Thermal effects and enthalpy of formation. Introduction to thermochemistry

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 engaged in the study of the temperature indexes of various reactions and their dependence on various physico-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 the most stable state.

For example, for hydrogen, nitrogen or oxygen, this is a gaseous form, for carbon it is graphite, and for bromine, the state of the liquid. The enthalpy of formation is an extremely important thermochemical property, since it allows not only to successfully solve theoretical problems, but also to develop various chemical technologies and create the equipment necessary for their implementation. And this is possible only if there is reliable and scientifically grounded information about 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 properties of various substances as the enthalpy of formation is today one of the priority areas of thermochemistry, where thermodynamic reaction equations are often used to determine a given quantity. They necessarily indicate the aggregate state of the substance, and the thermal effect is considered as one of the interaction products.

The heat of reaction, including the enthalpy of the formation of matter, is determined by means of calorimeters of various types. The so-called calorimetric bomb is a well-closed container of thermostated type, in which the heat energy released as a result of a chemical reaction is used only to increase the temperature in the interior space. The calorimeter contains reacting substances, and the very course of the reaction is initiated in a variety of ways. Then, the temperature of the internal medium 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 received data are scrupulously and pedantically recorded in special guides and tables. Also, the enthalpy of formation of various substances can be calculated mathematically, using similar quantities of reacting substances and products of their reaction. For this, the above-mentioned thermodynamic equations are used.

Simple substances almost always have an enthalpy of formation equal to zero. What 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 (s) + ₁ 1 C ₃ = PC (x)

For gaseous metals (Fe, AL), the enthalpy of formation is different from zero and represents a thermochemical transformation upon transition from the solid state (standard) to the gaseous state. Such a value is perceived as the enthalpy of evaporation, also called atomization.