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Thermodynamics — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryThermodynamics1 Mark
MCQ
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.
  • ✓
    May be positive or negative.
  • D
    Is never negative.
✓

Answer

Correct option: C.
May be positive or negative.
Heat of formation of a compound may be positive or negative, e.g.,
$\text{C}(\text{s})+\text{O}_2(\text{g})\rightarrow\text{CO}_2(\text{g});$
$\Delta\text{H}^\circ=-393.5\text{kJ}\ \text{mol}^{-1}$
$\text{N}_2(\text{g})\frac{1}{2}\text{O}_2(\text{g})\rightarrow\text{N}_2\text{O}(\text{g});$$\Delta\text{H}^\circ=+92\text{kJ}\ \text{mol}^{-1}$

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