is hcl a stronger acid than H2SO4. Why?

Answer 1

Amy F you should really reconsider your answer.

1.Bonds can break heterolytically as well, so bond strength is not a measure only of homolytic breaking. Thus the weaker the bond the more easy it is to give H+ and thus the stronger the acid
http://members.aol.com/logan20/acid_str.html

2.The strength of an acid is expressed by the dissociation constant Ka and NOT the valency of the acid. When you are comparing a monoprotic acid with a polyprotic you compare the Ka of the monoprotic with the Ka1 (first dissocation constant) of the polyprotic since that is the greater of the Ka values of the polyprotic acid.
For HCl pKa= -8.0 (where pKa=-logKa)
For H2SO4 pKa1= -3.0
According to wikipedia

The lower the pKa, the higher the Ka and thus the stronger the acid.

Thus HCl appears stronger acid than H2SO4
It is not easy to justify why this is the case since it is difficult to compare 2 acids that are so different.

In fact I would expect the opposite. H2SO4 should be stronger because O is more electronegative than Cl, it is also bound to other electronegative atoms, and there are resonance structures stabilizing the anion.

According to http://www.usm.maine.edu/~newton/Chy251_253/Lectures/AcidBase/pK.html

pKaHCl=-7.0
pKa1H2SO4=-9.0

or pKa1H2SO4=-10 according to http://chemweb.unp.ac.za/chemistry/Physical_Data/pKa_values.htm

Meaning that in this case H2SO4 is STRONGER acid than HCl.
You can read at http://members.aol.com/logan20/acid_str.html
how the strength of the acid is affected by the electronegativity of the atoms and the stability of the conjugate base.

[Edit] To be honest I don't know which source is correct, some sources give pKa1=-3 and others around -9 while always for HCl it is -7 or -8. The strongest acid has the lowest pKa so it is a matter of which source you trust more.

Answer 2

pKa values will show which is stronger:

HCl pKa = -8
H2SO4 pKa = -2
Different authors will give different pKa values, however almost all will show HCl as strong acid with a larger dissociation constant.

Perhaps a better way to determine acid strength is to compare stability of conjugate bases. Acid strength is directly related to conjugate base stability:

HCl + H2O <----> H30+ + Cl-
H2SO4 + H2O <----> H30+ + HSO4-

Now compare Cl- to HSO4-. The more stable conjugate base is Cl- because Cl- is a larger anion than O- (isolating the charge on sulfate anion the charge is placed squarely on an O). The larger, more diffuse the orbital is the better it can accept the charge. Being in the 3rd row, Chlorines valence shell is farther from its nucleus (and positive charge) than 2nd row O.

It should be noted that the sulfate anion is stabilized by another feature, resonance, but this feature is secondary to the size of atom bearing the charge.

Answer 3

HCl is stronger because it completely dissociates into its 2 ions. H2SO4 is slightly weaker because HSO4- can act as a proton acceptor as well, but most of it will dissociate. Thus it is slightly weaker. Also the SO4- makes the H bind more strongly than Cl.

Answer 4

H2SO4 is much stronger than HCl
condensed solution of H2SO4 can react with Ag,Hg,Cu, while HCl can not

Answer 5

Congratulations to Pocket Rocket, who actually seems to know what he's talking about.

For the record, the strength of the H bond does not affect the strength of an acid because bond strength is a measure of how hard it would be to break the bond in the middle, with one electron going to each atom, resuting in two neutral radicals. Acid strength, however, is a measure of how much H+ the acid creates, not how much H.

And whatever unpleasant effects the other half of the molecule may have, the only thing acid strength measures is the H+ concentration. The effects of Cl- have nothing to do with it. And besides, Cl- doesn't burn. It's in table salt for Pete's sake! Bryan probably got it mixed up with chlorine gas, which is poisonous.

Answer 6

Given equal parts of each acid in the same amount of water, the sulfuric acid will be stronger.

Sulfuric acid is stronger since it has twice as many hydrogen ions readily available than just one in HCl.

Answer 7

The weaker the bond to the H atom in any acid, the stronger that acid is.

Answer 8

Yes. because chloride burns