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 find out the differences between simple and complex substances. To fix the concept of โheat of formationโ, we consider specific chemical equations.
Standard enthalpy of substance formation
In the reaction of the interaction of carbon with gaseous hydrogen, 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 a methane molecule from simple substances. "Why?" - you ask. This is because the starting components were carbon and hydrogen. 76 kJ / mol will be the energy that chemists call "heat of formation."
Data tables
In thermochemistry, there are numerous tables that indicate the heats of formation of various chemicals from simple substances. For example, the heat of formation of a substance, the formula of which is CO 2 , in a gaseous state has an indicator of 393.5 kJ / mol.
Practical value
Why are these quantities needed? The heat of formation is a value 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 during the implementation of a chemical reaction. During the interaction, qualitative changes are observed in the reacting system. Some substances disappear, new components appear instead of them. Such a process is accompanied by a change in the system of internal energy, it manifests itself in the form of work or heat. The work that is associated with expansion for chemical transformations has a minimum indicator. The heat released during the conversion of one component into another substance can be large.
If we consider a variety of transformations, for almost all the absorption or release of a certain amount of heat is observed. To explain the occurring phenomena, a special section was created - thermochemistry.
Hess Law
Thanks to the first principle of thermodynamics, it became possible to calculate the thermal effect depending on the conditions of the chemical reaction. The calculations are based on the basic law of thermochemistry, namely on the law of Hess. Let us state its formulation: the thermal effect of the chemical transformation is associated with nature, the initial and final state of matter, it is not associated with the way of the interaction.
What follows from this formulation? In the case of obtaining a specific product, there is no need to apply only one interaction option, you can conduct 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, it is necessary to summarize the thermal effects of all intermediate transformations. Thanks to the Hess law, it became possible to carry out calculations of numerical indicators of thermal effects, which is impossible to carry out in the calorimeter. For example, the heat of formation of a carbon monoxide substance is quantified according to the Hess law, but by ordinary experiments you will not be able to determine it. That is why special thermochemical tables are so important, in which numerical values โโfor various substances are entered, determined under standard conditions
Important points in computing
Considering 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 conducting measurements, it is customary to consider graphite rather than diamond as the standard state of carbon. Also take into account pressure and temperature, that is, those conditions in which the reacting components were initially located. These physical quantities can have a significant effect on the interaction, increase or decrease the amount of energy. In order to perform basic calculations, it is customary to use specific indicators of pressure and temperature in thermochemistry.
Standard conditions
Since the heat of formation of a substance is the determination of the magnitude of the energy effect under standard conditions, we will single them out separately. The temperature for the calculations is 298 K (25 degrees Celsius), pressure - 1 atmosphere. In addition, an important point worth paying 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, because it is here that important calculations are carried out, concrete results are obtained that are used in the power system. In thermochemistry, there are many concepts and terms that it is important to operate in order to obtain the desired results. Enthalpy (ฮ) indicates that the chemical interaction took place in a closed system, there was no influence on the reaction from other reagents, the pressure was constant. Such a refinement allows us to talk about the accuracy of the calculations.
Depending on what kind of reaction is considered, the magnitude and sign of the resulting thermal effect can differ significantly. So, for all transformations involving the decomposition of one complex substance into several simpler components, heat absorption is assumed. The reactions of combining many 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 reaction products, not forgetting the state of aggregation. Further, armed with the Hess law, they compose an equation for determining the desired value.
Particular attention should be paid to taking into account the stereochemical coefficients present in front of the starting or final substances in a particular equation. If there are simple substances in the reaction, then their standard heats of formation are zero, that is, such components do not affect the result obtained in the calculations. Let's try to use the information received on a specific reaction. If we take as an example the process of formation of pure metal from iron oxide (Fe 3+ ) by interaction with graphite, then in the reference book you can find the standard heat of formation. For iron oxide (Fe 3+ ), it will be โ822.1 kJ / mol, for graphite (a simple substance) it is zero. As a result of the reaction, carbon monoxide (CO) is formed, for which this indicator has a value of 110.5 kJ / mol, and the heat of formation for zero iron is zero. The record of the standard heat of formation of a given chemical interaction is characterized as follows:
ฮ about 298 = 3 ร (โ110.5) - (โ822.1) = โ331.5 + 822.1 = 490.6 kJ.
Analyzing the numerical result obtained according to Hess's law, it is possible to draw a logical conclusion that this process is an endothermic transformation, that is, it involves the expenditure of energy on the reaction of reduction of iron from its trivalent oxide.