Free Hydrogen Ions do not Exist in Water
Owing to the overwhelming excess of (H_2O) molecules in aqueous solutions, a bare hydrogen ion has no chance of surviving in water. The hydrogen ion in aqueous solution is no more than a proton, a bare nucleus. Although it carries only a single unit of positive charge, this charge is concentrated into a volume of space that is only about a hundred-millionth as large as the volume occupied by the smallest atom. (Think of a pebble sitting in the middle of a sports stadium!) The resulting extraordinarily high charge density of the proton strongly attracts it to any part of a nearby atom or molecule in which there is an excess of negative charge. In the case of water, this will be the lone pair (unshared) electrons of the oxygen atom; the tiny proton will be buried within the lone pair and will form a shared-electron (coordinate) bond with it, creating a hydronium ion, (H_3O^+). In a sense, (H_2O) is acting as a base here, and the product (H_3O^+) is the conjugate acid of water:
Although other kinds of dissolved ions have water molecules bound to them more or less tightly, the interaction between H+ and (H_2O) is so strong that writing “H+(aq)” hardly does it justice, although it is formally correct. The formula (H_3O^+) more adequately conveys the sense that it is both a molecule in its own right, and is also the conjugate acid of water. However, the equation
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[ce{HA → H^{+} + A^{-}} nonumber]
is so much easier to write that chemists still use it to represent acid-base reactions in contexts in which the proton donor-acceptor mechanism does not need to be emphasized. Thus it is permissible to talk about “hydrogen ions” and use the formula H+ in writing chemical equations as long as you remember that they are not to be taken literally in the context of aqueous solutions.
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Interestingly, experiments indicate that the proton does not stick to a single (H_2O) molecule, but changes partners many times per second. This molecular promiscuity, a consequence of the uniquely small size and mass the proton, allows it to move through the solution by rapidly hopping from one (H_2O) molecule to the next, creating a new (H_3O^+) ion as it goes. The overall effect is the same as if the (H_3O^+) ion itself were moving. Similarly, a hydroxide ion, which can be considered to be a “proton hole” in the water, serves as a landing point for a proton from another (H_2O) molecule, so that the OH- ion hops about in the same way.
The hydronium ion is an important factor when dealing with chemical reactions that occur in aqueous solutions. Because hydronium and hydroxide ions can “move without actually moving” and thus without having to plow their way through the solution by shoving aside water molecules as do other ions, solutions which are acidic or alkaline have extraordinarily high electrical conductivities. The hydronium ion has a trigonal pyramidal geometry and is composed of three hydrogen atoms and one oxygen atom. There is a lone pair of electrons on the oxygen giving it this shape. The bond angle between the atoms is 113 degrees.
As H+ ions are formed, they bond with (H_2O) molecules in the solution to form (H_3O^+) (the hydronium ion). This is because hydrogen ions do not exist in aqueous solutions, but take the form the hydronium ion, (H_3O^+). A reversible reaction is one in which the reaction goes both ways. In other words, the water molecules dissociate while the OH- ions combine with the H+ ions to form water. Water has the ability to attract H+ ions because it is a polar molecule. This means that it has a partial charge, in this case the charge is negative. The partial charge is caused by the fact that oxygen is more electronegative than hydrogen. This means that in the bond between hydrogen and oxygen, oxygen “pulls” harder on the shared electrons thus causing a partial negative charge on the molecule and causing it to be attracted to the positive charge of H+ to form hydronium. Another way to describe why the water molecule is considered polar is through the concept of dipole moment. The electron geometry of water is tetrahedral and the molecular geometry is bent. This bent geometry is asymmetrical, which causes the molecule to be polar and have a dipole moment, resulting in a partial charge.
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