[parachute]

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)  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.

[magician4]

I would strongly advise you to look up what the definition of "free energy " (aka "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
- the dimension of S (or ΔS ) would be (usually) J/mol*K

regards

Ingo

[Dan]

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

[Big-Daddy]

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°, ΔS_f°, and ΔG_f°?

[Dan]

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°, ΔS_f°, and ΔG_f°?

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

Δ_fH°[O₂(g)] = Δ_fS°[O₂(g)] = Δ_fG°[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

[Big-Daddy]

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

Δ_fH°[O₂(g)] = Δ_fS°[O₂(g)] = Δ_fG°[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.