[Rutherford]

I know how to balance chemical equations, but I really don't understand how to know the product of some, for me, very difficult equations if only the reactants are given, for example:
H2S+K2Cr2O7+H2SO4--->
Cu+HNO3(concetrated)--->why the products aren't Cu(NO3)2+H2
Please help me, I have dozens of examples but I can't solve a single one.

[Arkcon]

Its very hard to predict what will happen,  There are some facts you can use, like a table of electronegativity, and an evolved gas or an insoluble solid product can give you hints.  But historically, the information was determined by trial and error.  If you've been assigned these problems, you should have some more information, like that it is a re-dox reaction, or some hint as to one product.  As for the second one, you can read more about it here to see what I mean: http://www.angelo.edu/faculty/kboudrea/demos/copper_HNO3/Cu_HNO3.htm

[Rutherford]

I am preparing for an exam and I won't be able to use any table. The first reaction is a re-dox one (I forgot to write it), so are there some rules for guessing the product? By the way, thank you for the link!

[Arkcon]

Well, red-ox is a good guess.  Some of those chemicals are well known oxidizers, and at least one is a well known reducing agent.  Maybe you were supposed to memorize those?  Now, given that something ends up oxidized, and something else is reduced, you know the charge on the atom will change ... but still, unless you're called on to memorize all these outcomes, your exam will have to give you some hints.