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Question
The enthalpies of elements in their standard states are taken as zero. The enthalpy of formation of a compound ______.
Options
is always negative
is always positive
may be positive or negative
is never negative
Solution
The enthalpies of elements in their standard states are taken as zero. The enthalpy of formation of a compound may be positive or negative.
Explanation:
Heat of formation of a compound may be positive or negative, e.g.,
\[\ce{C(s) + O2(g) -> CO2(g); ΔH° = - 393.5 kJ mol^{-1}}\]
\[\ce{N2(g) + 1/2 O2(g) -> N2O(g); ΔH° = + 92 kJ mol^{-1}}\]
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