Geochemist's Workbench Support Forum

# [OLD] Modelling solid and gas phases in C-O-H system

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From: Laiq Rahman

Subject: modelling solid and gas phases in C-O-H system

I'm having some trouble using GWB to investigate the relative redox potentials of solid-gas and gas-gas phase equilibria for the C-O-H system. I've arbitrarily chosen a few carbon redox couples from a C-O-H ternary diagram, and listed the couples below, in the order that I think is correct (most reducing on the left, most oxidising on the right) i.e., C/CH4, C/CO, C/CO2, CH4/CO, CH4/CO2, CO/CO2, I'm using RXN to fix log f O2(g) for each reaction by generating the basic equations and swapping in the required phases if necessary e.g.,

react CH4(g)

swap O2(g) for O2(aq)

swap CO2(g) for HCO3-

long

Which gives CH4(g) + 2 O2(g) = CO2(g) + 2 H2O

Setting fugacities of CH4 and CO2 to unity, and activity of H2O to unity, I get log K = -2 log f O2(g). I divide the log K values at each temperature (0-300C) for this reaction by 2 to get log f O2(g), and use a spreadsheet to plot a graph of log f O2(g) vs each temperature.

As another example, C/CO, has a log K = -1/2 log f O2(g), and for which I multiply log Ks by 2. By repeating a similar procedure for the other redox couples, I can then view their relative positions on a graph. I would expect the results to show that CH4/CO2 is relatively more reduced than CO/CO2, and CO/CO2 to be the most oxidised couple in the C-O-H system. However, from the calculated log f O2(g) values below, CO/CO2 appears to be the most reduced!

For example, at 25C, I get the following log f O2(g) values (fugacities for all gases except O2(g) = 1bar, activity H2O =1)

C/CH4 log K = log f O2(g)

log K = - 74.2192

therefore log f O2(g) = - 74.2192

C/CO log K =

- 1/2 log f O2(g) log K = 24.0309

therefore log f O2(g) = - 48.0618

C/CO2 log K = - log f

O2(g)

log K = 69.0888

therefore log f O2(g) = - 69.0888

CH4/CO log K = -1.5 log f

O2(g) log K = 98.2502

therefore log f O2(g) = - 65.5001

CH4/CO2 log K = - 2 log f

O2(g) log K =

143.3080

therefore log f O2(g) = - 71.654

CO/CO2 log K = - 1/2 log f

O2(g) log K = 45.0578

therefore log f O2(g) = - 90.1156

Is my treatment of the solid and gas phases fundamentally wrong? Perhaps I'm not fixing the correct phase(s)? I'm using GWB v3.0 and log Ks are from the thermo.com.v8.r6.full database.

From: Craig Bethke

Subject: Re: modelling solid and gas phases in C-O-H system

I think you might have gotten a little turned around in your thinking. Look at it this way: if the equal fugacity point of the CO2/CO reaction is at higher redox potential than that of the CO/CH4 couple, then CO would be the stable gaseous form of carbon over a range of redox conditions, and we would all probably be dead!

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