Biothermodynamics - potentials
Entropy as a measure of disorder and probability. Third law. Thermodynamic potentials - definitions, changes, consequences
Entropy as a measure of disorder stems from the second law of thermodynamics (Entropy always increases in isolated systems). It measures the number of microstates within the macro state (i.e. how many random formations there are within the thermodynamic system, more on this regarding thermodynamic potentials.)
Third law of thermodynamics (you can’t get out of the game)
The entropy of a perfect crystal at 0 Kelvin is zero.
0 Kelvin is the lowest theoretical temperature. It is the point at which there is no kinetic movement within the substance, it can be approached in the laboratory but never fully reached.
A perfect crystal, by definition, has one set crystalline structure, as such if the crystal structure is perfect, and there is no kinetic energy of the molecules, there is only one possible structure of the crystal, meaning the total entropy (disorder/randomness) is zero.
Thermodynamic potentials
Thermodynamic potentials are used to measure the energy of a system in terms of different variables which we may not be able to measure directly. For instance we may know the pressure and temperature of a system but not the volume or entropy. Thermodynamic potentials allow us to measure additional variables to get this information.
The Thermodynamic potentials are: Internal energy, enthalpy, Gibbs Free Energy and Helmholtz free energy.
Consequence: living systems work at nearly constant temperature and pressure, so Gibbs free energy is the most useful. A process runs spontaneously only if ΔG is negative, and at equilibrium ΔG is zero.