Question

I am trying to make sense of these two graphs. They appear to have similar axes, yet display different curves. Please let me know if this understanding is correct: In the top image (x-axis = "Reaction coordinate"), this shows the free energy as we convert X equivalents of A to Y equivalent of B (with X and Y relating to the stoichiometric coefficients). So this is looking at an individual reaction essentially. And for the bottom image (x-axis = "Reaction progress"), this shows the free energy versus the reaction quotient (Q) essentially. A large part of my confusion is why the bottom image does not indicate activation energy yet the top one does. Does this have to do with Gibbs free energy being a state function. It is true that over the course of the reaction, reactants require some activation energy, but it is not represented in the bottom image since Gibbs free energy is a state function?

Answer 1

The free energy diagram with the 'Reaction coordinate' axis illustrates the reaction's activation energy and transition state, whereas the diagram with the 'Reaction progress' axis shows the change in Gibbs free energy relative to the reaction quotient (Q). Activation energy is not depicted in the latter because it is a kinetic factor, while Gibbs free energy is a thermodynamic state function.

Step-by-step explanation:

Your understanding of free energy diagrams is essentially on the right track. The diagram with the x-axis labeled 'Reaction coordinate' indeed represents the energy changes during the course of a specific chemical reaction. This diagram is useful for visualizing the transition state, the relative energies of reactants and products, and the activation energy (Ea) required to reach the transition state from the reactants.

On the other hand, the diagram with the x-axis labeled 'Reaction progress' relates to changes in Gibbs free energy as a function of reaction progress or reaction quotient (Q).

Activation energy is not depicted here because this diagram is focused on the thermodynamics of the reaction—namely, the change in Gibbs free energy—which is a state function and is independent of the path taken, including the activation energy barrier overcome during the reaction.

Activation energy is pertinent to the kinetics of the reaction, whereas Gibbs free energy changes are pertinent to the thermodynamics and the equilibrium position of the reaction. Both diagrams provide different, but complementary, information about a chemical reaction.