In both physics and chemistry a concept is used to measure the energy contained in the body. We are talking about the enthalpyThis is a type of measurement that tells us the amount of energy contained in a body or system with a given volume, subjected to pressure, and which can be exchanged with the environment. The enthalpy of a system is represented by the letter H, and the physical unit associated with it to indicate energy values ​​is the joule.
In this article we are going to tell you all the characteristics and importance of enthalpy.
Key features
We can say that the enthalpy is equal to the internal energy that the system has plus the pressure times the volume of the same system. When we see that the energy of the system, the pressure and the volume are functions of state, the enthalpy is also. This means that, when the time comes, it can occur in certain final initial conditions so that the variable can help to study the entire system as a whole.
The first thing is to know what the enthalpy of formation is. Its about absorbed heat forgotten by the system when 1 mole of product substance is produced from the elements in normal state. These states can be solid, liquid, gaseous, or in the case of a solution. The allotropic state is the most stable state. For example, the most stable allotropic state of carbon is graphite, and it is also found under normal conditions where the pressure values ​​are 1 atmosphere and temperature is 25 degrees. The importance of knowing enthalpy is crucial to understanding the energy involved in various chemical reactions, as well as in the analysis of impacts of heat waves.
We emphasize that the enthalpies of formation according to what we have defined are for 1 mole of compound produced. In this way, depending on the amount of existing reagent products, the reaction will have to be adjusted with fractional coefficients.
Formation enthalpy
We know that in any chemical process, the enthalpy of formation can be both positive and negative. This enthalpy is positive when the reaction is endothermic. That a chemical reaction is endothermic means that it can absorb the heat of the medium. On the other hand, we have a negative enthalpy when the reaction is exothermic. That a chemical reaction is exothermic means that it emits heat from the system to the outside.
For an exothermic reaction to occur, the reactants must have greater energy than the products. Conversely, for an endothermic reaction to occur, the reactants must have less energy than the products. For the chemical equation for all of this to be written correctly, the law of conservation of matter must be fulfilled. That is, the chemical equation must contain information about the physical state of the reactants and products, which is key to the study of .
You also have to keep in mind that substances that are pure have an enthalpy of formation equal to zero. These enthalpy values ​​are obtained under standard conditions, as mentioned above, and in their most stable form. In a chemical system with reactants and products, the enthalpy of reaction is equal to the enthalpy of formation under standard conditions.
Enthalpy of reaction
We have already mentioned what enthalpy of formation is. Now we are going to describe what the enthalpy of reaction is. It is a thermodynamic function that helps to calculate the heat that has been gained or the heat that has been delivered during a chemical reaction. A balance is sought in the heat exchange between both reactants and products. One of the aspects that must be met when calculating the enthalpy of reaction is that the reaction itself must occur at constant pressure. That is, throughout the entire time it takes for the chemical reaction to occur, the pressure must remain constant.
We know that enthalpy has dimensions of energy and that is why it is measured in joules. To understand the relationship of enthalpy to the heat that is exchanged during a chemical reaction it is necessary to go to the first law of thermodynamics. And it is that this first law tells us that the heat that is exchanged in a thermodynamic process is equal to the variation of the internal energy of the substance or substances involved in the process plus the work done by said substances during the process.
We know that all chemical reactions are nothing more than various thermodynamic processes that occur at a given pressure. The most common pressure values ​​occur under standard atmospheric pressure conditions. Therefore, all thermodynamic processes that occur in this way are called isobaric, since they occur at constant pressure.
It is very common to call enthalpy heat. However, it must be clear that it is not the same as heat, but heat exchange. That is, it is not the heat that can teach a lesson or the internal heat that the reactants and products have. It is the heat that is exchanged throughout the chemical reaction process.
Relationship with heat
Unlike what we discussed before, enthalpy is a state function. When we calculate the enthalpy change, we are actually calculating the difference of two functions. These functions usually depend exclusively on the state of the system. This state of the system varies depending on the internal energy and the volume of the system itself. As we know, the state remains constant throughout the chemical reaction, the enthalpy of reaction is nothing more than a state function that depends on both internal energy and volume.
Therefore, we can define the enthalpy of the reactants in a chemical reaction as the sum of each of them. On the other hand, we define the same thing but in the products as the sum of the enthalpy of all the products.
I hope that with this information you can learn more about enthalpy and its characteristics.