Brønsted-Lowry Theory of Acids and Bases
J.N. Brønsted and T.M.Lowry independently proposed a new acid-base theory. An acid is a proton donor, and a base is a proton acceptor. What this means is an acid goes from being neutral to having a negative charge, and a base goes from being neutral to being positively charged, because they take in a proton.
NH3 + H2O -> NH4+ + OH-
In this reaction H2O acts as an acid, giving up a proton, H+, which is taken up by NH3, a base.
Because NH3 is a weak base, we also need to consider the reverse of that reaction, where NH4+ is an acid, and OH- is a base. So, the pairs NH3/NH4+ and H2O/OH- are called conjugate pairs.
So, to write the equilibrium constant, we did we what we learned in Chapter 15 [OH-][NH4+]/[H2O]/[NH3].
However, water is present in such overwhelming numbers compared w/ NH3 molecules and NH4 and OH-, that water is essentially a pure liquid, so it's ignored.
Called Kb it equals [NH4+][OH-]/[NH3]
That constant is called the base ionization constant.
The ionization of acetic acid can be expressed as
HC2H3O2 + H2O = C2H3O2- + H3O+
Using the same logic from the previous reaction, the water is ignored, this time however, it acts as a base, so by taking the base out, and calculating the equilibrium value using everything else, you get a constant Ka, or the acid ionization constant.
Self-Ionization of Water and the pH Scale
Water can be an acid and a base, so some water molecules donate protons and others accept protons. In the self-ionization of water, for each H2O molecule that acts as an acid, another H2O molecule acts as a base.
H2O + H2O = H3O+ + OH-, and we can write: K = [H3O+][OH-]
The equilibrium constant for the self-ionization of water is called the ion product of water. It is symbolized as Kw. At STP Kw = [H3O+][OH-] = 1.0 x 10^-14.
So, if [OH-] increases, [H3O+] decreases.
pH and pOH
In 1909, the term pH was proposed to refer to the "potential of the hydrogen ion". He defined pH as the negative logarithm of [H3O+]
Here are the three main formulas of pH:
pH = -log[H3O+]
pOH = -log[OH-]
pH + pOH = 14.00
Find the pH of a solution of 0.0025 HCl. The chemical equation of this equation is H2O + HCl -> H3O+ + Cl-
So, the concentration is the same, meaning the pH is -log(0.0025), or around 2.6.