This isn't a homework question per se, but more of a coursework question. Specifically, I'm a bit at a loss as to how to go about learning a particular section of the coursework for a physical chemistry (i.e. thermochemistry) class I am taking. The section in question is that pertaining to calculations involving entropy (S). For example, take the following question: The heat capacity of a given ideal gas can be expressed as shown below. CP = (10 + 0.006T) J K-1 mol-1 Calculate the change in entropy when 4 moles of this gas are isobarically heated from 200 K to 300 K. Step 1 of the solution (as given by the solutions manual): ΔS = ∫dqrev/T Where are they getting this equation from? But more importantly, what is it actually saying, conceptually? I haven't done calculus in years, so I am more than rusty when it comes to integration, meaning I don't even understand what "dqrev" means. Thus, I can't just look at the given expression and figure it out for myself. The actual mechanics of working with integrals, I can figure out on my own; the conceptual part, and where equations are being derived from, not so much. If someone could help me out, it would be appreciated. Note that I am not looking for a solution to the problem given, as I already have that. Rather, I am requesting an explanation as to why the steps that are involved actually work (an explanation of the integration shown above would be a great start). For the record, the solution ends up converting dqrev to CpdT, and integrates from there (200 K to 300 K). But again, I don't know what dqrev signifies to begin with, so I can't really make sense of any further steps. I do know, however, that q signifies heat transfer in other questions, and "rev" signifies that it is a reversible process (i.e. theoretical), while T is obviously temperature.