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Extracting Metals + Energy Diagrams


Hi,

I am having difficulty in understanding how this diagram calculates Gibbs energy??? It's just given like this in my book, without data values. My homework consists of similar diagrams which show Gibbs energy so I was just wondering...
(edited 9 years ago)
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Original post by ps1265A

Hi,

I am having difficulty in understanding how this diagram calculates Gibbs energy??? It's just given like this in my book, without data values. My homework consists of similar diagrams which show Gibbs energy so I was just wondering...


Gibbs free energy is just like any other energy and Hess' cycles can be applied.

Energy is an extensive property (depends on amount reacting) and must be conserved.

The Gibbs free energy change in route 1 must equal the Gibbs free energy change in route 2.
(edited 9 years ago)
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ps1265A
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Original post by charco
Gibbs free energy is just like any other energy and Hess' cycles can be applied.

Energy is an extensive property (depends on amount reacting) and must be conserved.

The Gibbs free energy change in route 1 must equal the Gibbs free energy change in route 2.


Oh, thanks! So why is enthalpy of formation used in the cycle? Or is it better to ask, what do the values actually mean?
Original post by ps1265A
Oh, thanks! So why is enthalpy of formation used in the cycle? Or is it better to ask, what do the values actually mean?


It isn't enthalpy of formation, it's standard Gibbs free energy of formation, ΔGºf.
(edited 9 years ago)
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ps1265A
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Original post by charco
It isn't enthalpy of formation, it's standard Gibbs free energy of formation, ΔGºf.


We haven't learn this at all! What does the other number signify?

And is there a way to work out ΔGºf using enthalpy/entropy changes or is there a formula or is it given in a data book?
Original post by ps1265A
We haven't learn this at all! What does the other number signify?

And is there a way to work out ΔGºf using enthalpy/entropy changes or is there a formula or is it given in a data book?


The other number?

The left hand value gives the standard free energy of formation of iron(III) oxide. It is multiplied by 2 because there are 2 moles of iron(III) oxide created in the equation.

You can work out standard free energy changes using the formula:

ΔGºf = ΔHºf - TΔSºf
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ps1265A
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Original post by charco
The other number?

The left hand value gives the standard free energy of formation of iron(III) oxide. It is multiplied by 2 because there are 2 moles of iron(III) oxide created in the equation.

You can work out standard free energy changes using the formula:

ΔGºf = ΔHºf - TΔSºf


The number on the right.

And thanks, it makes so much sense now!
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ps1265A
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Original post by charco
The other number?

The left hand value gives the standard free energy of formation of iron(III) oxide. It is multiplied by 2 because there are 2 moles of iron(III) oxide created in the equation.

You can work out standard free energy changes using the formula:

ΔGºf = ΔHºf - TΔSºf


The number on the right?

ΔGº = -ΔHºf + x
(edited 9 years ago)
Original post by ps1265A
The number on the right?

ΔGº = -ΔHºf + x


You mean the 'x' in your equation?

That's the product of the absolute temperature and the entropy change of the system.

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