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Homework Help: How do you find the product of a chemical reaction?

  1. Feb 21, 2013 #1
    1. The problem statement, all variables and given/known data

    In a general sense, how do you determine what the product will be in a chemical reaction? In the first example, a gas evolution, you combine the cation of each compound and pair it with the anion of the other:

    HNO3(aq) + Na2CO3(aq) ----> H2CO3(aq) + NaNO3(aq)

    However, in other examples, its less intuitive. I understand what synthesis, decomposition and displacement reactions are, more or less...But I still cant figure out why the results are the way they are. For instance, the next example shows:

    CaO(s) + CO2(g) ----> CaCO3(s)

    Here it shows a synthesis reaction, rather than the cation and anion mix and match (Double displacement). And the last example shows a similar one to the CaCO example:

    SO3 + H2O ----> SO + H2O3

    In this example, they did use double displacement rather than what they used for the CaCO reaction.

    Why is that? And how does one determine what the result will be by looking at the reactants?

    2. Relevant equations

    Synthesis ---> A+B = AB
    Decomposition ---> AB = A+B
    Displacement ---> A+BC = AC+B
    Double displacement ---> AB+CD = AD+CB

    3. The attempt at a solution
  2. jcsd
  3. Feb 21, 2013 #2


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    I don't know what H2O3 is.

    I do know that SO3 + H2O --> H2SO4, otherwise known as sulfuric acid.

    You talk about examples, as if you found these reactions in some textbook.
    Could you provide the reference, please?
  4. Feb 22, 2013 #3
    The problem isnt what the resulting compounds are, but rather how to find out what they are.

    The book is "Introductory Chemistry" by Nivaldo J. Tro.

    The examples are taken from chapter 7 which discusses chemical reactions. Lets say I didnt give you the product section of the reaction, would you be able to tell me what the products would be based off of the reactants?
  5. Feb 22, 2013 #4


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    Many chemical reactions fall into a certain type. I think your textbook is trying to show how one would determine which type of reaction is present, which would determine the products of the reaction.

    I'm not sure what the author was trying to show with the SO3 reaction. The compound H2O3 is extremely unstable, and breaks down within a matter of minutes. It's much more likely that H2SO4 will result, and this compound is quite stable.
  6. Feb 23, 2013 #5
    But I still dont know how to find out what the product(s) would be from certain reactants. How do you know when to use synthesis, displacement, double displacement or decomposition? If I looked on the internet for random reaction equations and find:

    Fe + Cl2 ---> ?

    You could deduce that it wouldnt be a double displacement or decomposition reaction because they are free elements and it doesnt make physical sense. So that reaction is kind of intuitive, it stands out and appears to be a synthesis reaction. But what about reactants that are compounds:

    KMnO4 + HCl ---> ?

    How do you find out what this reaction will create? It doesnt immediately stand out, It would seem a double displacement would take effect, but how can you be sure? How do you know it doesnt decompose or do something else?

    And I cant solve questions like this:

    K4Fe(CN)6 + H2SO4 + H2O ---> ?

    Becuase I dont know what happens and why it happens up there ^.
  7. Feb 24, 2013 #6


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    The products of any reaction are not always discernible from the reagents. There are cases where the products are different depending on whether one of the reagents is dilute or concentrated. In another case, the reaction may differ according to whether it takes place in an acidic medium (even though there is no mention of acid in the equation). Or whether it takes place under high pressure, etc.
  8. Feb 24, 2013 #7


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    Staff: Mentor

    There is no simple answer. There are some classes of reactions, there are properties that help predict the outcome, there are things that you have to remember. The more you know, the more experience you have, the more obvious the reaction results are - and then there are surprises, when things go completely different.

    Note: it is not fault of chemistry (science), it is just that the world is made this way.
  9. Feb 25, 2013 #8
    I see. From what I've learned, there are many cases such as precipitation reactions which form a solid from 2 aqueous solutions, usually determined by a table of soluble/insoluble compounds/elements.

    Combustion reactions generally involve O2 and form CO2 and H2O.

    Acid-base reactions involve and acid and a base of course which generally create water and a salt.

    Gas evolution reactions form from doing double displacement on the reactants and checking to see if the product would form one of many unstable compounds which would then decompose into things like H2O and CO2.

    The above circumstances could help in finding the products, but like you guys have said, you sometimes cannot determine the products from their reactants. What if the reaction in question was under standard temperature and pressure, an ideal situation I guess you could say. Would you then be able to find the product(s)?

    Another small example would be:

    2H2(g) + O2 ---> 2H2O(g)


    CH4(g) + 2O2 ---> CO2(g) + 2H2O(g)

    The first one can be determined rather easily, but the second does not involve displacement or double displacement. You take Oxygen and combine it with both C and H. Is there a way to find out what they would be, whether it be mathematical in nature, or based off of higher chemistry knowledge? Its just hard to believe that the only way to find them - reliable or otherwise - is to do a physical experiment.
  10. Feb 25, 2013 #9


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    In the end it is nature that is always right, not us.
  11. Feb 26, 2013 #10
    The SO3 equation is part of an acidic oxide reaction, acidic oxides react with hydroxide ions to produce oxyanions (negative ions of the element in the equation and oxygen) and water molecules. If soluable in water they react to product oxyacids (acids consisting of the element in the equation combined with hydrogen and oxygen)

    this is how you can get equations such as...

    SO3 + H20 ---> H2SO4
    SO3 + 20H- ---> SO43- + H20
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