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How to do decompostion equations

  1. Mar 22, 2006 #1
    Ok...I'm not entirely sure how to do decompostion equations (balancing) since i just learned it today but...

    Al2O3------> ?

    I think that the oxygen becomes an O2because it is a diatomic molecule. That I understand.

    Do I have to carry the Al2 over so that the factors end up being 3Al2 + O2.....the thing is...that wouldn't work with the beginning of the equation because the first Al would be 4 while the factors Al would be 6..when I say this i mean the final equation would be like......2Al2O3----> 3Al2 + O2

    I hope that makes sense!! I need this for homework and that is how I figured the problem above...if someone could help me...that'd be amazing!!
  2. jcsd
  3. Mar 22, 2006 #2
    Nope, try again. Hint: you should start by determining if aluminum oxide is an ionic compound or a molecular compound.

    And a side note: aluminum is not a diatomic element. The diatomics are: H2, N2, O2, F2, Cl2, Br2, and I2.
  4. Mar 22, 2006 #3
    would it just be......2Al2O3-----> 2Al2 + 3O2?
    Last edited: Mar 22, 2006
  5. Mar 22, 2006 #4
    ohh shoot! i see wut i did! i think i was suppose to put the 3 in front of the O2...not the Al2...i think i did that cause i thought that thre wut affect the oxygen if i put it at the beginning
  6. Mar 22, 2006 #5
    Al2O3 is a ionic compound so it will dissociate into a cation (a metal substance) and an anion (a non-metal substance). This is:

    Al2O3 --> 2Al3+ + 3O2-
  7. Mar 22, 2006 #6
    but how does that balance? unless im totally missing the point
  8. Mar 22, 2006 #7
    It is balanced. You have equal amounts of each element on the reactants and on the products. Also same charge is present on both sides of the equation.
  9. Mar 22, 2006 #8
    ohhh so the charge takes the place of the subscript? but it cant...cause the aluminum has to have a subscript of 2.....no.....shoot...im confused!
  10. Mar 22, 2006 #9
    If you want a tip the charge of O2- takes the subscript of Al (in absolute value, of course) and the charge of Al3+ takes the subscript of O. But think and try to figure out why.
  11. Mar 22, 2006 #10
    yeah so i got that part....that the 2- acts for the Al....is the reason why related to the bonding that occurs? the 3+ from the Al couldnt work with the O cause the O has to be a diatomic molecule?
  12. Mar 22, 2006 #11
    Not quite. I'll give you an example...write the formula for potassium sulfide. KxSy

    Think about the charges of K and S. K has a 1+ charge and S has a 2- charge. You should've been taught that you can figure that out by looking at the periodic table and counting to see how many electrons it needs to gain or lose to have an electron configuration of a noble gas.

    The positive charges and negative charges have to be equal so that the compound is neutral. Thus, x = 2 and y = 1, so the answer would be K2S. Do you see why?

    Here's another example, but this one invovles oxygen. Write out the formula for calcium oxide. Ca has a 2+ charge and oxygen has a 2- charge, therefore, it will simply be CaO.

    I hope that didn't confuse you at all.
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