If the reaction gives off heat (releases energy), then the change in internal energy change (∆U) is negative. If reaction absorbs heat, internal energy change is positive.
Because of 1st Law of Thmdcs, if energy is absorbed/released by the system, then energy is equally and oppositely released/absorbed by the surroundings. ∆Usys = -∆Usurr
In determining energy changes in the system, we want to calculate the sum of the heat exchange (q) between the sys and surr and the work done (w) on or by the system.
∆Usys = q + w
- q is positive for endothermic process, negative for exothermic
- w is positive for work done on the sys by the surr, negative for work done by sys on surr
--- q and w are NOT state functions