enthalpy of formation as an important thermodynamic properties of chemical substances, which indicates the amount of energy that is in its molecular structure can be converted into heat, it is one of the key concepts of thermodynamic chemistry.This branch of science has been studying the temperature characteristics of different reactions and their dependence on all kinds of physical and chemical parameters, heat capacity and phase transitions.The enthalpy of formation (from the Greek. "Entalpio" - "heated up") determines the chemical compounds required amount of energy to create the complex substance from simple elements, taken as a rule, in its most stable state.
For example, hydrogen, nitrogen or oxygen such a gaseous form, for carbon - graphite and bromine - liquid state.The enthalpy of formation - an extremely important thermochemical properties because it allows not only to successfully solve the problem in theory, but also to develop a variety of chemical technologies and create the necessary equipment to implement them.And this is possible only if there is reliable and scientifically based information on the physico-chemical and thermodynamic properties of various compounds.Such information often make up more than fifty percent of the initial data in the design of complex equipment for scientific experiments or industrial purposes.
Therefore, the study of the properties of different substances, such as the enthalpy of formation, is now one of the priorities of thermochemistry, which often use the thermodynamic equations of reactions for determining this value.They must indicate the physical state of the substance, and the thermal effect is regarded as one of the products of the interaction.
heat of reaction, including the enthalpy of formation of a substance is determined by different types of calorimeters.The so-called bomb calorimeter is a well thermostated vessel of the closed type, in which the thermal energy released by a chemical reaction, is only consumed to increase the temperature of the interior space.The calorimeter are reactive substance itself and the reaction initiated by a variety of methods.Then, the inner temperature of the medium is measured by a thermometer or thermocouple.
this way is measured standard enthalpy of formation of many thousands of substances.All the data carefully and meticulously recorded in special directories and tables.Also indicators enthalpy of formation of various substances can be calculated mathematically, using the same quantity of the reacting substances and the products of their reaction.For this purpose and used the above-mentioned thermodynamic equations.
Simple substances almost always have a zero enthalpy of formation.What is also taken into account in the calculations.And this thermodynamic properties, such as phosphorus trichloride can be calculated using the following formula:
P (tv.) + 1½ C3 = RS (f.)
For gaseous metals (Fe, AL) enthalpy of formation is different from zero and representsthermochemical transformation at the transition from the solid state (the standard) in the gaseous state.This value is seen as the enthalpy of vaporization, also called atomization.
