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Name: Unknown
Status:  student
Age:  30s
Location: N/A
Country: N/A
Date: 2000


Question:
Hi I wonder why a proton (ion) from hydrogen in HCl leaves and moves to the water molecule's (H2O) free electronpair and becomes a H3O-ion, because the HCl-molecule already has 8 electrones in it's outer shell,it should be stable. Is it because the oxide nuclei has less electronegativity than the chloride nuclei or has it something to do with the bindings between the hydrogen- and oxide atoms in water H2O molecule and the binding between hydrogen- and chloride atoms. I don't understand why the proton leaves and how it can, and when H2O takes the proton it builds a H3O-ion. Isn't it the whole hydrogen nuclei that moves and leaves an electron for the acid HCl.

Please help me and give me a well explained answer because I have trouble to understand the chemistry. I have already asked the teachers but they don't know it either.

I'm from Sweden so I'm sorry about my English but I hope you can understand what I mean.


Replies:
Your question is not at all trivial. First, we know HCl, HBr, and HI in reasonably dilute solution are all essentially completely ionized. We know this because the electrical conductance of dilute solutions of equal molar concentrations of these acids is very high, and almost identical. The explanation for this is that the (H3O)+ cation is the primary charge carrier in each case. THOSE ARE THE FACTS. Now how do we "explain" the facts. The high conductivity is attributed to the rapid "passing around" of the proton from water molecule to water molecule. Every other ion must diffuse, and this is a much slower process.

A neat BUT INCORRECT explanation is similar to what you suggested in your question. The halide ion is very electronegative and captures the electron it shares with the H atom producing a (H3O)+ and a halide ion with a completely closed shell - 1s2,2s2,2p6 and so on. And it leads exactly to the confusion you expressed.

The degree of ionization of an acid, ANY ACID, is dominated by its interaction with the solvent, water. These solvation effects are large, complex, not well understood, and cannot be "explained" by simple bond strength or filled orbital arguments you proposed -- it would be nice if it were true, but it isn't.

The strength of an acid ionizing in dilute aqueous solution according to the reaction:
      HA  =  H+     +    A-

is characterized by an equilibrium constant: Keq = (H+)*(A-)/(HA), which is defined in terms of a quantity called the standard free energy of reaction: delta(G). This can be divided into two terms: delta(G) = delta(H) - T*delta(S) T = temperature in K. where the delta(H) term accounts for the energy difference between the reaction products and reactants, and the T*delta(S) term accounts for the relative freedom of motion of the products compared to the reactants. This term is dominated by the relative solvation of the products and reactants, for which no simple model exists.

If you look at the data for some organic acids, such as acetic acid, what you find is that this second term SWAMPS the first in size. In fact, a good rule of thumb is that the delta(H) term for carboxylic acids is zero +/- 2 kcal/mol!!!! And the trends in acidity have no relation to simple structural trends.

So back to HCl. The effects of electron configuration, to the extent they are present, are dominated by solvation effects for which no simple models or trends are known.

Probably more about elephants than you wanted to know (American slang). But it is indeed a simple sounding question with a very complicated answer, which in the end is not very satisfying to our ambition to "explain" things in simple chemical terms.

Vince Calder



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