Dissociation and Ionization
Date: Sunday, December 08, 2002
What is the difference between dissociation and ionisation?
Strictly speaking, yes. Dissociation refers to the breaking of a chemical
bond without reference to whether the products are ions or neutral
fragments. Ionization (ionisation, alternate spelling) refers to breaking of
chemical bonds into charged species. The terms are sometimes both used when
referring to the ionization reaction HA -----> H(+) + A(-).
Dissociation of a molecule can either produce ions or neutral molecules.
If it produces ions, then the dissociation is also an ionization.
so HCl ---> H+ + Cl- is a dissociation reaction and also an
ionization reaction. However, H2 ----> 2H is just a dissociation
However, ionization is a general term which refers to the formation of
ions, and not necessarily from dissociation. You can form an ion
by removing an electron from it (a cation) or adding an electron
(forming an anion, as in F + e ---> F-). If you want to think of
cation formation as a dissociation reaction, fine- but the latter is clearly
not a dissociation.
hope this helps,
There is some overlap between the terms. Ionization is any process that
makes a particle electrically charged. Dissociation is when a single
particle becomes two or more. When chemists speak of a "dissociation" or
"ionization" process of a solvent such as water, for example, they mean the
splitting of the solvent into a positive and negative ion:
2H2O --> H3O+ + OH-
2 NH3 --> NH4+ + NH2-
I usually hear this process called "autoionization" rather than
"Dissociation" also refers to the loss of a proton by and acid in solution:
HA + S --> A- + HS
where HA is the acid and S is a molecule of solvent. In all these cases a
molecule is breaking apart.
"Ionization" can also refer to processes outside of solution, such as
ejection of an electron from a neutral atom upon absorption of light. That
sort of process is usually not referred to as "dissociation."
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012