The heat of education is what?

Let's talk about what constitutes the heat of formation, and also define those conditions that are called standard. In order to understand this issue, we will clarify the differences between simple and complex substances. To consolidate the concept of "heat of formation", we will consider specific chemical equations.

The standard enthalpy of the formation of substances

In the interaction of carbon with hydrogen gas, 76 kJ of energy is released. In this case, this figure is the thermal effect of the chemical reaction. But this is also the heat of formation of the methane molecule from simple substances. "Why?" - you ask. This is explained by the fact that the initial components were carbon and hydrogen. 76 kJ / mol will be the energy that chemists call "heat of formation".

Tables of data

In thermochemistry there are numerous tables, in which the heats of the formation of various chemicals from simple substances are indicated. For example, the heat of formation of a substance, the formula of which CO ₂ , in the gaseous state is 393.5 kJ / mol.

Practical meaning

Why do we need these values? The heat of formation is a quantity that is used when calculating the thermal effect of any chemical process. In order to carry out such calculations, the application of the law of thermochemistry will be required.

Thermochemistry

It is the basic law that explains the energy processes observed in the process of chemical reaction. During the interaction, qualitative transformations in the reacting system are observed. Some substances disappear, new components appear instead. Such a process is accompanied by a change in the internal energy system, manifested in the form of work or heat. The work that is associated with the expansion, for chemical transformations, has a minimum value. The heat released when one component is converted to another substance can be a large quantity.

If we consider various transformations, absorption or emission of a certain amount of heat is observed for almost all. To explain the phenomena taking place, a special section was created-thermochemistry.

The Hessian Law

Thanks to the first principle of thermodynamics, it became possible to calculate the thermal effect depending on the conditions for carrying out the chemical reaction. Calculations are based on the basic law of thermochemistry, namely, the law of Hess. Let us give its formulation: the thermal effect of chemical transformation is connected with nature, the initial and final state of matter, it is not connected with the way of interaction.

What follows from this formulation? In the case of obtaining a certain product, it is not necessary to use only one interaction option, it is possible to carry out the reaction in a variety of ways. In any case, no matter how you get the desired substance, the thermal effect of the process will be unchanged. To determine it, we need to summarize the thermal effects of all the intermediate transformations. Thanks to Hess's law, it became possible to perform numerical calculations of thermal effects, which can not be done in a calorimeter. For example, the heat of formation of a carbon monoxide substance is quantitatively calculated according to Hess's law, but by ordinary experiments you will not be able to determine it. That is why special thermochemical tables are very important, in which numerical values for various substances, determined under standard conditions

Important points in the calculations

Considering the fact that the heat of formation is the thermal effect of the reaction, the aggregate state of the substance in question is of particular importance. For example, when carrying out measurements, graphite is considered to be the standard state of carbon, and not diamond. Also take into account the pressure and temperature, that is, the conditions in which the reacting components originally existed. These physical quantities can exert a significant influence on the interaction, increase or decrease the amount of energy. In order to perform basic calculations, it is customary to use specific pressure and temperature indices in thermochemistry.

Standard Conditions

Since the heat of the formation of a substance is the determination of the magnitude of the energy effect precisely under standard conditions, we single out them separately. The temperature for the calculations is chosen 298 K (25 degrees Celsius), the pressure is 1 atmosphere. In addition, an important point to pay attention to is the fact that the heat of formation for any simple substances is zero. This is logical, because simple substances do not form themselves, that is, there is no expenditure of energy for their occurrence.

Elements of thermochemistry

This section of modern chemistry is of particular importance, since it is here that important calculations are carried out, and concrete results are obtained that are used in heat power engineering. In thermochemistry there are many concepts and terms that it is important to operate in order to obtain the desired results. The enthalpy (ΔH) indicates that the chemical interaction occurred in a closed system, there was no effect on the reaction from other reactants, the pressure was constant. This refinement allows us to talk about the accuracy of the calculations.

Depending on what kind of reaction they are considering, the magnitude and sign of the resulting heat effect can differ significantly. Thus, for all transformations involving the decomposition of one complex substance into several simpler components, absorption of heat is assumed. Reactions of combining multiple starting materials into one, more complex product are accompanied by the release of a significant amount of energy.

Conclusion

When solving any thermochemical problem, the same algorithm of actions is used. First, according to the table, the value of the heat of formation is determined for each initial component, as well as for the reaction products, without forgetting the aggregate state. Next, armed with Hess's law, make up the equation for determining the sought value.

Particular attention should be paid to taking into account the stereochemical coefficients available before the initial or final substances in a particular equation. If there are simple substances in the reaction, then their standard heat of formation is zero, that is, such components do not influence the result obtained in the calculations. We will try to use the information obtained on a specific reaction. If we take as an example the process of the formation of pure iron from iron oxide (Fe ³⁺ ) by interaction with graphite, then in the handbook we can find the values of the standard heat of formation. For iron oxide (Fe ³⁺ ), it is -822.1 kJ / mol, for graphite (simple matter) it is zero. As a result of the reaction, carbon monoxide (CO) is formed, for which this figure has a value of 110.5 kJ / mol, and for the liberated iron, the heat of formation corresponds to zero. The recording of the standard heat of formation of a given chemical interaction is characterized as follows:

ΔH _о 298 = 3 × (-110.5) - (-822.1) = -331.5 + 822.1 = 490.6 kJ.

Analyzing the numerical result obtained by Hess's law, one can draw a logical conclusion that this process is an endothermic transformation, that is, it involves the expenditure of energy on the iron reduction reaction from its trivalent oxide.