Question

What is reaction isotherm? Describe briefly.

Answer 1

The reaction isotherm is given by

`DeltabarG_(rxn) = DeltabarG_(rxn)^@ + RT ln ((Pi_(j) a_j^(nu_j))/(Pi_(i) a_i^(nu_i)))`,

where:

• `DeltabarG_(rxn)` is the molar Gibbs' free energy for the reaction.
• `DeltabarG_(rxn)^@` is the molar Gibbs' free energy for the reaction at the standard pressure (`"1 bar"`) and at the temperature of the reference reaction.
• `R` and `T` are known from the ideal gas law; `T` is in `"K"`.
• The argument of `ln` is the general form of the reaction quotient, `Q`, which for real gases and solutions, utilizes activities `a` for the `i`th reactant and `j`th product.
  An example for the activity is `a_i -= f_i/P^@` for gases, where `f` is the fugacity (the real-gas partial pressure). Or, if you know the mol fraction `chi_i` of substance `i` in the solution, `a_i = gamma_i chi_i`, where `gamma_i` is the activity coefficient of substance `i` at that `T` and `P`.

This is generally useful for determining `DeltabarG_(rxn)` at nonstandard temperatures.

It's also important that `DeltabarG_(rxn)^@` isn't necessarily at `"298.15 K"`, although it is usually convenient to specify that as the standard reference temperature because `DeltabarG_(rxn)^@` has been tabulated for many reactions at room temperature.

If you wish to derive this, I show it here.