And that's because there is no exchange in mass. So, if the number of moles stay the same, if the temperature is constant and pressure is constant, and according to the ideal gas law, the volume remains constant, so there is no volume change. So, according to the formula for change in enthalpy, if there's no volume change, then this guy, the PV work done, is zero. And so we can approximate the change in enthalpy to simply equaling change in internal energy or change in energy or heat. Now, before we jump into the formula, let's look at entropy and entropy. So what is entropy?