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How to know the products of a chemical reaction?

  1. Sep 1, 2015 #1
    1. The problem statement, all variables and given/known data
    I am having trouble on knowing what products are formed in a chemical reaction, such as these :
    i) aluminium oxide and sodium hydroxide
    Al2o3 + 2NaOH + 3H2O -> NaAl(OH)4
    How do we know that we have to add H2O? And how we know that the product is NaAl(OH)4 ? Why can't it be NaAlO3 ?
    ii) silicon dioxide with sodium hydroxide
    SiO2 + 2NaOH-> Na2SiO3 + H2O

    How do we know that the salt is Na2SiO3 ? Why can't it be Na2SiO2 ? I'm really confused.. And why this reaction no need to add water?
    iii) Al2Cl6 + H2O -> 2Al(OH)3 +6HCl
    SiCl4 + 4H2O -> SiO2 . 2H2O + 4H2O

    Why not Si(OH)4? And why not Al2O3. H2O?

    Are there any formula for these? Or they just happen naturally just like that? If so, how do we know the products during the exams?:oldsurprised:
     
  2. jcsd
  3. Sep 1, 2015 #2

    Borek

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    Staff: Mentor

    This is tricky, and although there is a logic behind, it is blurred. Produced salt is called an aluminate and its molecules must be neutral. It contains Na+ and Al3+, so it needs two O2- for neutrality, and the simplest formula becomes NaAlO2. Unfortunately, the real product almost always contains some water, and the anion tends to polymerize, so the exact formula is pretty hard to predict, and quite often listed in different forms. I would accept both NaAlO2 and NaAl(OH)4 as a correct answer, but that's just me.

    Once you know the formula of the final product, water is just necessary to balance the equation.

    Silicon is in a group IV (carbon group), so you can assume its charge to be 4+, see what the neutrality of the sodium salt molecule suggests about the simplest formula.

    These are both very specific cases. Actually I don't like the way Al2Cl6 reaction is written, I would prefer Al2O3 as a product. Product of the second reaction is a very highly stable silica (SiO2), written here as a hydrate.

    Sadly, nature doesn't care about being easy to follow. We try to categorize the reactions and derive simple rules that would help us predict what will happen, but while they work in typical cases, they often fail when applied blindly. Al and Si are amphoteric and the rules for acids and bases reactions become blurred.
     
  4. Sep 1, 2015 #3
    Thanks a lot for the help :)
     
  5. Sep 1, 2015 #4

    epenguin

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    Good questions, very good questions IMO.
    Sound chemical education trains us to stop asking them. :oldbiggrin:

    Going to a couple of high level chemistry meetings once it seemed to me you can force to combine almost any bunch of atoms that we don't think normally sit together, perhaps I exaggerate slightly., with forcing conditions and protection.

    But then ornery chemistry is about what can easily exist, at least stable enough to observe ordinarily, reactions that go easily, preferably products you can isolate.

    One of the things we've been trained and habituated not to think of is NaOH without any water. I can see how you could make some in a dry atmosphere. But with normal (or Normal) 1 M or 5 M NaOH of the ornery laboratory there is plenty of H2O - so I would not have thought of adding it. I guess then your answer to i is you're expected to know and assume the water's there anyway.

    ii is a good question - on analogy with carbon you might expect that to exist. It does, or at least metasilicate HSiO3- ions and salts (analogous to bicarbonates) do exist apparently - but it is fairly hard to find anything out about them - which seems to be already telling you something about their stability. Apparently hydration of the oxide or dehydration of silicon acid is hard to stop at the halfway metasilicate stage. BUT someone might think, now if I can find a metal ion that will make a particularly good fit with metasilicate anion the metasilicate could crystallise and so I would stabilise metasilicate. And apparently - I never heard of it before today - cadmium metasilicate does exist and you can buy some.

    I hope this gives some idea why in chemistry you hear of certain things and not others that seem at first sight to have equal right to exist and receive attention.
     
  6. Sep 1, 2015 #5
    Thanks for the help. But to be honest, the things you said are abit too advance for my level.. I am currently studying A levels and sometimes questions ask us to write the reaction of this and that, which I have no clue on what are the products.. I guess we have to memorize them before hand..
     
  7. Sep 1, 2015 #6

    SteamKing

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    When should you study ornery chemistry, before or after organic and inorganic?
     
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