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Name: Madhavi
Status: educator
Age: 30s
Location: N/A
Country: N/A
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(-).

Vince Calder

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 reaction.

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,
Prof. Topper

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 "ionization."

"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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