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Ionization vs. Dissociation for acids & bases

  1. Jun 25, 2015 #1
    1. The problem statement, all variables and given/known data
    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.

    2. Relevant equations


    3. 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-
     
  2. jcsd
  3. Jun 25, 2015 #2

    Borek

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    Staff: Mentor

    Do you have any given definitions of what ionization is and what dissociation is?
     
  4. Jun 25, 2015 #3
    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.
     
  5. Jun 25, 2015 #4

    Borek

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    Staff: Mentor

    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.
     
  6. Jun 26, 2015 #5

    epenguin

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    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. :oldsmile:
     
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