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Homework Help: Chemistry - Net ionic equations, ionic solids!

  1. Sep 12, 2006 #1
    I am having a review test soon and I was not taught anything about ionic stuff last year, I have no idea what it is at all.

    Sample questions look like:

    "which of the following compounds are considered to be ionic solids? If it is an ioni solid, write an equation showing the dissociation of the compound as it dissolves in water"

    eg. NiCl2, ClF5, NaOH

    First I have no idea what an ionic solid is. why is it even dissociating when it dissolves in water. How would I know if its ionic solid or not?

    I have to write an equation. Is it like decomposition, single replacement, synthesis that kind of equation? what becomes reactant or the product? I have no idea what is happening! Is part of this requiring memory work, or is it all mathematical, or do i have to refer to the periodic table, or is it logic, or do I have to think about it? I am totally lost:rofl:

    Also I have a question like this:

    Ca(OH)2 has low solubility. write the net ionic equation for the reaction that occurs when NaOH(aq) is mixed with Ca I2 (aq) Identify the spectator ions.

    I have the answer but I totally dont know how it came up. How do i know if something is positive or negative charge? What becomes solid or liquid?

    If someone could clarify this all up for me, that would be really appreciated
  2. jcsd
  3. Sep 12, 2006 #2
    There are some exceptions to the statements I am going to make, but for basic chemistry you should not need to worry about them.

    Ionic compounds are those compounds that have a metal and a nonmetal. You can use the periodic table to determine what is a metal/nonmetal/metalloid. An example is NaCl. Na (sodium) is a metal and Cl (chlorine) is a non-metal. Hence NaCl, sodium chloride, is an ionic compound.

    Ionic compounds break up into their respective ions when they dissolve in water. Sodium chloride breaks up into sodium ions and chloride ions. The +1 charge on the sodium ion and -1 charge on the chlorine ion cancel each other giving a neutral ionic solid of NaCl. So your equation showing its dissociation in water would look like this:

    NaCl(s) --> Na+(aq) + Cl-(aq)

    (s) means solid (aq) means aqueous (in water)

    Another example is aluminum sulfate, Al2(SO4)3. Now in this case you have more than 2 atoms in your ionic solid. SO4-2 is what is called a polyatomic ion. The naming of these compounds is something that you pretty much have to memorize. There are some patterns for naming them, however. Aluminum sulfate's dissociation in water would look like this:

    Al2(SO4)3(s) --> 2Al+3(aq) + 3SO4-2(aq)

    Aluminum sulfate is comprised of 2 aluminion ions each with a +3 charge and 3 sulfate ions each with a -2 charge. This, as you can see, gives a neutral ionic compound with no charge.

    The second question you ask has to do with a class of reactions called double displacement reactions. Double displacement reactions can occur if a) one of the potentially formed products is not very soluble (ie a precipitate is formed), b) if a gas is formed, or c) if it is a acid-base neutralization reaction. If the mixture of reactants does not belong to one of these three categories, then the reaction will not occur and all you will have is a bunch of ions in solution.

    We can see that this reaction is a) a precipitation reaction, because Ca(OH)2 has a low solbuility.

    First of all, write your two reactants that are given:

    NaOH(aq) + CaI2(aq)

    Now both NaOH and CaI2 are soluble (your teacher may or may not want you to memorize which compounds are soluble and which are not...there are rules for remembering these). There are both also ionic compounds. Since they are ionic compounds and are soluble in water, they dissociate into ions.

    NaOH(aq) + CaI2(aq) --> Na+(aq) + OH-(aq) + Ca+2(aq) + 2I-(aq)

    Now you know that Ca(OH)2 is not very soluble so the calcium ion and hydroxide ion, OH-, will react to form Ca(OH)2.

    Na+(aq) + OH-(aq) + Ca+2(aq) + 2I-(aq) --> Ca(OH)2(aq) + Na+(aq) + 2I-(aq)

    The spectator ions are the reactions that do not play a direct role in a double displacement reaction. They are the ones that are left on both sides of the equation. In this case they are the sodium ions and the iodide ions. You can cancel out the spectator ions to yield the net-ionic equation.

    Ca+2(aq) + OH-(aq) --> Ca(OH)2(aq)

    Metals form positive ions. Nonmetals form negative ions. Almost all polyatomic ions are negative. The major exception is the ammonium ion, NH4+
  4. Mar 14, 2009 #3

    Thanks a lot for this very meaningful and generous reply. But just one question. How do we find the spectator ions? Ca(OH)2 isn't very soluble, so does that mean its ions won't dissociate in solution and thereby, this is the way it should be written in an ionic equation?

    Also, can Ionic equations only be applied to Ionic solutions and not Ionic solids?
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