Thermochemistry enthalpy changes

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 ... 

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. 

The first law of thermodynamics leads to a broad array of physical and chemical consequences. In the following Sections 3.6.1-3.6.8, we describe the formal theory of heat capacity and the enthalpy function, the measurements of heating effects that clarified the energy and enthalpy changes in real and ideal gases under isothermal or adiabatic conditions, and the general first-law principles that underlie the theory and practice of thermochemistry, the measurement of heat effects in chemical reactions. 

The currently preferred method for the study of gas forming reactions as function of temp is DSC. Here the specimen and the ref sample are heated at programmed heating rates (in controlled atms, if desired) while the differential energy input to the specimens is recorded. Hence the pen movement is directly proportional to the heat capacity while the area under the curve represents the enthalpy change. New equipment is now on the market which can operate up to 1200° (Ref 79) and which is therefore adequate for expl and propint studies. Limitations on the use of DSC for kinetics studies of expls will be discussed under the entry Thermochemistry in this Vol... 

If a process can be imagined to occur in successive steps, AH for the overall process is equal to the sum of the enthalpy changes for the individual steps. This rule, sometimes called Hess s law of constant heat summation, has many applications in thermochemistry. 

The experimental study of heat produced or absorbed in chemical reactions is usually called thermochemistry. Such investigations are often best conducted by direct calorimetric measurements, but values for heat quantities and their time and temperature derivatives can also be obtained from other kinds of thermodynamic experiments. A heat quantity, Q, which is lost or gained in a process conducted under constant pressure, p (in chemistry and biology most frequently the atmospheric pressure), is defined as the enthalpy change, AH, accompanying the process. 

Thermochemistry is a branch of thermodynamics that deals with the change of heat (enthalpy) in chemical reactions. The heat absorbed or lost in chemical reactions usually occurs at constant pressure rather than at constant volume. The change of heat is mostly expressed by AH, the enthalpy change of a process from reactants to products ... 

Hypoiodous acid and iodine nitrate have been investigated with coupled cluster theory and the results extrapolated to the complete basis set limit. Together with revised thermochemistry for several ancillary molecules, the enthalpy changes of working reactions yields new thermochemistry for HOI and IONO2. The latter data, employed in unimolecular rate theory, appear to be consistent with kinetic measurements on the lO -l- NO2 reaction to within the uncertainties of the kinetic analysis. 

CHAPTER 12 Thermodynamic Processes and Thermochemistry 507 TABLE 12.2 Enthalpy Changes of Fusion and Vaporizationt... 

The enthalpy change for a chemical reaction in which all reactants and products are in their standard states and at a specified temperature is called the standard enthalpy (written AFf°) for that reaction. The standard enthalpy is the central tool in thermochemistry because it provides a systematic means for comparing the energy changes due to bond rearrangements in different reactions. Standard enthalpies can be calculated from tables of reference data. For this purpose, we need one additional concept. The standard enthalpy of formation AH° of a compound is defined to be the enthalpy change for the reaction that produces 1 mol of the compound from its elements in their stable states, all at 25°C and 1 atm pressure. For example, the standard enthalpy of formation of liquid water is the enthalpy change for the reaction... 

Tables of AH° for compounds are the most important data source for thermochemistry. From them it is easy to calculate AH° for reactions of the compounds, and thereby systematically compare the energy changes due to bond rearrangements in different reactions. Appendix D gives a short table of standard enthalpies of formation at 25°C. The following example shows how they can be used to determine enthalpy changes for reactions performed at 25°C and 1 atm pressure.

The heat that flows across the boundaries of a system undergoing a change is a fundamental property that characterizes the process. It is easily measured, and if the process is a chemical reaction carried out at constant pressure, it can also be predicted from the difference between the enthalpies of the products and reactants. The quantitative study and measurement of heat and enthalpy changes is known as thermochemistry... 

Note that arbitrarily assigning zero AHf for each element in its most stable form at the standard state does not affect our calculations in any way. Remember, in thermochemistry we are interested only in enthalpy changes, because they can be determined experimentally whereas the absolute enthalpy values cannot. The choice of a zero reference level for enthalpy makes calculations easier to handle. Again referring to the terrestrial altitude analogy, we find that Mt. Everest is 8708 ft higher than Mt. McKinley. This difference in altitude is unaffected by the decision to set sea level at 0 ft or at 1000 ft. 

Table 4.1 Thermochemistry and volumetric changes of a selection of oxidation reactions of sodium alanate by oxygen or water vapor. Free energy and enthalpy changes are given per mol of NaAIH4. Volumetric changes refer to the solid compounds only and are percentages based on NaAIH4...

We have seen that the heat supplied to a system at constant pressure is equal to the enthalpy increase. For many chemical reactions it is possible to make a direct determination of the heat change at constant pressure for other reactions indirect methods (to be discussed later) can be used. In these ways it has proved possible to build a considerable body of data on the enthalpy changes of chemical processes of all kinds and these data have been of great value in an understanding of molecular structure and in various other ways, The study of enthalpy changes in chemical systems is known as thermochemistry. 

The subject of thermochemistry deals with the heat changes resulting from chemical processes its laws are direct consequences of the first law of thermodynamics. As most reactions are carried out under constant pressure, our treatment will be restricted to a discussion of enthalpy changes. A corresponding set of equations could easily be obtained for the internal energy. In this section we discuss heat changes in chemical reactions and the thermochemistry of solutions. 

The negative value of the enthalpy change for Equation 8 when n = 0 is defined (7) as the electron affinity (EA) of the oxidized species when the oxidized and reduced species are in their ground rotational, vibrational and electronic states (0 K). At any temperature for any value of n (0, positive, or negative), the thermodynamic state functions for l uation 8 are given by aX (X = G, H, or S), and the thermochemistry of electron attachment can be defined in the ion convention ("stationary electron convention") (7). The relationship between EA and aG is given by Equation 9. A similar relationship applies for adiabatic ionization energies. 

Step 1 is the decomposition of reactants into elements in their standard states. But this is just the opposite of the formation reaction of the reactants, so the enthalpy change of the process is -AHf°(reactants). Similarly, Step 2, the formation of the products from elements in their standard states, has an enthalpy change of AHf°(products). Remember, however, that the formation reaction is defined for the generation of one mole of the compound. Consequently, to use tabulated heats of formation we must multiply by the stoichiometric coefficients from the balanced equation to account for the number of moles of reactants consumed or products generated. Taking these factors into account leads to one of the more useful equations in thermochemistry. 

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