posted by mel .
Combustion reactions involve reacting a substance with oxygen. When compounds containing carbon and hydrogen are combusted, carbon dioxide and water are the products. Using the enthalpies of combustion for C4H4 (-2341 kJ/mol), C4H8 (-2755 kJ/mol), and H2 (-286 kJ/mol), calculate H for the reaction.
C4H4(g) + 2 H2(g) C4H8(g)
What they want you to do here is to use Hess' Law. Write the reactions as combustion reactions.
C4H4 + 5O2 ==> 4CO2 + 2H2O -2341 kJ/mol
2H2 + O2 =>2H2O -286 kJ/MOL= -572 rxn
C4H8 + 6O2 ==> 4CO2 + 4H2O -2755 kJ/mol
Add the sum of equation 1 and equation 2 to the reverse of equation 3 to give you the reaction you have written (You shouldn't omit the arrow--it makes us guess where the arrow should be). Delta H for the reaction you have written will then be the sum of delta H for equation 1 and equation 2 + delta H for equation 3 (with the sign changed since you changed the direction of the reaction). I've almost worked the entire problem for you.
Post your work if you get stuck.