How do we know which base/acid is stronger?

[Reactive]

I had read that we can compare the strengths of different oxyacids by counting the number of oxygen atoms borne by them, e.g.: HNO3 is stronger than HNO2, likewise H2SO4 is stronger than H2SO3. But what can we say about the comparison of HNO3 with H2SO4? Moreover, shoudn't H2CO3 be a strong acid as its also got 3 oxygen atoms? Kindly post in your explanations Im eagerly awaiting one..

(If one of you can give a fundamental approach to resolve most acid and base strengths, it would be highly appreciated)

[JHAQ]

Would it not be due to the electronegativity of the oxygen inducing higher positivity on the N or S etc which allows easier ionization of H+ ? Surely pH which is log base 10 of the reciprocal of the H+ ion concentration ( g mol / liter ) is the relative measure of acidity ? HF may be an exception due to the extreme electronegativity of the F - entity

[frosch45]

You look at the Ka (acid dissociation constant).

 

This might be helpful to you

 

http://en.wikipedia.org/wiki/Strong_acid

 

Very basically put (and ignoring some other factors), an acid is stronger when it will ionize more completely in water and form the hydronium ion.

[Reactive]

HeY! Look U said that we gotta consider the Ka for determining the strength of an acid or a base..Rite..

But thats wat exactly i wanna know, wat effects the Ka, if the weaker acids have it low then wat is it in them which causes this? I mean if CH3COOH is weak wat makes it so? For HF I know its the electronegative F atom which has a small size.. Smaller the size and lesser wud b the charge dispersion(which it gains from H+). Larger the size more wud b the charg dispersion like in HCl Hence the trend HF<HCl<HBr<HI.. And I also know more the number of O atoms stronger the acid is.. Like H2SO4>H2SO3.. But beyond this there has to be a way a clear cut one, which would give a base reasoning for comparison of any acid with any other, like we have the different concepts which define acids..Arrhenius, Lewis and Lowry-Bronstd. These define all types of acids and bases and mention the exceptions if any!! We say this is based on electronegativity. But how? How does it explain the weakness of H3PO4, H2CO3, H3O+, CH3COOH ; and the strength of H2SO4, HNO3,etc (Electronegativity of O,C,or P?). I guess I made my point clear now that I lookin for some fundamental theory which explains the strength of all acids and bases.. I just dont know where to look for it.. I think I hav written a pretty long thing Well I really need an answer to this..Or even if any of u thinks its kinda irrelevant thing im banging my head into, then also feel free to comment..thanks

[Fuzzwood]

Reactive: paragraphs, look up what that word means and please employ it.

 

In water, the strongest possible acid is always H⁺

[CaptainPanic]

It all has to do with electronegativity, as you already knew. The order of electronegativity is important: F, O, N, Cl, Br, I, S, C, H.

 

The stronger the bond between the H and any other atom, the weaker the acid (takes more energy to break the bond). In HF, the electron pair that makes the bond is practically stuck to F, due to its high electronegativity... so it will not take a lot to completely break the bond, and create a H+ ion, leaving all electrons on the F...

 

With electronegativity, you can reason where electrons will be drawn to. If they are drawn to the H more, it’s a weak acid. If they are drawn to the other atom, then it is a strong acid.

 

Example:

 

Cl3COOH vs. Cl2HCOOH

Please look at the picture below. In the 1st structure, I have filled in with triangles all the electronegativities that you can just fill in straight away if you know the F, O, N, Cl, Br, I, S, C, H list. The big question is: what will the C-C bond do?

 

I must admit that this is a guess, but the direction is in fact irrelevant… so I kept it “neutral” (meaning that the chlorides on one side and the oxygens on the other side pull equally hard).

 

Now, in the 2nd structure, the H on the left side is replaced by another Cl. There are now 3 chloride atoms on the left side. These pull harder on the carbon’s electrons than the two chlorides in the first picture. Therefore, relatively to the 1st structure, the left carbon will be more negative.

 

This also means that the left carbon will start pulling a bit on the right carbon’s electrons.

 

And that in turn means that the oxygen to which the Hydrogen is attached will attract the electrons less… meaning that this time the oxygen is just a little more positive… and it is able to attract the electrons between itself and the hydrogen just a bit harder.

