Why HCl is a much stronger acid than H2O?

[jaumzaum]

I want to know why HCl is a much stronger acid than H2O.They have similar bond energies (H-Cl = 427 kJ/mol, O-H = 467 kJ/mol), so there should be easy to ionize both. Besides, O is more electronegative than Cl, so it should "steal" the electron easier. Why is HCl Ka so superior than water Kw?

 

[member 392791]

Consider the size of chlorine and oxygen

 

[symbolipoint]

Consider what kind of bond each is. HCl is, at least when dissolved in water, ionic. H-OH is not mostly ionic, so ionizes only weakly.

 

[jaumzaum]

Ok, O-H is more covalent than Cl-H, but what contributes for that? Why is HCl ionic and H2O covalent? Is it the size of O and Cl? F is also very small (smaller than O) and HF is still much stronger than H2O (the bond energy of HF is superior too).

 

[lightarrow]

HCl and HF are stronger acids than H₂O because Cl^- and F^- are much weaker bases than OH^-. Those bond energies you wrote refers only to homolitic dissociation: H-Cl --> H. + Cl. and not to ionic dissociation: H-Cl --> H⁺ + Cl^-. Ionic dissociation energies are much different; looking for them you should find that OH- is much more reactive than Cl- or F- with H₃O⁺, for this reason the dissociation costant of water is much lower.

 

[Borek]

Also, to properly analyze the situation, you have to remember all processes involved take place in water, and ion solvation (hydration) plays crucial role.

 

[lightarrow]

Exactly, thanks for clafication (you have anticipated me ).
Infact ion dissociation is favoured in a polar solvent as water.

 

[epenguin]

Well I think the student should be a bit alert and critical.

It seems to me that #3 and 5 have just restated in different terms the fact that HCl is a much stronger acid than H₂O, not explained it.

Though it is true that homolytic dissociation energies, if that is what the quoted figures are, are not relevant.

 

[sgstudent]

I remember my book stated that we should look at the conjugate base to determine the equilibirum. Even though O is more electronegative than Cl, Cl has a larger volume so the charge density is greater allowing the negative charge to be spread out. So this makes Cl- more stable than OH- as a result at equilibrium there is more Cl- and H3O+ than for water with OH- and H3O+. Furthermore, the HCl bond is weaker than the HO bond further attributing to the equilibrium favouring the forward reaction for HCl than for H2O.

I recall the H in OH- having an effect on the stability of the charge but I can't seem to recall it. Hope this helps.