For the reaction: #2H_2O + 137 kcal -> 2H_2(g) + O_2(g)# how many kcal are needed to form 2.00 moles #O_2(g)#?
Start by taking a look at the thermochemical equation given to you
#2"H"_ 2"O"_ ((l)) + "137 kcal" -> 2"H"_ (2(g)) + "O"_ (2(g))#
Notice that this thermochemical equation describes an endothermic reaction. You can say that because you have a heat term added to the reactants' side.
This essentially tells you that heat is needed in order for the reaction to take place. This can also be shown by using the enthalpy change of reaction,
#DeltaH_"rxn" = +"137 kcal"#
The enthalpy change of reaction is positive because the reaction is endothermic
Now, take a look at the stoichiometric coefficients. You can say that it takes
The problem wants you to figure out how much heat is needed in order for the reaction to produce
You can use the fact that
#2.00 color(red)(cancel(color(black)("moles O"_2))) * overbrace("137 kcal"/(1color(red)(cancel(color(black)("mole O"_2)))))^(color(blue)("given by the thermochemical equation")) = color(darkgreen)(ul(color(black)("274 kcal")))#
The answer is rounded to three sig figs.