Thermochemistry

After reading the article, you will find out in which direction the reaction is directed, what constitutes the internal energy systems and what is enthalpy

Thermochemistry is a branch of physics and chemistry that studies thermal phenomena in chemical reactions and physico-chemical processes.

Direction of the chemical reaction

The direction of the chemical reaction is determined by the Gibbs free energy, which will be described below. Any process can occur by itself, or it may require the application of external forces, for example, if you release the book, it will fall, this is a natural process, expect the spontaneous lifting of the book in it is not possible under normal conditions. So are chemical reactions, under normal conditions they flow in one direction, and under certain conditions they can flow in the opposite direction, for example, the reaction sodium and chlorine under normal conditions (25° and 1 atm.) flows from left to right:

2Na + Cl₂ → 2NaCl (1)

The following reaction will not occur under the same conditions:

2NaCl → 2Na + Cl₂ (2)

But! it is possible to create conditions in which reaction (2) will proceed, but reaction (1) will not, for example, if let the current flow through the heated sodium chloride.

The study of energy conversion is engaged in thermodynamics, thermodynamics, which deals with solving problems chemistry is called thermochemistry.

Internal energy of the system

The internal energy of the system is the sum of the potential and kinetic energies of each of the particles of the system. Thermodynamics describes macroscopic systems, i.e. not the behavior of molecules, but the behavior of some system, a system is "a part of the universe allocated for its (system) study in a convenient form." Sun, tree or a cup is an example of a system. Everything that is not included in the system is called the environment. The energy inside the system can change in various ways, for example:

Heat q

When the system comes into contact with the environment, heat exchange occurs, heat from a more heated body passes to a less heated one. So, the system gives or receives heat, the change in the energy of the system is equal to the difference received and given energy: ΔE = q_in - q_out = E₁ - E₂. If we consider the room as a system, then through the battery the heat q_in enters the room and through window, walls, floor, ceiling and doors heat q_out goes away.

Work W

If we heat the gas in a rubber ball, then part of the heat will be spent on increasing the ball, this energy consumption is called work. Any action against external forces is work. Work it is indicated by a minus sign, since work is a loss of energy.

Thus, the change in the energy of the system is the heat received minus the work done:

ΔE = q - w

Enthalpy H

Enthalpy is a function of the state of the system, showing the amount of heat that the system possesses. In any process, heat is absorbed or released, so if heat is released as a result of the reaction, then the reaction is called exothermic. If heat is absorbed during the reaction, the process is called endothermic. The standard enthalpy of formation (H^o_f) is a value showing the change in enthalpy during the formation of a substance from simple components, for example, during the formation of one mole of aluminum oxide Al₂O₃ 400.5 kcal is absorbed from aluminum Al and oxygen O. Measure the amount of released/absorbed substance using a special device - a calorimeter.

2Al + 3O → Al₂O₃

The total enthalpy change consists of the difference between the energy released/absorbed when the bonds of the reagents are broken and the energy released/absorbed when the bonds of the products are created:

Hess's Law:
ΔH_o = ΣΔH^o_{f products} - ΣΔH^o_{f reagents}

Hess's law is formulated as follows: the thermal effect of a chemical reaction carried out at constant temperature and pressure or constant volume and temperature, depends only on the type and condition of the starting substances and reaction products and it does not depend on the way it flows.

The standard enthalpy of formation is measured in laboratory conditions, the data can be found in various reference books.