I too was bothered by the books reasons for why some acids are stronger
than others. I think I have come up with reasonable explinations (which
may or may not be correct), largely on the input of this happy e-mail
family(except for Mr. No-Reply man).
The first supposition the book puts forward is F-H is a weaker
acid than H-Cl and H-Br and H-I. The reason for this (by the book) is
that because F is more electronegative, the bond strength is stronger
between the halide (florine) and the Hydrogen, as compared to halides and
hydrogens further down on the periodic table.
At the same time, the book says that when hydrogen is bonded to
weaker electronegative atoms in the same row, it is more acidic (or more
ready to let go of the H in the complex). This not only seems like a
contradiction, it IS. That's why I looked to some other aspects of the
atoms involved besides electronegativity
The first case, when you decend the periodic table the acids
become stronger (are more willing to get rid of a proton), I believe can
be explained by a kind of 'Insulation' factor. Br has several layers of
electrons between it and hydrogen (4 to be precise) while F only has a
few (2). Thus, when a hydrogen is attached to Florine, its bond is
relativitaly strong because of two factors. The first is that the
hydrogen is actually closer to the florine in H-F than in H-Br or H-Cl
etc... (Take this statement with a grain of salt, I did not look up the
actual bond lengths, I am assuming this is so). This closer relationship
means that the bond is exponentially stronger (an electrostatic property
is that the attraction between opposite charges increases by a power of
two the closer it gets, like gravity or magnatism does). Thus Hydrogen
is more firmly attached to the electrons it is sharing with florine, and
florine is REALLY attached to the electrons it is sharing with hydrogen.
In Br-H however, the distance is close to twice as far (by the same
assumption as before). The other factor is that the electrons in the
bonding orbital of Br-H are have far more energy than the electrons in
the bonding orbital of F-H. (Recall that the higher the shell number
[Br's bonding orbitals are in the 4th and F's are in the 2nd] the higher
the energy of these electrons. If you don't recall just trust me on this
one) This extra energy means they are zipping around much farther out
from the Br than from the F. If an electron goes out too far, it will
loose its bonding property with the Hydrogen atom more often, while still
retaining the electron's attraction to the Br or F because of its
increased electronegativity (ability to hold electrons!!!). For visual
people this may help:
X
Br X
BrBrBr
H BrBrBrBr F
BrBrBr H FFF
Br F
X=An example bonding electron
Looking at the pictures you can see that the electron can get much
farther away from Hydrogen in the H-Br (because the energy allowes it to
fly farther out from the Br) than the electron in the F-H. The electron
in the Br may not even have much of a bonding effect on H at some points
because of it's incresed distance from it. Thus, the H is held less
strong in Br-H than F-H because the F electron cannot fly so far away.
Now to deal with moving ACROSS the periodic table.
When an H is bonded to elements in the same row of the table, the
bond lengths and strengths are relativitally the same. (The book
actually said this!). There aren't any more shells of electrons in the
way, and the electrons can zip about the same distance away from atoms
than from any other atom in the same row because the energies are about
the same. Thus bond strength is no longer the dominant factor. Instead
electronegativity takes it's place. To start off with, we need a solid
definition of electronegativity. ELECTRONEGATIVITY: the ability of an
atom to hold electrons to it, or the affinity of an atom for electrons.
With that in mind we'll continue. IF a bond between a C-H were to break,
the Carbon would be left with an extra electron. The electronegativity
of Carbon is about 2.5. That means it doesn't mind having the extra
electron around by a factor of 2.5. (Like rating how much you like to
eat icecream. If you like to eat icecream 2.5 on a scale from 0 to 4.1,
you don't mind eating them but you could do without it too). Now imagine
an H-F bond breaking. The Florine is now left with an extra electron
that it doesn't mind having by a factor of 4.1. It doesn't mind having
it around alot more!!! (Like a friend who likes an Icecream 4.1 one a
scale of 0-4.1, he's the guy who likes icecream more than anyone else in
the world!!!) When comparing the two, Carbon doesn't like being in the
extra-electron state as much as Florine (you don't like icecream as much
as your friend). Naturally, it is not going to want to give away it's
Hydrogen (and pick up the extra electron) as much as Florine. Thus
Florine is more willing to give up the Hydrogen, or in other words, is
more acidic. (Your friend eats icecream more than you). Also, IF both
the H-C and the F-H lose there Hydrogen (and pick up an extra electron),
the F is more likely to stay in the state and not re-pick up a hydrogen
than the C. (You are less likely to eat icecream all day than your friend).
To summerize: As you move down the periodic table, H-A ('A'
being a species in a particular COLUMN of the table) is more acidic
because the bond between A and H is weaker than ones above it on the
table, due to a longer bondlength and the extra energy the electrons in
the valence shell have (allowing them to be too far from the small
Hydrogen atom.) However, as you move right to left across the table, H-B
('B' being an atom in the same ROW of the table) becomes less acidic
because as electronegativity decreases, an atom is less willing to have
an extra electon around, and is more willing to pick up a H+ to remove
itself from that state if it does have an extra electron.
I hope this helps, because it was awful to try to
understand whay the book was saying. I hope it didn't drawl on too, but
ya' gatta do what ya' gatta do. Also, although this explination makes
the reason for acidity differences across and up and down make sense,
some of my assumptions may be unfounded. In other words, it works, but
it works for the wrong reasons. IF I AM WRONG*** PLEASE LET ME KNOW.
We can't have a blithering idiot just spouting out garbage all across the
world (thanks to the power of the internet). Later...