Gibbs Free Energy and Transition State Energy
Gibbs Free Energy (ΔG) is a thermodynamics concept, and describes the thermodynamic potential of a system. It measures the "work" that can be obtained from a system, for us a chemical reaction, if that system is held at constant temperature and pressure. The goal is to determine whether a given reaction is spontaneous in the direction written, i.e., whether the reaction will proceed in the given direction without the input of energy.
Mathematically, Gibbs Free Energy is described as:
- ΔG The change in Free Energy
- ΔH The change in Enthalpy
- ΔS The change in Entropy
Gibbs free energy is a measure ONLY of the difference in free energy of the products and reactants, and does not tell us about the rate of the reaction. It only tells us whether the reaction is Exergonic or Endergonic.
- ΔG < 0 Exergonic: The reaction is considered spontaneous in the direction written.
- ΔG = 0 The system is in equilibrium
- ΔG >0 Endergonic: To carry out this reaction as written, there needs to be an addition of free energy (NOTE: this means that the reverse reaction is spontaneous).
Transition State Energy is not Gibbs Free Energy.
Gibbs Free Energy does not describe the path of transformation or the mechanism of transformation. It does not describe the rate of transformation. It only describes whether the reaction is spontaneous, non-spontaneous or at equilibrium.
The breakdown of the dissacharide sucrose to glucose and fructose has a ΔG of -5.5 kcal/mol. This is a spontaneous (Exergonic) reaction in terms of ΔG. Yet you can store sucrose in your kitchen and it remains sucrose. There is no spontaneous degradation into monosaccharides. Why?
Sucrose is a stable molecule. In order to force the breakdown (catabolism) of sucrose, we need to destabilize the molecule. This destabilization is the transition state of the reaction, and in a closed system, requires the input of energy. This is termed the Activation Energy of the reaction (you will also hear this described as the Transition State Energy). If we were to look at our breakdown of sucrose, we would see the following:
The energy of the transition state is noted as either EaΔG‡. These two expression are from different formula for calculating activation energy. ΔG‡ is used in a formula that relates activation energy to Gibbs Free Energy (the Eyring equation). In either case, the expression describe transition state energy (aka, activation energy).
In biochemistry, enzymes are used to reduce the activation energy (just like catalysts are used in chemistry). Catalysts and Enzymes reduce the activation energy ΔG‡; they do not alter Gibbs Free Energy ( . This is a critical concept!
Transition state deals with the rate of the reaction. By lowering the activation energy (transition state energy), you increase the rate of the reaction. In essence, you are making the reaction more likely to happen. But the enzyme does not change Gibbs Free Energy.
|Original Image from ChemWiki,|
Take home message:
Activation Energy (ΔG‡) does not alter ΔG; it does not determine whether a reaction is spontaneous or non-spontaneous. Activation energy does help determine the rate of the reaction.