question_answer

The heat of formation is the change in enthalpy accompanying the formation of a substance from its elements at 298 K and 1 atm pressure. Since the enthalpies of elements are taken to be zero, the heat of formation \[(\Delta H)\] of compounds        [NCERT 1978; KCET 1993]

A)                 Is always negative

B)                 Is always positive

C)                 May be negative or positive

D)                 Is zero