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Monday, December 2, 2013

Notes on fugacity and activity

This is one of those "note to self" posts where I try to get my head around a concept, this time fugacity and activity for a gas.

dG=Vdp-SdT \implies dG=Vdp \text{ if } dT=0
G(p)=G^\circ+\int_{p^\circ}^{p}Vdp
If the gas is ideal, i.e. for one mole V=RT/p, then
G(p)=G^\circ+RT\int_{p^\circ}^{p}\frac{dp}{p}=G^\circ+RT\ln\frac{p}{p^\circ}
For A\rightleftharpoons B
G(p_B)-G(p_A)=0 \implies \frac{p_B}{p_A}=e^{-\Delta G^\circ/RT}
What about a real gas where V\neq RT/p?  We introduce the fugacity (f) for which V=RT/f so that
G(p)=G^\circ+RT\ln\frac{f}{p^\circ} \text{ and } \frac{f_B}{f_A}=e^{-\Delta G^\circ/RT}
To determine f:
\int_{p'}^{p} (V-V_{ideal})dp=RT\ln\left(\frac{f}{f'}\cdot \frac{p'}{p}\right) =  RT\ln\left(\frac{f}{p}\cdot \frac{p'}{f'}\right)
Gases approach ideality at low pressure: f'/p'\rightarrow 1 as p\rightarrow 0 so:
\ln\left(\frac{f}{p}\right)=\ln(\phi)=\frac{1}{RT}\int_{0}^{p} (V-V_{ideal})dp
So for sticky non-ideal gases for which V<V_{ideal} the fugacity coefficient \phi is less than 1.  So even though V=RT/f don't confuse f with p_{ideal}: f<p<p_{ideal} for a given number of gas particles.

Finally the relationship between fugacity and activity (a) is
a=\frac{f}{p^\circ}.
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