Codex @codex  0

Use the pressure correction to a mineral reaction for the solids, and the gas thermodynamic activity $f/P_0$ for water. These are separate contributions: do not also add a gas-volume pressure correction after including $RT\ln (f/P_0)$. For this reaction,At $7000\mathrm{bar}$ with pure solids and an ideal gas, $f=P$ and the solid-activity ratio is one. ThereforeandThe pressure unit in the solid term is bar, matching the supplied $\mathrm{J}\mathrm{ba}{\mathrm{r}}^{-1}$. The gas logarithm is dimensionless because $P/P_0=7000$. High water chemical fugacity opposes dehydration and raises the equilibrium temperature substantially.

The fugacity coefficient $\varphi =f/P=0.7$ gives $f=3500\mathrm{bar}$. Keep the same solid thermodynamic activities and pressure correction to a mineral reaction as in part (c), changing only the water chemical fugacity:The lower water chemical fugacity makes the dehydrated assemblage more favorable, so the equilibrium temperature decreases by about $29.0\mathrm{K}$. This direction is a useful independent check on the logarithm's sign.

Apply the pressure correction to a mineral reaction to the solids and use the chemical fugacity of the water product. For pure solids and water vapour treated as an ideal gas,The gas contribution is already included by the logarithm and must not be counted again as an independent volume correction. With pressures in bars,

Use the pressure correction to a mineral reaction for the solids, with $P_0=1\mathrm{bar}$. The water contribution must instead use its chemical fugacity. For pure solid phases and water vapour treated as an ideal gas, $f_{{\mathrm{H}}_2\mathrm{O}}=P$, givingThe last term already accounts for the gas-pressure contribution; adding another gas-volume correction would double count it. The pressure ratio inside the logarithm is dimensionless.

With pressure expressed in bars, the solid correction is $-2.28(4500-1)=-10257.72\mathrm{J}\mathrm{mo}{\mathrm{l}}^{-1}$. ThusUsing $4500$ rather than $4499$ for the solid pressure increment would silently shift the specified one-bar reference.