## 11.19

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

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

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

Caitlin Mispagel 1D
Posts: 47
Joined: Tue Oct 10, 2017 7:13 am

### 11.19

Can someone explain how to do 11.19? The question asks Calculate the equilibrium constant at 25 C for each of the following reactions, using data in Appendix 2A:
(a) the combustion of hydrogen: 2 H2(g) + O2(g) <--> 2 H2O(g). I tried using the equation deltaGrxn = -RTlnK and using the free energy of formation of water but I am not getting the right answer.

Kathleen Vidanes 1E
Posts: 62
Joined: Fri Sep 29, 2017 7:07 am

### Re: 11.19

You are correct in that you must use deltaG °rxn = -RTlnK and the free energy of formation of H2O.

1. Make sure that you are using the free energy of formation of water as a GAS / VAPOR (-228.57kJ/mol).
2. Multiple this number by 2, since there is a coefficient of 2 in the balanced chemical equation. Your answer for delta G ° should therefore be -457.14 kJ/mol, or -457140 J/mol.
3. Then use the equation deltaG °rxn = -RTlnK, and solve for ln K, which is 184.42. Make sure your units match and cancel. I used Joules for deltaG°rxn, so that they cancel with the Joules in R=8.314 J/ K.mol .
4. To get K by itself, raise both sides so that they are a power of e. e and ln will cancel and you should be left with K = e^184.42 = 1x10^8

Hope this helps!