Dissociation of Water

• Acids have a sour or tart taste (but don't use this method to identify if a compound is an acid)
• Bases have a bitter taste, and have a slippery feel
• When bases are added to acids, they lower the amount of acid. When acids and bases are mixed together in appropriate proportions, they are completely neutralized (and their characteristic acid/base properties disappear altogether)

1830 - It was known that all acids contain hydrogen, but not all hydrogen-containing compounds were acids

1889 - Svante Arrhenius (he of the Arhhenius equation) connected acidic properties with the presence of H⁺ions and basic properties with the presence of OH^- ions

• If a solution contained more H⁺ than OH^- ions, then it was acidic
• If a solution contained more OH^- than H⁺ ions, then it was basic
• H⁺ and OH^- ions can react with each other to form H₂O molecules during neutralization reactions

Pure water consists almost entirely of H₂O molecules. While this may seem like a redundant statement, the point is that H₂O is essentially a non-electrolyte.

• Pure water is a poor conductor of electricity because it does not ionize to any great extent.

What little ionization of water does occur results in the production of H⁺ and OH^- ions. At room temperature, one in a billion water molecules will ionize:

H₂O(l) ó H⁺(aq) + OH^-(aq)

This process is termed the auto-ionization of water

• The equilibrium expression for the auto-ionization reaction is:

• The concentration of pure H₂O is about 55M. The concentration of water is essentially unchanged during chemical reactions involving dilute aqueous solutions (i.e. where the "solute" is in very low concentrations compared to the pure 55M water).
• As a result, the term for the concentration of water is considered to be a constant in the equilibrium equations involving dilute aqueous reactions. Therefore:

• The constant, K_w, is called the ion-product constant. At 25°C, the value of K_w is 1 x 10^-14

• The presence of other ions in dilute aqueous solution does not significantly perturb this equilibrium constant. Therefore, this value is used for pure water and such solutions.
• Based on this equilibrium constant, if we know the concentration of either H⁺ or OH^- ions in solution, then we can derive the concentration of the other ion

[H⁺] = 1 x 10^-14 / [OH^-]

[OH^-] = 1 x 10^-14 / [H⁺]

• A solution for which [H⁺] = [OH^-] is said to be neutral

[H⁺][OH^-] = 1 x 10^-14

if at neutrality [H⁺] = [OH^-] then we can substitute into the above equation to yield:

[H⁺][H⁺] = 1 x 10^-14

[H⁺]² = 1 x 10^-14

• Most aqueous solutions are not neutral
• As the concentration of one of these ions increases, the other must decrease according to the equilibrium constant

The auto-ionization of water:

H₂O(l) ó H⁺(aq) + OH^-(aq)

An H⁺ ion is a hydrogen atom that has lost its (one and only) valence electron. In other words, it is a naked proton.

• This proton is quite reactive; it wants two valance electrons in order to form a stable noble gas valence electron configuration (like He)
• The proton in solution has a high affinity for a non-bonding pair of electrons in neighboring water molecules, and will form a covalent bond to the oxygen atom in water molecules:

• The H₃O⁺(aq) ion is called a hydronium ion
• Chemists use H⁺ and H₃O⁺ terms interchangeably to represent the hydrated proton in acidic aqueous solutions
• The H⁺ ion is most often used because its simple and convenient, but H₃O⁺ is more closest the physical state of this hydrogen ion in solution
• Production of the hydronium ion from the auto-ionization of water is often written not in two distinct steps, but as resulting from the interaction of two water molecules:

H₂O(l) ó H⁺(aq) + OH^-(aq)

H₂O(l) + H₂O(l) ó H₃O⁺(aq) + OH^-(aq)

Or

• The equilibrium equation for this ionization would be:

• This would seem to give a different constant than the above expression for the equilibrium constant arising from a single water molecule. However, since the concentration of H₂O(l) is constant, the [H₂O]² term is also a constant.

• Thus, the value of the equilibrium constant is the same (i.e. it depends on the product of the concentration of the OH^- ion and the H⁺ or H₃O⁺ ion). And [H⁺] is interchangeable with [H₃O⁺]