In the early weeks of most general chemistry courses, we are taught these definitions of acids and bases (Svante Arrhenius, 1859 - 1927):
- Acid: a substance that produces H+(aq) when dissolved in water
- Base: a substance that produces OH-(aq) when dissolved in water
Acid-base reactions then are reactions that involve H+(aq) reacting with OH-(aq) to produce H2O.
However, two things are wrong with this scheme:
- H+(aq) is NOT an accurate description of the positive ion actually present in acidic aqueous solutions
- Many proton transfer reactions make sense as acid-base chemistry, but don't fit the Arrhenius definitions.
Let's look at these points in turn.
- A hydrogen ion in water is instantaneously attached to a water molecule, and becomes indistinguishable from the two H already there:
The computed van der Waals surface of H3O+
The picture shows clearly that H+(aq) is a fiction. Furthermore, H3O+ then becomes tightly solvated by several more water molecules. Two waters stick especially tightly, meaning that the dominant species in aqueous solutions containing "H+" are H3O+ and H7O3+
The computed van der Waals surface of H7O3+
Can you look at either of these pictures and decide which H is the H+ ?
