Ionization vs. Dissociation for acids & bases

[jumbogala]

Homework Statement

Hi everyone. I'm a relatively new high school chemistry teacher. I'm teaching modified arrhenius theory to my students, and I'm unclear on ionization vs. dissociation.

Homework Equations

The Attempt at a Solution

Here is my understanding:

- Dissociation occurs when an ionic compound breaks down. Example: NaCl --> Na⁺ + Cl^-

- Ionization occurs when a compound reacts with water to gains or lose a hydrogen ion. Example: HCl + H₂O --> Cl^- + H₃O⁺

I found something in the textbook that says acids always ionize, but bases always dissociate.

However, wouldn't NH₃ (a base) ionize in the following situation? NH₃ + H₂O --> NH₄⁺ + OH^-

 

[Borek]

Do you have any given definitions of what ionization is and what dissociation is?

 

[jumbogala]

According to the textbook:

Dissociate: to break apart to form separate ions (as when ionic substances such as NaOH break apart). This process is not called ionization because the substances are already made of ions.

Ionize: the process in which a molecular substance, often an acid, dissolves in water and separates into ions.

So my understanding would be that NaOH dissociates but NH₃ ionizes, even though they are both bases.

However this is also confusing because my students would probably think that H₂SO₄ is an ionic compound. Therefore, it dissociates, but the textbook claims that all acids ionize.

 

[Borek]

Sigh, I have a feeling these definitions ask for confusion. No wonder you have problems.

I would say ammonia neither dissociates nor ionizes. It reacts with water producing ions, but it is even another process than the two mentioned.

 

[epenguin]

You could think of it like the language is not that of tables issued by an official Standards Bureau, the language is just English. In normal writing you might naturally vary words a bit just to avoid too much repetition.

Dissociation is a more general term than ionisation - it is just breaking apart as your definition says, but that doesn't have to be into ions, it can also be into neutral molecules or into free radicals. (The reverse reaction is 'association' which you might sometimes call 'reassociation'.)

It would be natural to me to say ammonia 'is protonated' to form NH₄⁺ (to an extent that varies with pH).. A protonated molecule can 'deprotonate'.

I thin it is good to quite often use the word 'transfer'. An Arrhenius acid transfers a proton to a base, rather than 'loses' it, though you will inevitably often say it loses it, because you are talking English not always Chemspeak. Good work and don't worry.

 

[Lilly Frank]

As far as my understanding goes, dissociation takes place when ions are already present in the molecule, i.e. there is an ionic or electrovalent bond. So when it reacts with water, it just breaks down, which is the literal meaning of dissociate. In ionization on the other hand, new ions are being formed, so that indicates a presence of a covalent bond. NH4OH is an exception since it contains both ionic and covalent bonds, but as a general rule, acids contain ionic bonds whereas bases contain covalent bonds. Thus, acids ionize and bases dissociate. In covalent compounds also, there is a distinction: polar and non polar. Polar compounds also have an ionic character, so they also dissociate.

 

[Borek]

Trying to apply rigid logic to poorly defined concepts never works. I am not aware of any precise distinction between dissociation and ionization (which is exactly what @epenguin referred to above) - they are rather broad concepts that try to classify processes into categories. Trick is, these processes occupy continuum, so putting them in different drawers is artificial and will always produce problems.