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Homework Help: Ionic equations

  1. Oct 5, 2015 #1
    1. The problem statement, all variables and given/known data
    An ionic equation is an equation that shows the chemical species that actually take part in a reaction.
    Ex. Pb(NO3)2 (aq) + 2KI (aq) -> PbI2 (s) + 2KNO3 (aq)
    So the ionic equation for this reaction is
    Pb2+ (aq) + 2I- (aq) -> PbI2 (s)
    How do we know that Pb2+ and I- takes part in the reaction? What about K+ and NO3- ?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 5, 2015 #2


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    KNO3 is pretty soluble. So is KI. Perhaps Pb(NO3)2 as well. So except the ions that precipitate, the ions in solution don't even notice what's "happening".
  4. Oct 5, 2015 #3
    Ok...then what about NaOH (aq) + HCl (aq) -> NaCl (aq) + H2O (l)
    How do we know that H+ and OH- are the ions that react? And not Na+ and Cl-?
  5. Oct 5, 2015 #4


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    We know because -- if everything happens in solution, so full ionization (*) and no solids --
    the NaOH (aq) on the left stands for Na+ plus OH-
    The HCl (aq) on the left for H+ plus Cl-
    NaCl (aq) on the right stands for Na+ plus Cl-
    And if we leave out the common things on left and right all that happens is H+ + OH- → H2O

    So much for the simplistic answer. Reality is a lot more complex, but this case can be untangled by considering it as a set of equilibria:

    NaOH (aq) ↔ Na+ + OH-
    HCl (aq) ↔ H+ + Cl-
    H2O ↔ H+ + OH-
    with an equilibrium constant for each of these.
    And the k for water is so small compared to the other two that it makes the last equilibrium reaction "shift" to the left.
  6. Oct 5, 2015 #5


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    Knowing what reacts and when is what chemistry is about. Some things you just have to remember - H+ + OH- is a neutralization reaction, quite common.

    What do the solubility rules say about NaCl solubility? Do you expect it to precipitate?
  7. Oct 5, 2015 #6
    Oh I see. Thanks!!
  8. Oct 5, 2015 #7
    Nope.. Ok I think I've got it thanks
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