Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: vegard85 on May 16, 2020, 05:42:24 PM
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Hi
What are the Gibbs free Energy of a system containing multiple species in both gas and liquid phase?
The specific system I'm trying to solve by minimizing the Gibbs free Energy are:
SO2(g) ::equil:: SO2(aq)
SO2(aq) + H2O(l) ::equil:: HSO-3(aq) + H+(aq)
CO2(g) ::equil:: CO2(aq)
CO2(aq) + H2O(l) ::equil:: HCO-3(aq) + H+(aq)
I don't want to solve it using Henry's Law & Equilibrium constants because I want to add constraints on some of the variables later on.
My best guess are the below function but I'm not sure if it's correct. (The pressure are assumed constant at 1atm)
[tex] \Delta T = T - 298.15 [/tex]
[tex] n_{gas} = n_{SO_{2(g)}} + n_{CO_{2(g)}} [/tex] [tex] n_{liq} = n_{SO_{2(aq)}} + n_{HSO^-_{3(aq)}} + n_{CO_{2(aq)}} + n_{HCO^-_{3(aq)}} + n_{H_2O_{(l)}} + n_{H^+_{(aq)}} [/tex]
[tex] \mu_{SO_{2(g)}} = G_{f,SO_{2(g)}}^0 - S^0_{SO_{2(g)}} \Delta T + RT \ln \left\{ \frac{n_{SO_{2(g)}}}{n_{gas}} \right\} [/tex] [tex] \mu_{CO_{2(g)}} = G_{f,CO_{2(g)}}^0 - S^0_{CO_{2(g)}} \Delta T + RT \ln \left\{ \frac{n_{CO_{2(g)}}}{n_{gas}} \right\} [/tex] [tex] \mu_{HSO^-_{3(aq)}} = G_{f,HSO^-_{3(aq)}}^0 - S^0_{HSO^-_{3(aq)}} \Delta T + RT \ln \left\{ \frac{n_{HSO^-_{3(aq)}}}{n_{liq}} \right\} [/tex] etc...
[tex] G = \mu_{SO_{2(g)}} n_{SO_{2(g)}} + \mu_{CO_{2(g)}} n_{CO_{2(g)}} + \mu_{HSO^-_{3(aq)}} n_{HSO^-_{3(aq)}} + \mu_{HCO^-_{3(aq)}} n_{HCO^-_{3(aq)}} + \mu_{H^+_{(aq)}} n_{H^+_{(aq)}} + \mu_{H_2O{(l)}} n_{H_2O{(l)}} [/tex]
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After some digging I found out - made an error by using molar fractions everywhere
The standard potentials given in tables (Hf°,S°,Gf°) for all the species are not necessarily in molar fractions.
For solutes dissolved in water they are in molality
For solvent (H2O) it's in molar fraction
For gas it's in partial pressure
Ref: https://www.chem.wisc.edu/deptfiles/genchem/netorial/modules/thermodynamics/table.htm
This seems to work...
[tex] \mu_{SO_{2(g)}} = \mu^0_{SO_2(g)} - \bar{S}^0_{SO_{2(g)}} \Delta T + RT \ln p_{SO_{2(g)}} [/tex] [tex] \mu_{H^+_{(aq)}} = \mu^*_{H^+_{(aq)}} - \bar{S}^*_{H^+_{(aq)}} \Delta T + RT \ln m_{H^+_{(aq)}} [/tex] [tex] \mu_{H_2O_{(l)}} = \mu^0_{H_2O_{(l)}} - \bar{S}^0_{H_2O{(l)}} \Delta T + RT \ln x_{H_2O_{(l)}} [/tex]