Standard Enthalpy of Formation and Reaction

The sea level reference point for all enthalpy expressions is called the standard enthalpy of formation (Affy). Substances are said to be in the standard state at 1 atm, hence the term standard enthalpy. The superscript represents standard-state 

The importance of the standard enthalpies of formation is mat once we know meir values, we can readily calculate me standard enthalpy of reaction, defined as the enthalpy of a reaction carried out at 1 atm. For example, consider the hypometical reaction 

TABLE 6.4 Standard Enthalpies of Formation of Some Inorganic Substances at 25°C  

To use Equation (6.18) to calculate AH% we must know the AHf values of the compounds that take part in the reaction. These values can be determined by applying the direct method or the indirect method. 

This method of measuring AHf works for compounds that can be readily synthesized from their elements. Suppose we want to know the enthalpy of formation of carbon dioxide. We must measure the enthalpy of the reaction when carbon (graphite) and molecular oxygen in their standard states are converted to carbon dioxide in its standard state  

A quantity of 1.00 X 10 mL of 0.500 M HCl was mixed with 1.00 X 10 mL of 0.500 MNaOH in a constant-pressure calorimeter of negligible heat capacity. The initial temperature of the HCl and NaOH solutions was the same, 22.50°C, and the final temperature of the mixed solution was 25.86°C. Calculate the heat change for the neutralization reaction on a molar basis 

Assume that the densities and specific heats of the solutions are the same as for water (1.00 g/mL and 4.184 J/g °C, respectively). 

Strategy Because the temperature rose, the neutralization reaction is exothermic. How do we calculate the heat absorbed by the combined solution What is the heat of the reaction What is the conversion factor for expressing the heat of reaction on a molar 

Solution Assuming no heat is lost to the surroundings, q ys = soin + 9rxn = 0, so = 4 soinj where q otn is the heat absorbed by the combined solution. Because the density of the solution is 1.00 g/mL, the mass of a 100-mL solution is 100 g. Thus, 

From the molarities given, the number of moles of both HCl and NaOH in 1.00 x 

---

Appendix D lists standard enthalpies of formation and standard entropies. Here are the values for the substances involved in this reaction ... 

The enthalpy of reaction 2.45 cannot be determined directly. As shown in figure 2.5, it is calculated by using several experimental quantities the standard enthalpy of formation of the solid alkoxide, the standard sublimation enthalpy and the ionization energy of lithium, and the standard enthalpy of formation and the adiabatic electron affinity of gaseous methoxy radical (equation 2.47). 

When calculating aH° and AS° values for a reaction, make sure you multiply the standard enthalpy of formation and the standard entropy of each substance by its corresponding stoichiometric coefficient. 

However, if you know both the standard enthalpies of formation and standard entropies for every substance in a reaction, you can estimate the free energy of reaction at other temperatures by using the equation... 

A Born-Haber cycle is the application of Hess s Law to the enthalpy of formation of an ionic solid at 298 K. Hess s law states that the enthalpy of a reaction is the same whether the reaction takes place in one step or in several. A Born-Haber cycle for a metal chloride (MCI) is depicted in Figure 1.56 the metal chloride is formed from the constituent elements in their standard state in the equation at the bottom, and by the clockwise series of steps above. From Hess s law, the sum of the enthalpy changes for each step around the cycle can be equated with the standard enthalpy of formation, and we get that ... 

The enthalpy change on formation of Portland cement clinker cannot be calculated with high precision, mainly because of uncertainties associated with the clay minerals in the raw material. Table 3.1 gives data for the main thermochemical components of the reaction, almost all of which have been calculated from a self-consistent set of standard enthalpies of formation, and which are therefore likely to be more reliable than other values in the literature. The conversion of the clay minerals into oxides is an imaginary reaction, but valid as a component in a Hess s law calculation. Few reliable thermochemical data exist for clay minerals those for pyrophyllite and kaolinite can probably be used with sufficient accuracy, on a weight basis. 

Oppermann et al. studied the ternary system Bi-O-Se. Six crystalline phases were prepared by high temperature synthesis from Bi203(cr) and Se02(cr). The phases were characterised by X-ray powder diffraction and IR spectroscopy. The standard enthalpies of formation and entropies of the compounds were obtained from the temperature dependence of the equilibrium constants of the decomposition reactions. The standard enthalpies of formation were also found from solution calorimetric work in which the enthalpies of dissolution of the compounds, Bi203(cr), and Se02(cr) in 4 M HCI were measured. It should benoted that the results in the paper were obtained with a correct value of the enthalpy of formation of Se02(cr) and not the erroneous value in Table II of tha paper. 

The thermodynamic data AG , AH and S° of tellurium halides were calculated from the e.m.f. s of the galvanic cells and their temperature dependences (see IX.B) by taking into account the data for the other compounds which were involved in the total cell reactions Literature data of the standard enthalpies of formation and standard entropies of silver , silver iodide , silver chloride silver bromide and tellurium have been used in the calculations. 

Analyze We are given the reactants (P4S3 and O2 from air) and the products (P4O10 and SO2) for a reaction, together with their standard enthalpies of formation, and asked to calculate the standard enthalpy change for the reaction. 

As explained in Section 2.2, for applications where chemical reactions are involved, it is advantageous that the reference points are set to the standard enthalpy of formation and to the absolute entropy, which is done in process simulators (Section 6.2). 

A particular class of enthalpies of formation is constituted by standard enthalpies of formation at a temperature T (written as ), which are those obtained when the compound and its elements are all in their standard state at that temperature. Thus, we have the relation between standard enthalpy of formation and enthalpy associated with the reaction of formation f. 

Strategy Obtain the relevant standard enthalpies of formation and standard entropies from the Resource section. Then calculate the standard reaction enthalpy and the standard reaction entropy from... 

Thermodynamic cycles are applications of Hess s law, which states that the total enthalpy change for (or heat of) a reaction is independent of the pathway followed from reactants to products. Max Born and Fritz Haber applied Hess s law to an ionic solid in 1917. The Born-Haber cycle for a general alkali-metal halide (M X) is shown in Figure 8.5. Equation (8.13) at the top of the cycle shows the formation of MX(j) from its constituent elements in their standard states and therefore corresponds to the standard enthalpy of formation. The reactions in the box of the figure... 

The heat of combustion of any substance can be determined from its standard enthalpy of formation and from the standard enthalpies of formation of the oxidation products, such as carbon dioxide and water. Let us consider the oxidation of glucose (reaction 9). Glucose has a rather complicated structure, but for the moment we ll just use its empirical formula ... 

---