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### How do we find out if a system is favorable?

Posted: Fri Feb 02, 2018 1:29 pm
Dr. Lavelle mentioned during Wednesday's class that we can use entropy in order to find out if a system is favorable. He said there where two ways in order to find find this out, but I was not able to catch it.

### Re: How do we find out if a system is favorable?  [ENDORSED]

Posted: Fri Feb 02, 2018 1:38 pm
To find if a reaction is favorable/spontaneous, you need to calculate ΔG. If ΔG is negative, then the reaction is favorable. There are 2 methods to calculate Gibbs free energy:

Method 1: use standard enthalpy of formation and standard entropy values of reactants and products.
ΔG°=ΔH°-TΔS°
To find ΔH° and ΔS°, you can subtract the standard enthalpies/entropies of formation of the products and subtract the standard enthalpies/entropies of formation of the reactants to get the standard enthalpies/entropies of the reaction to use in the Gibbs free energy equation.

Method 2: Hess-type approach: ΔG° of the reaction can also be calculated by subtracting the standard Gibbs free energy of the reactants from the standard Gibbs free energy of the products. You can use this approach because like enthalpy, Gibbs free energy is also a state function.

### Re: How do we find out if a system is favorable?

Posted: Fri Feb 02, 2018 1:38 pm
I’m not completely sure, but I vaguely remembering him saying spontaneous processes are favorable, but correct me if I’m wrong

### Re: How do we find out if a system is favorable?

Posted: Fri Feb 02, 2018 3:03 pm
We also learned a new equations today in lecture, such as the Van't Hoff equation, which can be used to find the G of a reaction. When G is negative, the system is favorable, or spontaneous.

### Re: How do we find out if a system is favorable?

Posted: Sun Feb 04, 2018 3:39 pm
Adding to the comment above, the system is spontaneous when delta G is negative because G is energy, and decreases in energy are spontaneous.

### Re: How do we find out if a system is favorable?

Posted: Sun Feb 04, 2018 3:46 pm
A system is favorable when it is spontaneous. Spontaneity depends on wether deltaG is negative (giving off heat) or positive (requiring heat).

### Re: How do we find out if a system is favorable?

Posted: Sun Feb 04, 2018 4:53 pm
Hey guys great discussion!
Also, if the delta(S)total is positive then the system is favorable. This is because, according to the laws of thermodynamics, the universe tends to maximize entropy.

delta(S)total = + (spontaneous process)

delta(G)system = - (spontaneous process)

Hope this helps! :))

### Re: How do we find out if a system is favorable?

Posted: Sun Feb 04, 2018 10:23 pm
Since delta G = delta H - T delta S, a system is favorable if:
- delta G is negative
- delta H is negative and delta S is positive
- delta H is negative and delta S is negative and delta H is greater than T delta S
- delta H is positive and delta S is positive and delta H is less than T delta S
A system can not be favorable when delta H is positive and delta S is negative.