Textbook Question 4J.13

[Erika Patel 3I]

This problem asks us to determine which of the compounds are stable with respect to decomposition into their elements under standard conditions. I wrote my first equation as PCl₅ (g) P(s) + 5/2 Cl₂ (g). However, the textbook has the equation reversed. I am confused why that would be the case given that the question asks for decomposition, which is a compound becoming its elements. Could someone explain why we set the problem up like this?

[806145776]

for this we would compare the standard gibbs energy of formation for PCl5 if it is negative then it is spontaneous in the elements to the PCl5 direction and if it is positive then it is spontaneous in the other direction so the PCl5 would decompose into its constituent elements hence by seeing its gibbs free energy of formation we can figure out if it would decompose or not based on its sign.