Free Energy Diagrams: Understand Endo/Exothermic Reactions

[lollol]

It's thermo.. so I just posted it in the physics forums

but.. a free energy diagram is supposed to represent G right?

We normally conclude whether a rxn is endo/exothermic depending on whether the product is at a "lower" or higher energy than the reactants just by looking at the diagram

But how can we do that? especially if Delta G = Delta H - (T * Delta S)
Free energy depends on Entropy and Temperature as well

A "negative" change in free energy surely does not necessarily mean an exothermic reaction... but somehow, we are able to conclude whether something is exo/endo simply by looking at a free energy diagram.. how?

 

[russ_watters]

The diagram you have there doesn't have any numbers on it, but if it did it would be valid for only one set of initial conditions.

I'm not sure I understand your last sentence. A negative delta G is exothermic because it means energy is released. The initial hump there doesn't have anything to do with if the final result is exothermic or endothermic - it is just the required activation energy (in this case, it looks like the energy required to disassociate the constituents of the compound).

 

[sir-pinski]

First of all, let's clarify what a free energy diagram represents. It is a graph that shows the change in free energy (ΔG) of a reaction as it progresses from reactants to products. This means that the x-axis represents the progress of the reaction, while the y-axis represents the change in free energy. The shape of the curve on the diagram can tell us a lot about the nature of the reaction.

Now, to address your question about how we can determine if a reaction is endo/exothermic just by looking at a free energy diagram, it's important to remember that the change in free energy (ΔG) is directly related to the enthalpy change (ΔH) and the entropy change (ΔS) through the equation ΔG = ΔH - TΔS. This means that the shape of the curve on the free energy diagram is influenced by both ΔH and ΔS.

If the reaction is exothermic (releases heat), then the enthalpy change (ΔH) will be negative. This means that the curve on the free energy diagram will slope downwards as the reaction progresses, indicating a decrease in free energy. On the other hand, if the reaction is endothermic (absorbs heat), then the enthalpy change (ΔH) will be positive and the curve on the diagram will slope upwards, indicating an increase in free energy.

However, as you mentioned, free energy also depends on entropy (ΔS) and temperature (T). If the reaction has a positive entropy change (ΔS), then the curve on the diagram will slope downwards even if the reaction is endothermic. This is because the increase in entropy contributes to a decrease in free energy. On the other hand, if the reaction has a negative entropy change (ΔS), then the curve will slope upwards even if the reaction is exothermic, because the decrease in entropy contributes to an increase in free energy.

In summary, we can determine if a reaction is endo/exothermic by looking at the shape of the curve on a free energy diagram, taking into consideration both the enthalpy change (ΔH) and the entropy change (ΔS). It is important to note that a negative change in free energy does not always mean an exothermic reaction, as it also depends on the entropy change.