calculating standard gibbs free energy of reation

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

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

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

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Rachael_1H
Posts: 31
Joined: Fri Sep 25, 2015 3:00 am

calculating standard gibbs free energy of reation

Calculate standard gibbs free energy of a reaction for the reaction showing the decomposition of mercury (II) oxide at 298 K.

2HgO(s)---> 2Hg(l) + O2(g)

Standard reaction enthalpy of formation for HgO = -90.83
standard molar entropy HgO=70.29, Hg=76.02, and O2=205.14

When finding the standard gibbs free energy of this rxn, I believe that the answer key in our practice quiz booklet changes the Standard RXN Enthalpy of formation of HgO= +90.83

Why is the Standard RXN Enthalpy of formation of HgO made positive?

Fariha Hameed 1D
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Re: calculating standard gibbs free energy of reation

You make the value positive because you are not forming HgO but rather doing the reverse, which is decomposing 2HgO to make 2Hg and O2. Hope this helps!

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