[crushedcorn]

I am getting confused about the different ΔG values and was hoping someone could help me fill in the gaps in my understanding and correct any misconceptions I may have.

• ΔG is the energy available to do work and tells us if a reaction is spontaneous or non-spontaneous.
• ΔG° is free energy available to do work under the standard conditions of 25°C, 1 atm, and an initial reactant concentration of 1.0 M.
• ΔG± is the activation energy of a reaction without catalyst intervention.
• ΔG°' is similar to ΔG°, but it functions in biological systems where the pH=7 (concentration of H⁺ is 1 x 10^-7. My confusion with this term is how the pH of the system plays into any of the variables used to calculate our ΔG values, and is there a different way to calculate this term or would you just use, for example, the following equation: ΔG°=-RTln(Keq) and sub ΔG°' in for ΔG°? Is ΔG°' only interchangeable with ΔG° or can it also be used in place of ΔG?
  

Thank you for your knowledge.

[Babcock_Hall]

Is ΔG°' only interchangeable with ΔG° or can it also be used in place of ΔG?

Thank you for your knowledge.

Of the three classes of Gibbs' free energy which one(s) is (are) tabulated for constant concentrations of reactant and products, and which one(s) is (are) dependent on the those concentrations?  I am not sure exactly what you are asking in the rest of your question.  Are you asking why ΔG°' and ΔG° take on different values for the same reaction or something else?

[crushedcorn]

ΔG is based on the concentration of the reactants, while ΔG° and ΔG°' are based on the standard initial concentrations of 1 M for each reactant.

What I'd like to know is why there is a need to have a ΔG° and a ΔG°' value if the only difference between the two is that the latter refers to reactions at a pH of 7? Is the ' there just to let you know that a reaction is occurring in a living organism or is it there because it will have some bearing on calculations of ΔG° and its related equations?

[Babcock_Hall]

I suppose that biochemists chose the standard state to be defined at pH 7 because ΔG°' is closer to the value that a living cell would experience, than ΔG° is.  That having been said, there are some sub cellular locations where the pH is considerably different from 7, such as the lysosome and the chloroplast.  I can think of several reasons why the two quantities are sometimes numerically different.  Imagine we have a simple isomerization, such as HA  HB.  If the two pK_a values are different, then the ratio between the total amount of A and the total amount of B will be different at different pH values.  The same idea holds true for substitution reactions involving HA and HB.  ATP and ADP do not have identical pK_a values, for example.

[Irlanur]

Redox reactions including hydrogen/protons depend on the pH. You can also have a look at the Nernst Equation if you wish.