The spontaneous course of processes in open and closed systems is described through a special criterion, called Gibbs energy. It is a function of state. D.U. Gibbs, working with thermodynamic systems, managed to bring it through entropy and enthalpy. Gibbs energy, in particular, allows us to predict the direction of the course of spontaneous biological processes and evaluate their theoretically achievable efficiency.
If we apply the Gibbs conclusions to the second thermodynamic law, the formulation will be as follows: at constant (const) pressure and temperature without external influence, the system can support the spontaneous occurrence of only such processes, the consequence of which is to reduce the Gibbs energy level to the value that occurs when it reaches its steady state minimum. The equilibrium of any thermodynamic system means the invariance of the indicated energy (minimum). Therefore, the Gibbs energy is a potential (free enthalpy) in isobaric-isothermal systems. Let us explain why the minimum is indicated. The fact is that this is one of the most important equilibrium postulates in thermodynamics: this state, at a constant temperature and pressure, means that for the next change it is necessary to increase the energy level, and this is possible only when any external factors change.
The letter designation is G. Numerically equal to the difference between the known enthalpy and the value of the product of temperature and entropy. That is, Gibbs energy can be expressed through the following formula:
G = H - (S * t),
where S is the entropy of the system; t is the thermodynamic temperature; H is enthalpy. The entropy of the system in this formula is included in order to take into account the fact that high temperature leads to a decrease in the ordered state of the system (disorder), while low temperature - on the contrary.
Since both Gibbs energy and enthalpy are some of the functions of the system in thermodynamics, the occurring chemical transformations can be characterized by changing G or H. If the reaction equation and the Gibbs energy change are given, then it is classified as a thermochemical.
In relation to this energy, the Hess Rule can be formulated: if pressure and temperature are unchanged, then the creation of new substances from the initial (basic reagents) leads to the fact that the energy in the system changes, while the type of reactions and their number do not affect the result.
Since the energy referred to in the article is a variable quantity, the concept of “standard Gibbs energy” was introduced to perform calculations. This value is present in any chemical reference book, numerically equal to 298 kJ / mol (note that the dimension is exactly the same as for any other molar energy). This value allows you to calculate the change for almost any chemical process.
If in the process of a chemical reaction an external influence is exerted on the system (work is done), then the value of the Gibbs energy increases. Such reactions are classified as endergonic. Accordingly, if the system itself does the work, spending energy, then we are talking about exergonic manifestations.
The concept of Gibbs energy has found wide application in modern chemistry. For example, polymer synthesis is based on addition reactions. When they are carried out, several particles are combined into one, while the value of entropy decreases. Based on the Gibbs formula, it can be argued that external action (for example, high temperature) can reverse a similar exothermic reaction of addition, which is confirmed in practice.