}

Obviously, K_a1 > K_a2

thanks for that, so should I use the first value for sulphuric (i.e. when the first H+ gets pulled off?)

Reply 6

Plato's Trousers

Original post by illusionz

Apologies. I misunderstood you.

Original post by BioIz

thanks for that, so should I use the first value for sulphuric (i.e. when the first H+ gets pulled off?)

Well it depends what question you're trying to answer.

But if you want some comparison between the two, it would be reasonable to use the first value, yes (as it means it's at least that strong)

(edited 11 years ago)

Reply 7

Big-Daddy

Original post by illusionz

Surely there is no fair way to compare them at all, considering that conc sulphuric acid is not even dissolved in the same solvent?

Would there be potential for the Ka values of sulphuric acid to change if the solvent is changed?

Reply 8

Plato's Trousers

Original post by Big-Daddy

Surely there is no fair way to compare them at all, considering that conc sulphuric acid is not even dissolved in the same solvent?

Eh? Sure you can compare them. On the table I linked to, the pKa values all refer to aqueous solution. (Apart from the ones in brackets which are DMSO)

Original post by Big-Daddy

Would there be potential for the Ka values of sulphuric acid to change if the solvent is changed?

Yes, as I said, pKa depends on the solvent

(edited 11 years ago)

Reply 9

Big-Daddy

Original post by Plato's Trousers

Eh? Sure you can compare them. On the table I linked to, the pKa values all refer to aqueous solution. (Apart from the ones in brackets which are DMSO)

Yes, as I said, pKa depends on the solvent

No, what I meant was that you can't compare concentrated H₂SO₄ with concentrated HCl, as the latter is still in aqueous solution whereas the former is in a different solvent.

To the OP: with reference to general solution, a good rule of thumb is probably that in a concentrated solution HCl will be stronger than H2SO4 (concentrated aqueous solution of either, is what I mean now) but in a solution of medium/low concentration H2SO4 will be stronger. By stronger I mean produce more H+ ions. Which is not the normal definition of "stronger": generally a stronger acid is one with a greater Ka value, to be completely plain.

(edited 11 years ago)

Reply 10

Plato's Trousers

Original post by Big-Daddy

No, what I meant was that you can't compare concentrated H₂SO₄ with concentrated HCl, as the latter is still in aqueous solution whereas the former is in a different solvent.

Please don't confuse the OP. What do you mean by conc. sulfuric acid being in a different solvent? Why would you put it in a different solvent? (are you maybe confusing it with "pure" sulfuric acid? Which is in fact a pure liquid not an aqueous solution). As I have said twice before now, we are talking about aqueous solutions. That is the only meaningful way to compare them, it is the table of pKa I provided and it is what the OP was asking for!

(edited 11 years ago)

Reply 11

BioIz

Original post by Plato's Trousers

Apologies. I misunderstood you.

Well it depends what question you're trying to answer.

But if you want some comparison between the two, it would be reasonable to use the first value, yes (as it means it's at least that strong)

Original post by Big-Daddy

Will try and get my head round this, I'm too sure about what the difference between mono and diprotic means in terms of strength... I'm just trying.to understand why my reaction with Mg occurred faster with sulphuric acid than with hydrochloric acid at the same temperature /:

Reply 12

Plato's Trousers

Original post by BioIz

That will be because at the same concentrations of each acid, sulfuric acid will produce twice the quantity of protons (as it's diprotic) compared to hydrochloric acid. The different degrees of dissociation isn't the issue here as the magnesium is constantly going to be using up protons (reducing them to hydrogen) in either case, and so pulling the equilibrium over to the right.

(edited 11 years ago)

Reply 13

Big-Daddy

Original post by Plato's Trousers

Please don't confuse the OP. What do you mean by conc. sulfuric acid being in a different solvent? Why would you put it in a different solvent? (are you maybe confusing it with "pure" sulfuric acid? Which is in fact a pure liquid not an aqueous solution). As I have said twice before now, we are talking about aqueous solutions. That is the only meaningful way to compare them, it is the table of pKa I provided and it is what the OP was asking for!

I am talking about H₂SO₄ (l). So yes, pure sulphuric acid, though I thought it could be referred to as concentrated sulphuric acid too?

Reply 14

illusionz

Original post by Big-Daddy

I am talking about H₂SO₄ (l). So yes, pure sulphuric acid, though I thought it could be referred to as concentrated sulphuric acid too?

My comparison is because that is the most concentrated form each acid comes in. Many people on the internet (just go look at yahoo answers) confuse strength and concentration, which is why I made my point.

Conc. sulfuric acid is 36N (ie the concentration of hydrogen ions is 36M), whereas conc. HCl is 12N (12M hydrogen ions). When a chemist says conc. sulfuric acid, they mean sulfuric acid (l).

Perhaps I am confusing you all :biggrin:

I merely tried to point out the difference in amount of hydrogen ions each solution provides, as there are many people who will then say that sulfuric is a 'stronger' acid because of this - as a potential source of the OP finding conflicting opinion ont he web.

(edited 11 years ago)

Reply 15

Plato's Trousers

Original post by illusionz

Granted. I completely accept your point and think it may well be right as an explanation of the OP's confusion.

I was just getting a bit concerned we were piling in a lot more info than the OP necessarily wanted/needed. Esp Big Daddy's introduction of "different solvents" which I didn't think was particularly helpful.

I don't think "pure" sulfuric acid should be referred to as "concentrated" sulfuric acid. The former has a precise meaning, the latter does not (for example, in a lab, most people would consider 5N sulfuric acid to be "concentrated").

But no prob. I think (hope) the OP is clear now.

(edited 11 years ago)

Reply 16

illusionz

Original post by Plato's Trousers

I don't think "pure" sulfuric acid should be referred to as "concentrated" sulfuric acid. The former has a precise meaning, the latter does not (for example, in a lab, most people would consider 5N sulfuric acid to be "concentrated").

While I agree there is inconsistency between what 'concentrated' means when considering different acids, no organic chemist would consider 5N sulfuric acid to be concentrated. In the literature, if a procedure states the use of conc. sulfuric acid, they mean 98% (or thereabouts).