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Hess's Law for Gibbs Energy
Concepts tested here
- Combining Gibbs energies
All Questions
2000 AIPMT 1 question
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$2Zn + O_2 \rightarrow 2ZnO$, $\Delta G^\circ = -616$ J
$2Zn + S_2 \rightarrow 2ZnS$, $\Delta G^\circ = -293$ J
$S_2 + 2O_2 \rightarrow 2SO_2$, $\Delta G^\circ = -408$ J
$\Delta G^\circ$ for the following reaction is : $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$The required reaction = (first) $-$ (second) + (third):
$2Zn + O_2 \rightarrow 2ZnO$ (–616)
$2ZnS \rightarrow 2Zn + S_2$ (+293, reversed)
$S_2 + 2O_2 \rightarrow 2SO_2$ (–408)
Adding: $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$
$\Delta G^\circ = -616 + 293 - 408 = -731$ J