PS3, problem 5b/c

Here's a slightly clarified explanation of this part of the problem. Here, by constant pressure process, we mean that the entire system is at the same pressure throughout the process. That is, even though it may not be at equilibrium, the pressure that it exerts on the container walls remains constant. This should be all that you need to simplify the first law (differential form) to show that constant pressure processes are also constant enthalpy processes. Note that we don't need to assume quasi-static or reversible behavior; we only need to know that the pressure is constant.
MSS