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Chapter 16 Notes: Acids and Bases

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. 

Example:

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

Example

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.

David Witten

Chapter 14: Chemical Kinetics

Quantum Numbers and Electron Orbitals