 

This means that in the 2nd structure, the hydrogen should come off easier. And it does.

 

The pKz of both acids:

 

Cl3C-COOH: pKz = 0.70; Kz = 2.0E-1

Cl2HC-COOH: pKz = 1.3; Kz = 5.0E-2

 

p.s. Reactive, I'd appreciate the use of normal font and color, and the use of the enter-button, creating paragraphs. Thanks!

[Reactive]

Well I am glad to read the reply by CaptainPanic as it sounded more logical than any!! Most of my doubts have been clarified..The structural representation was indeed a useful aid in understanding.

 

I would remember the key line: "If electrons are drawn to the H more, it’s a weak acid. If they are drawn to the other atom, then it is a strong acid."

 

I understood the role of electronegativity well, but a slight thing still bothers me : How can we apply the same explanation to H2SO4 and H2CO3, where the former is a strong acid and the latter a weak one?

 

The answer to this last query would make the concept crystal clear! And for the previous one - I am thankful!!

 

I guess my text presentation has improved by this post.. thanks again!

 

 

 

This one's specially for Fswd - I already know the meaning of paragraph:rolleyes:, was just new to the forum so wrote the whole thng in one go!!

 

Well u seem 2 b an old member.. but u cudn't get my ques right!

 

U told me the strongest acid's name (in water)- H+

 

But I was looking for a way to differentiate different acids, not just one name!! Anyways thanks for your concern!!

 

 

Below is a rewritten form of my previous ques, I have tried to put it in a more clear form! I feel it would be easier to understand now.. Please read on:

 

We know that a strong acid has high Ka and a weak acid a low one!

 

My ques was what effects the Ka? That is - if a weaker acid has it low then wat is it which makes it low? I mean if CH3COOH is weak wat makes it so?

 

You must have heard about the different concepts which define acids and bases..Arrhenius, Lewis and Lowry-Bronstd. These define all types of acids and bases and mention the exceptions if any!!

 

I was looking for similar kind of concept which would differentiate between the strength of any acid from the other acid.

 

I got a suitable reply from CaptainPanic, I guess if You understood my ques now U cud give it a TRY!!

Thanks for reading!

Edited August 13, 2008 by Reactive
multiple post merged

[John Cuthber]

The original post poses an interesting question.

Perhaps some more data will explain it better so I'm cheating and copying some directly from Cotton and Wilkinson's advanced inorg chem.

Acid strengths for acids of the form HnXOm

tabulated by m-n

m-n acid -log K

0 HClO 7.5

0 HBrO 8.7

0 H4GeO4 8.6

 

1 H3PO4 2.12

1 H3AsO4 3.5

1 H2SO3 1.9

1 HNO2 3.3

1 H2CO3 6.4

1 HClO2 1.94

 

2 HNO3 large negative value

2 H2SO4 large negative value

 

3 HClO4 very large negative value

3 HMnO4 very large negative value

 

 

There's clearly something about having more oxygens attaced to the central atom that makes these stronger acids- and it's the electronegativity of oxygen that does it.

There's also something about not having many hydrogens to share the "acidity" among that also contributes .

That's why, as Reactive pointed out, H2CO3 should be a lot stronger than it is.

 

It sticks out like a sore thumb.

Now I think I have identified the real question here- why is H2CO3 2 or 3 orders of magnitude weaker than the acids with a comparable structure?

 

The same book gives an answer. The usual given value for the acid strength of carbonate overlooks the fact that CO2 dissolves in water but only part of it hydrates to H2CO3. If you correct for the fact that much of what you expect to be H2CO3 is in fact CO2 in solution, you get a more sensible answer for log K of about 3.6 which is pretty much in the range you expect.

Hope that helps.

[Reactive]

Well now that too makes sense, CO2 from H2CO3 stays in a dissolved form in the aqueous solution.

 

Therefore we can say that H2CO3 doesn't stay in its original form, which is supposed to donate H+ ions. It breaks down to give CO2 in water which is soluble. Hence, lesser the amount of intact H2CO3 lesser ions formed,which makes it a weak acid..

 

I guess I have understood it right? Well thanks for that reasoning.. It really helped!!