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Hydrogen chloride

  1. Dec 22, 2011 #1
    When hydrogen chloride dissociates in water , does it form ions or atoms ?
     
  2. jcsd
  3. Dec 22, 2011 #2

    Borek

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    Ions.
     
  4. Dec 22, 2011 #3
    So it actually does dissociate in water?
     
  5. Dec 22, 2011 #4

    Borek

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    Yes.

    I thought you know it, you stated that it dissociates in your original question.
     
  6. Dec 22, 2011 #5
    Actually at first yes then no Because on the web some say it reacts with water and does not dissociate
     
  7. Dec 22, 2011 #6
    Or does it do both ?
     
  8. Dec 22, 2011 #7

    Borek

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    Dissociation - in a way - can be thought of as a reaction with water. But not like sulfur(III) oxide reacting with water and producing sulfuric acid, this is more about solvation (or more precisely hydration) of ions.

    Precise answer may depend on your level of education. Simple approach says it is just a dissociation:

    HCl -> H+ + Cl-
     
  9. Dec 22, 2011 #8
    Hmm well actually my textbook does say that but I don't agree in the sense that I still can't really accept the fact that a polar covalent compound dissociates to form ions . I mean why shouldn't it be atoms ? So I went to the web and found this explanation which i like and am able to understand . The term that HCl 'dissociates' in water actually doesn't mean that suddenly HCl molecule breaks into H+ and Cl-, because if that is the case one will need to supply enormous amount of energy. What actually happens is that HCl REACTS with water. since water is considered to be strong basic when compared to HCl, so what really happens is
    HCl + H2O --> [H3O+][Cl-]
    and this reaction is an exothermal reaction, that's why when you dilute HCl with water the you'll notice that the reaction glass will be slightly warmer.

    Since in the past time the most common solvent used in chemistry was water, the definition of acid was also used relatively to water (bronsted acidity concept) and in many case people start to leave out the complete reaction equation and shortened it with HCl ---> H+ + Cl- , it's not wrong though as long as you know that it only applies in aqueous solution and that what really happens is reaction between HCl and water and NOT self-breaking covalent bond of HCl
     
  10. Dec 22, 2011 #9
    Is it correct ?

    Actually I think I shouldnt try to understand things not really of my level as well. It's kind of tiring . But I can't stop thinking about it till I fully understand it .
     
  11. Dec 22, 2011 #10

    Borek

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    And hydration of H+ is what supplies the energy needed. Heat of hydration can be quite high, if you have ever seen one of these "instant hot coffee" cups they produce heat just by adding water to anhydrous calcium hydrate.

    Water is a strong base not when compared to the HCl, but when compared to the Cl-.

    If this explanation works for you, that's OK. It is not wrong. A little bit hand-wavy, but what I posted is a little bit hand-wavy too.
     
  12. Dec 23, 2011 #11
    Uhuh. Ok I think what u said makes sense too . Ok so both explanations are considered correct right ? Oh and thanks so much !!
     
  13. Dec 23, 2011 #12
    Oh and I still don't understand why it should be ions not atoms. Hcl is made up of h atom and cl atom covalent lu bonded together, just because it is polar and can be dissociated, it doesn't mean it dissociates into ions right? Shouldn't it dissociate into the original hydrogen atom and chlorine atom ? Or is it cos the atoms lose and gain electrons to become ions ?
     
  14. Dec 23, 2011 #13

    Borek

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    H and Cl atoms have unpaired electrons and they are highly reactive. That's not the case with H+ and Cl- - first one nicely fits into any lone pair of any water molecule, the latter has a noble gas configuration, which is very stable. Besides, being charged both can attract water molecules which have a dipole moment - that means they are surrounded by these molecules in an ordered way, which further stabilizes them.
     
  15. Dec 23, 2011 #14
    Uhuh... So in order to achieve stability hcl has to dissociate into ions not atoms?
     
  16. Dec 23, 2011 #15

    Borek

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    You can put it this way.

    All systems work this way - they try to get to the most stable configuration.

    They may have troubles getting there, in which case they can occupy configuration that is not so stable (some local minimum), but give them a nudge in the right direction and they will nicely move to the most stable.
     
  17. Dec 23, 2011 #16
    Ok , I get it now , thank you !
     
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