CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 349
S Symbols:When writing chemical equations we will use the symbol “→” to indicate complete
dissociation, and the symbol “ = ” to indicate a reaction proceeding until the state of a chemical
equilibrium.
Note:The reaction mentioned in the above example is often written in a different way:CH 3 COOH + H 2 O = CH 3 COO−+ H 3 O+where H 3 O+is called theoxonium cation. The reason given for this procedure is that
the hydrogen cation H+is actually a proton which cannot exist independently in the
solution and is always bound to water. On the other hand, we cannot say for sure whether
it is linked with one molecule of water (H 3 O+), two (H 5 O+ 2 ), or more. For the sake of
simplicity we will write H+while being aware of this ambiguity.Note: The boundary line between strong and weak electrolytes is not sharp. The term
strong electrolyte is usually applied when more than 90% of molecules are dissociated, the
term weak electrolyte indicates that the dissociation is less extensive.
The kind of solvent used is one of several factors deciding whether a substance is a strong
or a weak electrolyte. The solution of nitric acid in water, e.g., is a strong electrolyte, the
solution of nitric acid in methanol is a weak electrolyte.11.1.3 Ion charge number
The ion charge number is given by the ratio of the ion charge and the absolute value of the
electron charge. For example, the charge number of a bivalent zinc cation is +2, the charge
number of a sulfate anion is –2. Instead of the term “ion charge number” we will write simply
“ion charge” when there is no risk of ambiguity.
S Symbols:The symbolsz+andz−will be used to denote theabsolute valuesof charge numbers.
11.1.4 Condition of electroneutrality
It is obvious that after partial or complete dissociation of an electrolyte into ions the system
stays outwardly electrically neutral. This fact is called theglobal condition of electroneu-