text: The Clapeyron equation is first derived by considering the total differential of this quantity.
full_question: The Clapeyron equation is first derived by considering the total differential of this quantity. Maxwell relations are usually used to eliminate derivatives involving this quantity, since it can't be exactly measured. The inexact differential of reversible heat is converted to an exact differential in this quantity by dividing it by temperature. The derivative of this quantity with respect to time in a closed system is always greater than or equal to zero. For 10 points, name this quantity whose increase in a system is given by the Second Law.
first_sentence: The Clapeyron equation is first derived by considering the total differential of this quantity.