how can heat change between system and surrounding be reversible? du=q-w' in this case, heat change, i reckon a)since u is state function, there is no change in U, thus change in u is 0 b) 0=q-w' hence q=w' , all the heat transferred is spent on doing work c) in order to make this reversible, w' must be maximum, so does the q 1)does it mean that if i would like to get a reversible change, the amount of heat change between system and surrounding must be huge enough? a small amount of heat change will be irreversible? 2) i found that in order to have a infinitesimal steps in the exchange of heat, temperature difference of the system and the corresponding surrounding has to be infinitesimal. what if the amount of q is huge enough to make a difference in the temperature of system?