Question #b984a

Most substances that dissociate into constituent ions in aqueous solution are ionic compounds (#"KCl"#, #"NaBr"#, #"AlCl"_3#, etc.). These solvated ions carry a current between electrodes and conduct electricity.

However, there some very important covalent compounds that dissociate as well, and they are called acids.

Acids have a few different definitions, but they can be categorized into two main groups: strong acids and weak acids.

Strong acids are those that dissociate essentially completely in solution; i.e. there isn't really an equilibrium established between between the composite ions and the molecule itself. There are seven common strong acids:

• #"HCl"#

• #"HNO"_3#

• #"HBr"#

• #"HI"#

• #"HClO"_3#

• #"HClO"_4#

• #"H"_2"SO"_4#

(notice there is only one diprotic acid: sulfuric acid)

Weak acids essentially consist of all acids that are not the above seven. They do not dissociate to near completion, and have a measurable equilibrium between molecule and dissociated ions.

Weak acids have an acid dissociation constant, denoted #K_a#, that describes the ratio of concentration of component ions to concentration of the whole molecule, and the value will always be less than #1# (with the exception of the hydronium ion (#"H"_3"O"^+#), whose value is #1#).

For example, for the weak acid #"HF"#, we have the equilibrium expression

#K_a = (["H"^+]["F"^-])/(["HF"])#

The #K_a# value for hydrofluoric acid is #7.2xx10^-4#, which indicates the concentration of #"HF"# is greater than that of #"H"^+# and #"F"^-#; i.e. the equilibrium lies to the left.