Enthalphy problems?

Question

1). Determine ∆H˚ for the reaction

Zn(solid) + 2H^+(aqueous) -----> Zn^2+(aqueous) + H2(subscript)(gas)

2) From the following equations and enthalphy change:

a) (∆H˚ rxn= -393.585) C(graphite) + O2(subscript)(gas) -----> CO2(subscript)(gas)

b) (∆H˚ rxn= -2858) H2(subscript)(gas) + ½O2(subscript)(gas) -----> H20(subscript is 2)(liquid

c) (∆H˚ rxn= -2598.8) 2C2H2(subscript are 2nd and 3rd "2")(gas)+5O2(subscript)(gas) -----> 4CO2(subscript)(gas) + 2H2O(subscript is 2nd "2")(liquid)


3) Calculate the ∆H˚ of C2H2(subscript is 2) from its 2C(graphite) + H2(subscript)(gas) -----> C2H2 (subscript is 2)(gas)

questions:
1) what is the ∆H˚ of the following products and reactants

∆H˚=?

2) somewhere in the product or reactant is missing or unknown! so we must use the ∆H˚ in order to solve this! but i dont know which is unknown product/reactant!

3) problem 3 is somewhere the same at problem 1 but here is graphite and i dont know what is the value of that!

unfortunately i dont have the table of enthalphies... can someone send me a copy from the internet pls

Answer 1

For (1), you don't tell us what input data are you are allowed, and for (2) and (3), it looks as if (2) is your data input and (3) is the actual question.

Anyway, here is how to do it:

For any reaction, DeltaH0(rxn) = DeltaHof{products} - DeltaH0f{reactants}

If you have any set of reactions A, B, C such that carrying out A and B adds up to carrying out C, then

DeltaH0(C) = DeltaH0(A) + DeltaH0(B)

This is just a special case of conservation of energy.

Finally, if you reverse the direction of the reaction, DeltaH0 simply changes sign.

So you need to find some way of adding up (or subtracting) the reactions in (2) to give you the reaction in (3), and combining their enthalpies, to give you the answer.

I hope this removes the mystery.