Given a reaction that was exothermic as written, what enthalpy change would apply to the REVERSE reaction?
You mean the reverse of say a combustion reaction?
This is the standard combustion reaction. Now, ordinarily,
The reverse is simply:
So the take home message: if the reaction as written is exothermic, in the REVERSE direction it is necessarily endothermic. Conservation of energy laws assure this outcome. Come back for another serve if you are not completely sure of what I am saying.