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.
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
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
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.
Click here to return to the Chemistry Archives
Update: June 2012