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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: parachute on September 24, 2013, 09:38:54 AM

Title: Gibbs Free Energy Problem
Post by: parachute on September 24, 2013, 09:38:54 AM
Use the free energies of formation from standard thermodynamics tables to calculate the standard free-energy change for the decomposition of hydrogen peroxide in the following reaction:

2H2O2 (g)  :rarrow: 2H2O (g) + O2 (g)

Calculations:

ΔS°rxn = [2mol (S°f H2O (g)] + [1mol (S°f O2 (g)] = [2mol (S°f H2O2 (g)]

ΔS°rxn = [2mol (S°f (-228.6kJ/mol)] + [1mol (S°f (163.2kJ/mol)] = [2mol (S°f (-105.6kJ/mol)]

ΔS°rxn = (-457.2) + (163.2) - (-211.2)

ΔS°rxn = -82.8 kJ

I would just like reassurance to my calculations so I know I'm doing all the problems the right way.
Title: Re: Gibbs Free Energy Problem
Post by: magician4 on September 24, 2013, 10:12:58 AM
I would strongly advise you to look up what the definition of "free energy " (aka "Gibbs free energy" (http://en.wikipedia.org/wiki/Gibbs_free_energy))  is

furthermore , pls. be informed that

- free energy is labelled G ( i.e. the change of free energy is ΔG )
- the dimension of G  (usually) is kJ/mole (or any other equivalent relation of energy, amount of substance)
- S ( or, for that matter ΔS) stands for (change of) entropy (http://en.wikipedia.org/wiki/Entropy)
- the dimension of S (or ΔS ) would be (usually) J/mol*K

regards

Ingo
Title: Re: Gibbs Free Energy Problem
Post by: Dan on September 24, 2013, 10:20:07 AM
In addition, your value for Δ?[O2(g)] is incorrect. Where is your data from?
 
ΔH[O2(g)] = ΔS[O2(g)] = ΔG[O2(g)] = 0
Title: Re: Gibbs Free Energy Problem
Post by: Big-Daddy on September 24, 2013, 04:16:15 PM
Quote from: Dan on September 24, 2013, 10:20:07 AM
In addition, your value for Δ?[O2(g)] is incorrect. Where is your data from?
 
ΔH[O2(g)] = ΔS[O2(g)] = ΔG[O2(g)] = 0

Are you sure this true for S°, ΔSf°, and ΔGf°?
Title: Re: Gibbs Free Energy Problem
Post by: Dan on September 25, 2013, 02:46:15 AM
Quote from: Big-Daddy on September 24, 2013, 04:16:15 PM
Quote from: Dan on September 24, 2013, 10:20:07 AM
In addition, your value for Δ?[O2(g)] is incorrect. Where is your data from?
 
ΔH[O2(g)] = ΔS[O2(g)] = ΔG[O2(g)] = 0

Are you sure this true for S°, ΔSf°, and ΔGf°?

In a rush I forgot to specify these are standard changes of formation:

ΔfH°[O2(g)] = ΔfS°[O2(g)] = ΔfG°[O2(g)] = 0

Remember that all of these changes refer to formation from the constituent elements in their standard state. The standard state of elemental oxygen is O2(g), so there is no change in any of these processes.

I never suggested that S° = 0


http://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_(data_table)
http://en.wikipedia.org/wiki/List_of_standard_Gibbs_free_energies_of_formation
Title: Re: Gibbs Free Energy Problem
Post by: Big-Daddy on September 25, 2013, 07:48:50 PM
Quote from: Dan on September 25, 2013, 02:46:15 AM
In a rush I forgot to specify these are standard changes of formation:

ΔfH°[O2(g)] = ΔfS°[O2(g)] = ΔfG°[O2(g)] = 0

Ah of course. Formation is always formation from elements, and O2 is itself the elemental form so all thermodynamic values of formation for O2 will be 0.