I'm so confused!

Al + O2 -> Al2O3

This is supposed to be a balanced equations but how did it go from 2 oxygens to 3?

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  1. It's the FORMULA. Don't confuse balancing with writing a formula.
    Aluminum is Al.
    O2 is oxygen and it is almost always written as a diatomic molecule. Others, in their free state. written as diatomic molecules, are H2, O2, N2, F2, Cl2, Br2, I2 (and their cousins S8 and P4).
    For aluminum oxide, the valence of Al is +3, the valence of oxygen is -2 (in most cases), so the formula for aluminum oxide is Al2O3.
    Rule 1 in balancing equations is:
    You may NOT change any subscript to balance an equation. You may change coefficients ONLY.
    And no, the equation is not balanced. Why can I say that? Because we have 1 Al on the left and 2 on the right. Not balanced. We have 2 O on the left and 3 on the right. Not balanced. So how do we balance it.
    First we look on the right and we see 2 Al on only 1 on the left; therefore, we place a 2 in front (as a coefficient) of Al. That balances Al (we will need to change that later). Then we look at the 3 O on the right and 2 O on the left and say how do we do that. There is no integer I can multiply either by to get the other. So we use something that will get a common multiple. For example, if I place a 2 as a coefficient for Al2O3, that will give me 6 O atoms on the right. That lets me place a 3 as a coefficient for O2 on the left to have 6 O atoms on the left. Having done all that to balance the oxygen atoms, I realize I messed up on Al, which I already had done. But not to worry. We can fix that. Having O done, we look and see we need 4 Al so we can change the 2 Al on the left to 4 Al. The final equation looks like this.
    4Al + 3O2 ==> 2Al2O3. Now we check it.
    4 Al on the left and 4 on the right.
    6 O on the left and 6 on the right. All done. Let me know if there is anything you don't understand.

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  2. Thanks! I feel pretty stupid now..
    I was also curious about predicting products. How on EARTH would you do that when there are so many products?

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    posted by Jessica
  3. There are a few rules that govern many products. Some we just memorize but most are governed by sets of rules. For example, there are
    synthesis reactions (the 4Al + 3O2 ==> 2Al2O3 is a synthesis reaction; i.e., we take two atoms and make a molecule.)
    decomposition reactions. An example is
    2HgO ==> 2Hg + O2 where one substance breaks down to form two or more substances. Actually, you will see this is just the reverse of the synthesis reaction.)
    single replacement reactions.
    2Al + 6HCl ==> 2AlCl3 + 3H2 where a single substance, usually a metal, replaces the H in a compound.
    double displacement reactions.
    AgNO3 + NaCl ==> AgCl + NaNO3 where the positive and negative ions simply change places. The Ag goes with the Cl and the Na goes with the NO3^-.
    Acid + base gives a salt + water.
    There are others but you will learn them as you. So we try to learn rules to write equations (essentially predicting products) to keep from memorizing thousands of reactions. You're right! There are thousands.

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