how do I find the ÄH for C2H4 + O3 --> CH3CHO + O2?

Answer 1

If you want it at 25^o look up delta H_f for CH3CHO, O2, O3, and C2H4. Then
delta H_reaction = (delta H_products - delta H_reactants).
delta H_f for O2 will be zero

Answer 2

where would i look up the delta Hf for CH3CHO, O2, O3, and C2H4? online?

Answer 3

Be sure and read the prefix for the tables because they list different parts of the table in different units.

http://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_(data_table)