CRE: Gibbs Phase Rule for Reactive Systems

Gibbs Phase Rule for Reactive Systems

It is useful to recap the origins of Gibbs Phase Rule in order to see how the existence of reactions impacts the overall number of degrees of freedom available.

Recall that, we get adjustable intensive variables from (that is, we can independently set values for each of these intensive variables):

We then have Thermodynamic relations that relate these intensive variables (that is, they reduce our degrees of freedom):

Combining these two arguments, we get the non-reactive Gibbs Phase Rule that the number of degrees of freedom, $D$ is:

$D = (2+N\pi - \pi) - (N\pi -N) = 2+N-\pi$

If we now include a number of reactions ($r$), each must satisfy an equilibrium expression, so

Adding these "extra" Thermodynamic relations to our existing rule yields the reactive Gibbs Phase Rule where $D$ is now:

$D = 2+N-\pi-r$

Outcome:

Calculate the degrees of freedom using Gibbs Phase Rule (for reactive systems)