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| For a constant temperature process from eq.(21.22), |
| dG = dH –TdS |
| Since dH =dq +dw + d (PV) from equation eq.(21.9), we have |
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| dG = dq – PdV + dw ne + PdV +VdP– TdS |
(21.37) |
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| = dw ne +VdP |
(21.38) |
| At constant pressure, |
| dG = dw ne |
(21.39) |
From eq.(21.39), we conclude that the work (other than pV work) obtainable from the system, -dwne equals the decrease in the free energy dG. For a reversible process this is truly the maximum non PV work obtainable from the system, i.e., |
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| -dG = - (dw) ne, reversible |
(21.40) |
In a chemical cell, the work performed by or obtainable from the cell = n F , where n = no of moles electrons transferred during the process occurring at the electrode, F = Faraday = 96500 coulombs and = emf of the cell. From eq.(21.40), |
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 G = -n F |
(21.41) |
| This will be used in our studies in electrochemistry. |
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