So the question is: When 2.00 mol of SO2 reacts completely with 02 in a constant pressure reactor, the only material left in the reactor once the reaction has stopped is SO3 at 25 degrees celsius. The constant pressure in the reactor was 101kPa and 198 kJ of energy is released as heat. Assume Psys=Pext throughout the process. Calculate deltaH, deltaU, q and w per mole of O2 consumed for this reaction. So I know q = deltaH = -198kJ since its a constant pressure process and I know to solve for deltaU I use: deltaH = delta U + delta(PV) and in the solution apparently the delta(PV) = RTdelta(n). What I don't understand is why the temperature is assumed to be constant at 25 degrees celsius.