Enthalpy heat changes

The standard enthalpy of formation AHf for a compound is defined as the enthalpy (heat) change for the reaction of formation from the elements under standard conditions, i.e., T = 25°C and P = 1 atm. It either can be measured directly by calorimetric measurements on the direct reaction if possible or may be calculated by Hess s law, which is a consequence of the first... 

The heat absorbed in a process at constant pressure is equal to AH, the increase in the enthalpy of the system. It can thus be said that the heat change accompanying a chemical reaction is equal to the difference between the total heat content of the products and that of the reactants, at constant pressure and temperature conditions. This quantity is called the heat of reaction, AH, and can be expressed as follows... 

The enthalpy of formation of a compound is a so-called thermodynamic state function, which means that the value depends only on the initial and final states of the system. When the formation of crystalline NaCl from the elements is considered, it is possible to consider the process as if it occurred in a series of steps that can be summarized in a thermochemical cycle known as a Born-Haber cycle. In this cycle, the overall heat change is the same regardless of the pathway that is followed between the initial and final states. Although the rate of a reaction depends on the pathway, the enthalpy change is a function of initial and final states only, not the pathway between them. The Born-Haber cycle for the formation of sodium chloride is shown as follows ... 

First law change in enthalpy heat of formation heat of reaction... 

Background Thermochemistry is the study of heat changes and transfers associated with chemical reactions. In this thermochemical laboratory study, you will determine the enthalpy change that occurs when a strong base, sodium hydroxide, reacts with a strong acid, hydrochloric acid. Other mixtures studied will include ammonium chloride mixed with sodium hydroxide and ammonia mixed with hydrochloric acid. These three reactions are represented as ... 

Several material properties exhibit a distinct change over the range of Tg. These properties can be classified into three major categories—thermodynamic quantities (i.e., enthalpy, heat capacity, volume, and thermal expansion coefficient), molecular dynamics quantities (i.e., rotational and translational mobility), and physicochemical properties (i.e., viscosity, viscoelastic proprieties, dielectric constant). Figure 34 schematically illustrates changes in selected material properties (free volume, thermal expansion coefficient, enthalpy, heat capacity, viscosity, and dielectric constant) as functions of temperature over the range of Tg. A number of analytical methods can be used to monitor these and other property changes and... 

LIG. 34 Schematic illustrations of changes in selected material properties (free volume, thermal expansion coefficient, enthalpy, heat capacity, viscosity, and dielectric constant) as functions of temperature over the range of Tg. 

In this chapter, you learned about thermochemistry, the heat changes accompanying chemical reactions. You learned about calorimetry, the technique used to measure these heat changes, enthalpies, and the types of heat capacities that we can use in thermochemistry calculations. Finally, you learned about Hess s law and how we can use it to calculate the enthalpy change for a specific reaction. 

Many of the reactions that chemists study are reactions that occur at constant pressure. During the discussion of the coffee-cup calorimeter, the heat change at constant temperature was defined as qp. Because this constant-pressure situation is so common in chemistry, a special thermodynamic term is used to describe this energy enthalpy. The enthalpy change, AH, is equal to the heat gained or lost by the system under constant-pressure conditions. The following sign conventions apply ... 

From the definition of the heat of reaction, Qp will depend on the temperature T at which the reaction and product enthalpies are evaluated. The heat of reaction at one temperature T0 can be related to that at another temperature 7. Consider the reaction configuration shown in Fig. 1.1. According to the First Faw of Thermodynamics, the heat changes that proceed from reactants at temperature T() to products at temperature 7), by either path A or path B must be the same. Path A raises the reactants from temperature T0 to 7, and reacts at 7). Path B reacts at T0 and raises the products from T0 to 7). This energy equality, which relates the heats of reaction at the two different temperatures, is written as... 

The enthalpy change of a process is equivalent to its heat change at constant pressure. 

In section 5.1, however, you learned that an enthalpy change represents the heat change between products and reactants at a constant pressure. Therefore, the calorimeter you use to determine an enthalpy change should allow the reaction to be carried out at a constant pressure. In other words, it should be open to the atmosphere. 

DSC is a method associated with enthalpy (heat content) change in which the difference in energy inputs into a substance and a reference material are subjected to a controlled temperature programme. By convention, the enthalpy change is regarded as negative when a reaction produces heat. 

First law change in enthalpy, heat of formation, heat of reaction, Hess s Law, heats of vaporization and fusion, calorimetry... 

B) The formation of hydrogen sulfide from hydrogen gas and sulfur conforms to the definition of both standard enthalpy of reaction and standard enthalpy of formation since the heat change that is measured is for the formation of one mole of compound from the elements in their standard state. In Choice (D), diamond is not the stable aUotrope of carbon—it is graphite. 

In chemical reactions, the change in the enthalpies of molecules is measured as heat energy. The heat change at the end of a chemical reaction under constant pressure is called the enthalpy change of the reaction, and indicated by AH. 

The enthalpy change associated with formation of a thermodynamically ideal solution is equal to zero. Therefore any heat change measured in a mixing calorimetry experiment is a direct indicator of the interactions in the system (Prigogine and Defay, 1954). For a simple biopolymer solution, calorimetric measurements can be conveniently made using titra-tion/flow calorimeter equipment. For example, from isothermal titration calorimetry of solutions of bovine P-casein, Portnaya et al. (2006) have determined the association behaviour, the critical micelle concentration (CMC), and the enthalpy of (de)micellization. 

To avoid the use of the ambiguous term "heat" in connection with "heat content," it is customary to use the term enthalpy. At a given temperature and pressure, every substance possesses a characteristic amount of enthalpy (H), and the heat changes associated with chemical and physical changes at constant pressure are called changes in enthalpy (AH) AHT is the enthalpy of transition. Two common enthalpies of transition are AHf = 1435 cal/mole for the enthalpy of fusion (melting) of ice at 0°C, and AH, = 9713 cal/mole for the enthalpy of vaporization of water at 100°C. 

In the definition of enthalpy change (p 215) and in all of the examples of heat changes and transfers we have discussed, there has been the limitation of constant pressure. 

Because reactions carried out at constant pressure are so common in chemistry, the heat change for such a process is given a special symbol, AH, called the heat of reaction, or enthalpy change, of the reaction. The enthalpy (H) of a system is the name given to the quantity E + PV. 

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