question_answer 1)

  The enthalpies of elements in their standard states are taken as zero. The enthalpy of formation of a compound: (a) is always negative                                                     (b) is always positive (c) may be positive or negative                                  (d) is never negative

Answer:

  (c) Heat of formation of a compound may be positive or negative, e.g., \[C(s)+{{O}_{2}}(g)\to C{{O}_{2}}(g);\,\,\,\,\Delta H{}^\circ =-393.5kJ\,mo{{l}^{-1}}\]\[{{N}_{2}}(g)+\frac{1}{2}{{O}_{2}}(g)\to {{N}_{2}}O(g);\,\,\,\Delta {{H}^{{}^\circ }}=+92kJ\,mo{{l}^{-1}}\]