dissociation同ionization既分別

Dissociation and ionization are two different concepts, with certain overlapping.

"Dissociation" involves breaking of bonds : usually covalent bonds and ionic bonds.
"Ionization" involves the formation of charged species from uncharged species, or the formation of more highly charged species from charged species.

e.g. Na⁺Cl⁻(s) + aq → Na⁺(aq) + Cl⁻(aq)
It is dissociation but not ionization.
This is because it involves the separation of opposite ions by breaking of ionic bonds (dissociation), but the number and nature of charged species (ions) remain unchanged (not ionization).

e.g. HCl(g) + aq → H⁺(aq) + Cl⁻(aq)
It is dissociation and also ionization.
This is because it involves the breaking of H-Cl bonds in HCl molecules (dissociation) to form H⁺(aq) and Cl⁻(aq) ions (ionization).

e.g. HCO3⁻(aq) ⇌ H⁺(aq) + CO3²⁻(aq)
It is dissociation and also ionization.
This is because it involves the breaking of O-H bonds in HCO3⁻ ions (dissociation) to form H⁺(aq) + CO3²⁻(aq) ions (ionization as more highly charged species are formed).

e.g. Na(g) → Na⁺(g) + e⁻
It is ionization but not dissociation.
In the process, neutral sodium atoms release electrons to give sodium ions (ionization). It does not involve breaking of bonds (not dissociation).

e.g. Cl(g) + e⁻ → Cl⁻(g)
It is not dissociation and also not ionization.
The process does not involve breaking of bonds (not dissociation). The number of charges species (electrons or chloride ions) remain unchanged (not ionization).

2011-02-04 01:25:15 補充：
In case of acid in water, both the terms "dissoication" and "ionization" can be used.

2011-02-04 01:25:28 補充：
In F.4, it is usually said that "an acid is a substance which is ionized in water to give hydrogen ions".

In F.6, the term "dissociation constant" is more frequently used than the term "ionization constant" for the equilibrium constant of dissociation/ionization of a weak acid/base in water.

2011-02-06 08:29:27 補充：
己式庚辛, my first examples are based on F.4 level, and the last two examples are based on F.6 level.

Conventionally, the term "ionization" refers to the formation of charged particles rather than the formation of ions.

2011-02-06 08:29:38 補充：
In the 4th example, a neutral species (Na) gives two charged species (Na⁺) and e⁻), and thus it is ionization. In the 5th example, a neutral species (Cl) and a charged species (e⁻) combine to give a charged species. There is no increase in number of charged species, and thus it is NOT ionization.

i guess you're not talking about general meaning of "ionization" and "dissociation".
you're specifically talking about acid / base, right?

in that sense, "dissociation" of salts / bases means that, no matter in solid form or in aqueous form, identity is still the same.

for example, in solid form, sodium chloride exists as sodium ions and chloride ions (packed together); after dissolution, they're still sodium ions and chloride ions, just separated.
Na(+)Cl(-)(s) +aq ------> Na(+)(aq) + Cl(-)(aq)
same as potassium hydroxide. in solid, it exists as closely-packed potassium ions and hydroxide ions; after dissolving in water, it is still potassium ions and hydroxide ions, just separated and distributed in water.
K(+)OH(-)(s) +aq ------> K(+)(aq) + OH(-)(aq)


"ionization" means that before this process, the substance in NOT ionic; after process, that substance becomes ions.

for example, hydrogen chloride is a molecular compound, H & Cl bounded by a covalent bond. when not dissolved in water (as gas), it's just free, discrete molecules. NO ions are present.
when HCl is dissolved in water, ions are produced.
originally, there're NO ions; after ionization, it becomes ions.
H--Cl(g) +aq ------> H(+)(aq) + Cl(-)(aq)

notice that when ionic compound dissolves in water, it's NOT necessary to be a dissociation.
take calcium oxide as example.
Ca(2+)O(2-)(s) +aq ------> Ca(2+)(aq) + 2OH(-)(aq)
though calcium oxide is ionic, the oxide ions reacts to give hydroxide ions. thus this is better considered "ionization", or more exact, ionization of oxide ion.

after dissolving in water, ions either form or not-form. thus, in sense of acid-base concept, there's NO overlapping cases.


2011-02-06 01:15:41 補充：
let me add this: my answer is based on old F.4 chem.
in those days, "dissociation" and "ionization" are ONLY employed in acid-base concepts, not anywhere else.
these two processes are mutually exclusive.

2011-02-06 01:18:47 補充：
of course in higher levels, this's not the case;
but as the question's level is not so high (have to differentiate two words), i naturally assume that's at elementary level.

2011-02-06 01:24:00 補充：
Michael, your examples on sodium and chlorine seem contradicting.
sodium ionizes as ion is formed; why chloride-formation isn't? i don't understand that.


conclusion: the answer bases on question's level.

Products of ionization must be ions, but products of dissociation may not be ions.
e.g. N2O4 → 2NO2
is a reaction of dissociation.

However, there are some overlaps between dissociation and ionization.
e.g. HCl → H+ +Cl-
can be both defined as ionization or dissociation. But reactions happen in water are more likely to be defined as ionization.