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Homework Help: Figuring Subscripts in Single Replacement Equations

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

    When figuring out the product from a reaction using single replacement type of a reaction, we believe we understand what the elements and compounds will be produced but can't figure out how the subscripts are calculated/determined in some examples.

    #1 ZnS + O2 ---> ZnO + S
    #2 Fe + HCl ---> FeCl3 + H2
    #3 Ca + H2O ---> Ca(OH)2 + H2

    In #1, how do you get no subscript for the "S" element in the reactant?
    In #2, how do you get a subscript of 3 for the "Cl" portion of the compound in the reactant?
    In #3, how do you get the "OH" portion of the compound in the reactant with a subscript of 2?

    Are these things you just have to memorize or is there a way to figure them out.

    2. Relevant equations

    When dealing with a cation element or metal reacting with a compound: A + B C = B + A C
    When dealing with a anion element or non-metal reacting with a compound: A + B C = B A + C

    3. The attempt at a solution

    We based our attempts on solutions from other examples.

    For example, in the equation below we used the "cation" form from above to determine the product since Cu has a positive charge. Then, we figured the subscript for Cu(NO3) becomes "2" based on directions in the homework. In short, we determined that Cu has +2 charge and NO3 has a -1 charge. To "balance" them you turn it into 2 molecules to get the charge to -2.

    Cu + AgNO3 = Ag + Cu(NO3)2

    However, not sure that makes sense since we thought that compound was supposed to be ionic not neutral (or balanced).

    Figuring Subscripts in Single Replacement Equations
  2. jcsd
  3. Dec 1, 2015 #2


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    Staff: Mentor

    You are putting the cart before the horse. It is not about reactions, it is about compounds and their formulas, as these are what matters. Reaction is secondary (it is balanced to follow the formulas).

    First of all, formulas of compounds assume they are neutral. Then, you have to remember some of the common charges. For example, Ca(OH)2 - Ca in all compounds has a charge of 2+, OH in hydroxides has always charge of -1, so you have Ca2+ and OH-, the only way of making it neutral is to combine 1 Ca2+ with two OH-, yielding Ca(OH)2.

    Fact that the compound is ionic doesn't change the neutrality of the compound - Na+ and Cl- combine producing a neutral NaCl.

    Zn is in all compounds Zn2+, S alone means it is a sulfide, S2-, hence ZnS (ZnO is very similar - oxygen in oxides is in most cases present as O2-, hence ZnO, the difference between S2- and O2- is that the former can be observed in solutions, while the latter is way too reactive and reacts with water, producing OH-, so it is never observed alone, only present in solids).

    Fe is much more difficult, as it can be present either as Fe2+ or Fe3+, predicting which it is is something that you will learn later. Actually if you dissolve Fe in HCl solution you will produce FeCl2, not FeCl3.

    Cu is most often present as +2. Ag is almost always +1, so as NO3- has a charge of -1, both formulas you wrote are correct (but the reaction is not balanced).
  4. Dec 1, 2015 #3
    A couple questions. First in regards to this:

    How can a compound be ionic and neutral? Maybe I don't understand the distinction in definition. I thought an ion is something with a charge, either positive or negative. So if it's neutral, doesn't it have a net zero charge? Meaning it's not an ion?

    I'm looking at it this way (which is apparently incorrect):
    Using Na+ and Cl-.
    Each element is an ion because each has a charge, but when together they make a compound that has net zero charge - the charge from the Na nets against the charge from the Cl. Therefore, wouldn't it no longer have charge as a compound and not be an ion?
  5. Dec 1, 2015 #4


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    Staff: Mentor

    Ion is something charged, yes. Ionic compound is not an ion. Ionic compound is something you get when you combine ions of opposite charge. The ions in the ionic compound are still charged, but the molecules are not.

    Actually in the case of ionic compounds we typically don't speak about single molecules, rather about the crystal in which all ions are present and the crystal itself is neutral. But the logic behind is identical.
  6. Dec 1, 2015 #5
    Ok. Thanks again for your help
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