The combined statement of the first and second laws for the change in enthalpy of unary single-phase system may be written: dh' = tds' +v'dp +udn use this result to write an expression for the change in enthalpy of a two-phase (alpha + beta) system. if the entropy, pressure, and total number of moles are constrained to be constant, then the criterion for equilibrium is that the enthalpy is a minimum. paraphrase the strategy used to deduce the conditions for equilibrium in an isolated system to derive them for a system constrained to constant s', p and n. what happens to the condition for mechanical equilibrium?