# Two simple questions

1. Apr 6, 2005

### bomba923

Which of the strong acids: $$HCl , HClO_4, HNO_3, H_2 SO_4$$ is THE strongest?

HCl is a strong acid. HF is weaker (high bond energy of H-F single bond). Why then, are HBr and HI weaker than HCl as well? I mean, there is a lower bond energy between hydrogen and heavier halides (less electronegative than lighter halides) and much greater electron density away from the acidic hydrogen with heavier halides. Why then, is HCl the strongest hydrohalic acid? (does it satisfy a certain balance between bond energy and some other factors?)

Last edited: Apr 6, 2005
2. Apr 6, 2005

### pack_rat2

The correct order of strength for the acids listed is:
HI > HBr > HCl > HNO3 > H2SO4 >> HF

3. Apr 6, 2005

### bomba923

Hmm--isn't perchloric acid a strong acid?

So HI and HBr are stronger acids than HCl?

4. Apr 6, 2005

### pack_rat2

Oooops, I forgot to put it in. HClO4 is the strongest one there. The strength of the hydrohalic acids increases with the size of the anion. That makes sense--a bigger atom means less tightly bound electrons in the outer shell, a weaker bond and higher tendency to dissociate. HF is much less strong than the other, because of the small size of F.

5. Apr 6, 2005

### bomba923

So basically, there are really six strong acids: $$HCl , HClO_4, HNO_3 , H_2 SO_4 , HBr , HI$$

Right?

6. Apr 6, 2005

### dextercioby

That's right.As a general rule,monoprotic acids are the strongest.U missed the permanganic...

Daniel.

7. Apr 7, 2005

### bomba923

Seven strong acids then: $$HCl , HClO_4, HNO_3 , H_2 SO_4 , HBr , HI , HMnO_4 {?}$$

Interesting. My chemistry book (Zumdahl, 4th Ed) mentions only four strong acids: $$HCl , HClO_4, HNO_3 , H_2 SO_4$$ . And I used to think HCl was the strongest hydrohalic acid, until now: when I see the HBr and HI as well.

Last edited: Apr 7, 2005
8. Apr 7, 2005

### dextercioby

Chromic acid has the same (electronic) structure of chemical bonds as the sulphuric one,but only checking into a table for first acidity constants would tell you if you can add it to your list...

Chlorsulphonic acid is strong,too.

Daniel.

9. Apr 7, 2005

### Staff: Mentor

Some of pKa values for strong acids are here:

http://www.chembuddycom/?left=BATE&right=dissociation_constants [Broken]

and some are here:

pKa for HI = 10
pKa for HBr = 9

The most important remark I have is the fact that sulfuric acid is strong only if we are talking about first dissociation step. Second is a rather weak one, with pKa = 2.

Chemical calculators for labs and education
BATE - pH calculations, titration curves

Last edited by a moderator: May 2, 2017
10. Apr 7, 2005

### GCT

Usually the first element in any group is an exception in regards to the chemical trends pertaining to that group.

acidity is related to two factors (perhpas three, one which I do not know of); the polairity or the degree of ionic character between the electronegative atom and hydrogen (that is the more hydrogen resembles the its cation in transition state) and the strength of the bond between the two.

From what I remember, HF is an exception, in that while there is the greatest degree of ionic character, it is outweighed by the strength of the bond.

11. Apr 7, 2005

### pack_rat2

Of course, not in water solution; it decomposes into HCl & H2SO4. Do a web search on "magic acid"! Also, the haloacetic acids (like trichloroacetic acid, Cl3CCOOH) are pretty strong, too.