Let's talk about what the heat of formation is, 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 consolidate the concept of "heat of formation", consider specific chemical equations.
Standard enthalpy of formation of substances
In the reaction of interaction of carbon with gaseous hydrogen, 76 kJ of energy is released. In this case, this figure is the thermal effect of a chemical reaction. But this is also the heat of formation of a methane molecule from simple substances. "Why?" - you ask. This is due to the fact that the initial components were carbon and hydrogen. 76 kJ / mol will be the energy that chemists call the "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 whose formula is CO2, in the gaseous statehas an index of 393.5 kJ/mol.
Practical value
Why do we need these values? The heat of formation is a value that is used when calculating the heat effect of any chemical process. In order to carry out such calculations, the application of the law of thermochemistry will be required.
Thermochemistry
He is the basic law that explains the energy processes observed in the process of a chemical reaction. During the interaction, qualitative transformations are observed in the reacting system. Some substances disappear, new components appear instead. Such a process is accompanied by a change in the internal energy system, which manifests itself in the form of work or heat. The work associated with expansion has a minimum indicator for chemical transformations. The heat released in the transformation of one component into another substance can be large.
If we consider a variety of transformations, for almost all there is an absorption or release of a certain amount of heat. To explain the occurring phenomena, a special section was created - thermochemistry.
Hess' Law
Thanks to the first law of thermodynamics, it became possible to calculate the thermal effect depending on the conditions of a chemical reaction. The calculations are based on the basic law of thermochemistry, namely the Hess law. We give its formulation: the thermal effect of a chemical transformationassociated with the nature, the initial and final state of matter, it is not associated with the way the interaction is carried out.
What follows from this wording? In the case of obtaining a certain product, there is no need 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 obtain the desired substance, the thermal effect of the process will be the same value. To determine it, it is necessary to sum up the thermal effects of all intermediate transformations. Thanks to Hess's law, it became possible to perform calculations of numerical indicators of thermal effects, which is impossible to carry out in a calorimeter. For example, quantitatively, the heat of formation of carbon monoxide is calculated according to Hess's law, but you will not be able to determine it by ordinary experiments. That is why special thermochemical tables are so important, in which numerical values \u200b\u200bare entered for various substances, determined under standard conditions
Important points in calculations
Given that the heat of formation is the thermal effect of the reaction, the state of aggregation of the substance in question is of particular importance. For example, when making measurements, it is customary to consider graphite, rather than diamond, as the standard state of carbon. Pressure and temperature are also taken into account, that is, the conditions in which the reacting components were initially located. These physical quantities can have a significant effect on the interaction, increase or decrease the energy value. For basic calculations,thermochemistry, it is customary to use specific indicators of pressure and temperature.
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 calculations is chosen 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 formation.
Elements of thermochemistry
This section of modern chemistry is of particular importance, because it is here that important calculations are carried out, specific results are obtained that are used in thermal power engineering. In thermochemistry, there are many concepts and terms that are important to operate in order to obtain the desired results. Enthalpy (ΔH) 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. This clarification allows us to talk about the accuracy of the calculations performed.
Depending on what kind of reaction is considered, the magnitude and sign of the resulting thermal effect may 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 substances into one, more complex product are accompanied byreleasing a significant amount of energy.
Conclusion
When solving any thermochemical problem, the same algorithm of actions is used. First, according to the table, for each initial component, as well as for the reaction products, the value of the heat of formation is determined, not forgetting the state of aggregation. Further, armed with Hess's law, they compose an equation to determine the desired value.
Special attention should be paid to taking into account the stereochemical coefficients that exist in front of the initial or final substances in a particular equation. If there are simple substances in the reaction, then their standard heats of formation are equal to 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 (Fe3+) by interaction with graphite, then in the reference book you can find the values of the standard heat of formation. For iron oxide (Fe3+) it will be –822.1 kJ/mol, for graphite (a simple substance) it is equal to 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 for the released iron, the heat of formation corresponds to zero. The record of the standard heat of formation of a given chemical interaction is characterized as follows:
ΔHo298=3× (–110.5) – (–822.1)=–331.5 + 822.1=490.6 kJ.
Analyzingthe numerical result obtained according to the Hess law, we can make a logical conclusion that this process is an endothermic transformation, that is, it involves the expenditure of energy for the reaction of reduction of iron from its trivalent oxide.
