## Homework Problem 9.81

$\Delta G^{\circ}= \Delta H^{\circ} - T \Delta S^{\circ}$

$\Delta G^{\circ}= -RT\ln K$

$\Delta G^{\circ}= \sum \Delta G_{f}^{\circ}(products) - \sum \Delta G_{f}^{\circ}(reactants)$

Lily Guo 1D
Posts: 64
Joined: Fri Sep 29, 2017 7:03 am

### Homework Problem 9.81

"Which is the thermodynamically more stable iron oxide in air, Fe3O4(s) or Fe2O3(s)? Justify your selection." The solutions manual says to look at the reaction converting Fe3O4 to Fe2O3, calculate the delta G for that, and then based on the delta G/spontaneity of the reaction, you'll know which compound will form spontaneously and is therefore more thermodynamically stable. However, I tried doing this problem by finding the delta G for the formation reactions of each compound (one reaction forming Fe3O4 and one reaction forming Fe2O3). When I did that, I got the wrong answer. Does anyone know why the latter method doesn't work?

Tim Foster 2A
Posts: 73
Joined: Fri Sep 29, 2017 7:07 am

### Re: Homework Problem 9.81

Hey Lily
Fe3O4(s) + O2(g) = Fe2O3(s) This is the unbalanced equation for the oxidation of Magnetite to Hematite. If we balance the equation, we get
4Fe3O4(s) + O2(g) = 6Fe2O3(s). For your method to work, you would need to compare the deltaG of formation of six moles of Hematite to 4 moles of Magnetite; you will find that Hematite is more thermodynamically stable. Hope this helps!