FREE ENERGY
Free energy (total work, isothermical useful yield) is the energy, which is capable of producing work. To designate free energy the symbol G is used, in honour of Gibbs, Josiah Willard, the physicist of the XIX century, who introduced this term. Most often "free energy" stands for Gibbs’ free energy, though there is also the Helmholtz’ idea of free energy. It is designated with the symbol A. In the sphere of biology and physical chemistry the Gibbs’ free energy is more applicable. By definition,
G = H -TS, where
H is the enthalpy of the system;
S is its entropy,
T is the temperature.
The free energy increment in constant temperature
ΔG = ΔH - TΔS.
In biological systems both pressure and volume are usually constant, so that ΔH = ΔE, where E is internal energy of the system. Free energy defined this way is a function, which can be used for calculations in biological systems with constant temperature and constant pressure, as far as under such conditions it possesses properties of a potential. The general rule for this type of systems is the following:
If ΔG is equal to a negative value, then the reaction can proceed spontaneously.
If ΔG is equal to a positive value, then the reaction cannot proceed spontaneously.
If ΔG is equal to zero, then the system is in balance state and can neither acquire, nor lose free energy.
For the process of the formation of a chemical bond it is obvious, that the stronger the given bond (in other words, the more amount of free energy accompanies its formation), the more amount of atoms in the system are in bound state. It can be expressed quantitatively as follows:
ΔG = -RT ln Keq, where
R is the universal gas constant,
T is the absolute temperature,
Keq is the equilibrium constant, ln Keq is the logarithm of the equilibrium constant to base e (e ≈2,718)