## 6th edition 9.65

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

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

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

Sierra Cheslick 2B
Posts: 61
Joined: Fri Sep 28, 2018 12:27 am

### 6th edition 9.65

The question reads: Which of the following compounds become less stable with respect to the elements as the temperature is raised:
(a) PCl5(g); (b) HCN(g); (c) NO(g); (d) SO2(g)?

In the solutions manual, it shows that for each compound, you have to use the deltaS for each of the elements in the compound, and then look at whether the overall deltaS is positive or negative. Why can't you just use the deltaS value for the overall compound? You get different answers, but I don't understand why it doesn't work.

Neil Hsu 2A
Posts: 61
Joined: Fri Sep 28, 2018 12:16 am

### Re: 6th edition 9.65

What the solution is doing is looking at the decomposition reaction of each molecule. In essentiality, they are looking at deltaG = deltaH - TdeltaS; if the delta S is positive, then as temperature rises, then the reaction becomes more favorable. If the delta S is negative, then as temperature rises, then the reaction becomes less favorable. From the delta S value of the decomposition reaction, we can determine the stability of the molecule. If the reaction becomes more favorable, then the molecule is more likely to decompose and is thus less stable.

Kelsey Warren 1I
Posts: 60
Joined: Fri Sep 28, 2018 12:15 am

### Re: 6th edition 9.65

The book uses the delta S values of the elements that make up the compound because the question is asking about their stability "with respect to the elements." So just use those delta S values to see which becomes less stable as temp is raised.