Estimation of pH - More Detail About Acids

More Detail About Acids



We have defined an acid as a compound that donates a proton when in water to form hydronium ions (H3O+). Strong acids (like hydrochloric, nitric and sulfuric acids) donate all of their protons to water and are said to be 100% ionized. If considered in terms of equilibrium (shown below), the equilibrium lies extremely far to the right.

HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)

Weak acids (like acetic, formic and benzoic acids) do not donate all of their protons to water and are not 100% ionized. If considered in terms of equilibrium (shown below), the equilibrium lies predominantly to the left.

HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2-(aq)

We use equilibrium constants (K values) to identify whether the reactants (the equilibrium lies to the left of the equilibrium arrow) or the products (the equilibrium lies to the right of the equilibrium arrow) are favored. In the former case, values for K are small. In the latter, K values are high. Because we are dealing with an equilibrium which contains acids, the K value is called the acid dissociation constant and given the symbol Ka. As the strength of the acid decreases, so does its Ka value. Strong acids, have Ka values that are too high to measure. Weak acids have small Ka values, usually much less than one.

Some Ka values
hydrochloric acid (HCl), nitric acid (HNO3) infinite
formic acid (HCHO2) 1.8 x 10-4
lactic acid (HC3H5O3) 1.4 x 10-4
acetic acid (HC2H3O2) 1.8 x 10-5
boric acid (H3BO3) 5.8 x 10-10