The previous discussion should make it clear that it is possible to obtain the values of specific enthalpy of any substance at any temperature. It follows that if a process is carried out at a certain temperature—that is, both the feed and product streams are at that specified temperature—then a certain enthalpy change is associated with that process. The following generic reaction is an example:
A + (b/a) B → (c/a) C + (d/a) D
The enthalpy of reaction (or heat of reaction) is simply the difference between the enthalpies of products and enthalpies of reactants. The following shows this mathematically:
Here, v represents the stoichiometric coefficients of the species involved in the reaction. It should be noted that the equation for the reaction is written such that the stoichiometric coefficients of all the other species are normalized with respect to the stoichiometric coefficient of A; that is, the equation involves 1 mole of A and proportional moles of other species. Thus the enthalpy or heat of reaction, ΔHrxn is based on 1 mole of reactant A. Of course, the equation can be normalized on the basis of the stoichiometric coefficient of any other species involved in the reaction, with the enthalpy of the reaction changing proportionately.
If the process is conducted at standard conditions, then the enthalpy change is termed as the standard enthalpy change. For the reaction shown previously, the standard enthalpy of reaction follows:
If the standard enthalpies of the reactants are higher than those of the products, then the enthalpy of the reaction will be negative. The process involves starting with a material having higher chemical energy and ending up with a material with a lower energy. The difference between the two energies (or enthalpies) appears as the heat evolves during the transformation, making the process exothermic. Conversely, if standard enthalpies of the products are higher than those of the reactants, then the process involves starting with a material of lower energy and ending up with a material having higher energy. Such processes are termed endothermic. Figure 7.2 shows a conceptual schematic of the enthalpy changes in these two types of processes.
Figure 7.2 Conceptual schematic of enthalpy changes in endothermic and exothermic processes.
It is obvious that for an exothermic process, a mechanism for removing heat is necessary if it is desired to maintain a constant temperature. However, if the process is conducted adiabatically—that is, the system does not exchange heat with the surroundings—then the products will be at a higher temperature than the reactants. Conversely, if the process is endothermic, it will require heat input to maintain a constant temperature, and the adiabatic endothermic process will experience a decrease in temperature. Figure 7.3 shows the changes in temperature for an adiabatic system for both endothermic and exothermic processes.
Figure 7.3 Heat effects in transformations: temperature of adiabatic systems.
When the transformation involves a chemical reaction, the enthalpy effect is termed the enthalpy of reaction or the heat of reaction. The enthalpy (or heat) of reaction is termed enthalpy (or heat) of combustion when the reaction is of combustion of a substance. Transformations that are physical in nature—that is, transformations that do not involve chemical reactions—are also frequently (usually) accompanied by an enthalpy change. For example, heat effects accompany dissolution of a solute in a solution, and the change in enthalpy is termed enthalpy of solution or heat of solution. Similarly, enthalpy of mixing refers to the enthalpy change when the process involves mixing of different streams. These transformations can be endothermic or exothermic as well. In all these cases, the discussion presented above for reactive systems can be extended, mutatis mutandis, to other processes and transformations.
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