Given the values of Hrxn, Srxn, and T below, determine Suniv. I understand how t

Question

Given the values of Hrxn, Srxn, and T below, determine Suniv. I understand how to calculate this.

The second part of the question asks to predict whether or not the reaction is spontaneous. I know that I have to use the rules of Gibb's Free Energy equation, where I compare the signs and values of deltaH and deltaS, but I am unsure as to what delta S to look at. I have deltaS of the reaction, and deltaS of the universe. Do I use one of these, or do I have to use deltaS of the system. If so, how do I calculate that?

Here are one of the questions:

Hrxn= 85 kJ , Srxn= 145 J/K , T=750 K

I calcualted the deltaS universe was 30 J/K. The reaction is spontaneous. I do not understand why.

Explanation / Answer

G°system = Hsystem - T*Ssystem

Then use the one of the reaction

The interesting part of Gibbs free energy is that we can know if the reaction will or will not be spontanous with NO NEED to calculate the entropy of the universe (its kind of difficult measure the entropy of the universe some times)

G°system = Hsystem - T*Ssystem

G°system = 85000 J- (750K )*145 J/K = -23750 J/mol

NOTE that in reallity, you cant use the values of H° and S° at T = 750 sicne those values are for 298°... anyways you may assume its constant for this type of exercises

Since G°system < 0, this means it is spontanous... it can happen, since entropy of the universe INCREASES in overall value (this is 2nd law of thermodynamics)

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