The enthalpy of formation, being an important thermodynamic property of a chemical substance, which indicates how much energy in its molecular structure can be converted into heat, is one of the key concepts of thermodynamic chemistry. This branch of science is studying the temperature indicators of various reactions and their dependence on various physical and chemical parameters, heat capacity and phase transitions. The enthalpy of formation (from the Greek. "Entalpio" - "heat") of chemical compounds determines the necessary amount of energy to create a complex substance from simple elements, taken, as a rule, in its most stable state.
For example, for hydrogen, nitrogen, or oxygen, this is the gaseous form, for carbon, graphite, and for bromine, the state of the liquid. The enthalpy of formation is an extremely important thermochemical property, since it allows not only successfully solving theoretical problems, but also developing various chemical technologies and creating the equipment necessary for their implementation. And this is possible only if there is reliable and scientifically based information on the physicochemical and thermodynamic properties of various compounds. Such information often makes up more than fifty percent of the initial data when designing complex equipment for scientific experiments or industrial purposes.
Therefore, the study of such a property of various substances as the enthalpy of formation is today one of the priority areas of thermochemistry, where the thermodynamic equations of reactions are often used to determine this value. They necessarily indicate the state of aggregation of a given substance, and the thermal effect is considered as one of the interaction products.
The heat of reaction, including the enthalpy of formation of a substance, is determined using various types of calorimeters. The so-called calorimetric bomb is a well-thermostatic closed-type vessel in which the thermal energy released as a result of a chemical reaction is spent only on increasing the temperature in the inner space. In the calorimeter there are reacting substances, and the course of the reaction itself is initiated in a variety of different ways. Then the temperature of the internal environment is measured by means of a thermometer or thermocouple.
In this way, the standard enthalpy of formation of many thousands of substances is measured. All data obtained are carefully and meticulously recorded in special directories and tables. Also, the enthalpies of formation of various substances can be calculated mathematically using similar values of reacting substances and their reaction products. For this, the above-mentioned thermodynamic equations are used.
Simple substances almost always have an enthalpy of formation equal to zero. Which is also taken into account in the calculations. And this thermodynamic property, for example, for phosphorus trichloride can be calculated by the following formula:
P (tv) + 1½ C 3 = PC (g)
For gaseous metals (Fe, AL), the enthalpy of formation is nonzero and represents a thermochemical transformation during the transition from a solid state (standard) to a gaseous state. This value is perceived as the enthalpy of evaporation, also called atomization.