1. The problem statement, all variables and given/known data This is a basic conceptual question that i encountered when i was reading the section on Entropy on the book by K. Huang on statistical mechanics, it goes like this. In a reversible isothermal expansion of an ideal gas, i know that the change in internal energy is zero, which leaves, by the use of the first law an expression that says that the heat absorbed by the system is equal to the work done, which is a violation of the second law, because it's saying that the system converts all the heat into work, so why does this is a valid transformation? 3. The attempt at a solution I think that we need to add another term that involves dissipation.