Question:

Given
(i) 2Fe2O3(s) → 4Fe(s) + 3O2(g); \(\Delta_r G^\circ = +1487.0\ \mathrm{kJ\ mol^{-1}}\)
(ii) 2CO(g) + O2(g) → 2CO2(g); \(\Delta_r G^\circ = -514.4\ \mathrm{kJ\ mol^{-1}}\)
Free energy change, \(\Delta_r G^\circ\) for the reaction
2Fe2O3(s) + 6CO(g) → 4Fe(s) + 6CO2(g) will be :

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Add equation (i) and three times equation (ii); the O2 cancels, and the free energies add the same way.
Updated On: Nov 11, 2026
  • \(-112.4\ \mathrm{kJ\ mol^{-1}}\)
  • \(-56.2\ \mathrm{kJ\ mol^{-1}}\)
  • \(-168.2\ \mathrm{kJ\ mol^{-1}}\)
  • \(-208.0\ \mathrm{kJ\ mol^{-1}}\)
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The Correct Option is B

Solution and Explanation

Target: 2Fe2O3 + 6CO → 4Fe + 6CO2.
Take (i) as written: 2Fe2O3 → 4Fe + 3O2, \(\Delta G_1 = +1487.0\).
Take 3 × (ii): 6CO + 3O2 → 6CO2, \(\Delta G_2 = 3(-514.4) = -1543.2\).
Adding the two, 3O2 cancels and we get the target reaction.
\(\Delta_r G^\circ = 1487.0 - 1543.2 = -56.2\ \mathrm{kJ\ mol^{-1}}\).
Answer: (2) \(-56.2\ \mathrm{kJ\ mol^{-1}}\)
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