Dissociation and Ionization
Name: Tracy S.
What is the difference between dissociation and ionization? I thought it was
that ionization involved a reaction with water to produce ions whereas dissociation
involved an ionic substance simply separating into the ions of which it is made.
Like so many terms, the terms "dissociation" and "ionization" are frequently used
imprecisely, if the context is clear. "Ionization" refers to "dissociation" reactions in
which the products are ions -- regardless of whether that occurs in water, or in the gas
phase, or even the solid phase. The equilibrium constant for such reactions, unfortunately,
is sometimes referred to as the "dissociation" constant, which strictly speaking is
incorrect -- the proper term being "ionization" constant.
The term "dissociation" also refers to reactions in which the products are neutral species.
For example: HCl ----> H + Cl where "H" and "Cl" are atoms rather than ions. This is
different than the ionization reaction:
HCL -----> H(+) + Cl(-).
Dissociation refers to a reaction where a molecule or substance breaks apart into smaller
units. The units are not necessarily ions, although this is often the case.
Ionization generally refers to a reaction which forms ions from an uncharged species.
It does not have to be a reaction with water.
Ionization can sometimes be a particular kind of dissociation. Ionization can also be
achieved by using a photon of sufficient
energy to remove an electron (this is called photoionization).
So the gas-phase reaction
H2 (g) ----> 2 H (g) (R1)
is a dissociation,
whereas the gas-phase reaction
H2(g) -----> H- (g) + H+ (g) (R2)
is an ionization. The process
H2(g) + photon ----> H2+ (g) + electron (R3)
is a photoionization.
Depending on the temperature and other factors, all three of these reactions are possible.
Looking at solution-phase reactions, if you bubble HCl (g) into water, you make aqueous
HCl (g) -----> H+ (aq) + Cl- (aq) (R4)
This looks like a dissociation. However, there is no such actual species as H+ in solution
because H+ readily combines with H2O to form the hydronium ion H3O+:
H+ (aq) + H2O (l) -----> H3O+ (aq) (R5)
If you add reactions R4 and R5 together, you get the net outcome:
HCl (aq) + H2O (l) ---> H3O+ (aq) + Cl- (aq) (R6)
Reaction R6 appears to depict an ionization which is not a dissociation but is rather a
reaction with water. However, something had to dissociate (R4) in order for that to happen,
Finally, an ionic dissociation reaction in water might be the dissolution of an ionic
NaCl(s) -----> Na+(aq) + Cl- (aq)
This does not involve any subsequent reactions of the ions produced with water.
Hope this helps!
Best, Prof. Topper
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Update: June 2012