Standard enthalpy of formation listed

Using the standard enthalpies of formation listed in Table 9.4, calculate the standard enthalpy change for the overall reaction that occurs when ammonia is burned in air to form nitrogen dioxide and water. This is the first step in the manufacture of nitric acid. 

Calculate the heats of combustion for the following reactions from the standard enthalpies of formation listed in Appendix 3 ... 

The values of standard enthalpies of formation listed in Table 6.2 and in other tables are determined by direct measurement in some cases and by applying Hess s law in others. Oxides, such as water, can often be determined by direct calorimetric measurement of the combustion reaction. If you look back at Example 6.7, you will see an illustration of how Hess s law can be used to obtain the enthalpy of formation of tungsten carbide, WC. 

Now let us see how to use standard enthalpies of formation (listed in Table 6.2) to find the standard enthalpy change for a reaction. We will first look at this problem from... 

The standard enthalpies of formation of ions in aqueous solution listed at the bottom of Table 8.3 are relative values, established by taking... 

There are millions of possible reactions, and it is impractical to list every one with its standard reaction enthalpy. However, chemists have devised an ingenious alternative. First, they report the standard enthalpies of formation of substances. Then they combine these quantities to obtain the standard enthalpy of reaction needed. Let s look at these two stages in turn. 

The standard enthalpy of formation of diamond is therefore reported as AHt°(C, diamond) = + 1.9 kj-mol l. Values for a selection of other substances are listed in Table 6.5 and Appendix 2A. 

Again, it is convenient to follow the seven-step procedure to solve this problem. We are asked to find an enthalpy of formation. Because enthalpy is a state function, we can visualize the reaction as occurring through decomposition and formation reactions. Appendix D lists enthalpies of formation, and the experimental heat of combustion is provided. We can use Equation to relate the enthalpy of combustion to the standard enthalpy of formation for octane. 

The third law of thermodynamics establishes a starting point for entropies. At 0 K, any pure perfect crystal is completely constrained and has S = 0 J / K. At any higher temperature, the substance has a positive entropy that depends on the conditions. The molar entropies of many pure substances have been measured at standard thermodynamic conditions, P ° = 1 bar. The same thermodynamic tables that list standard enthalpies of formation usually also list standard molar entropies, designated S °, fbr T — 298 K. Table 14-2 lists representative values of S to give you an idea of the magnitudes of absolute entropies. Appendix D contains a more extensive list. 

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

A similar exercise can be made with other anions and cations, producing a list of relative values of standard enthalpies of formation, anchored on Af77°(H+, ao) = 0. This database is rather useful, because it allows the enthalpies of formation (equation 2.53) and the lattice enthalpies (equation 2.47) of many crystalline ionic salts to be predicted, since their solution enthalpies are usually easy to measure. 

Some standard molar enthalpies of formation are listed in Table 5.3. Notice that the standard enthalpies of formation of most compounds are negative. Thus, most compounds are more stable than the elements they are made from. 

Standard enthalpies of formation of some sulfur compounds [Reprinted with permission from Ref. (20). Copyright 1958 American Chemical Society.] are listed below, together with that for S g) from tables of the National Bureau of Standards ... 

As we mentioned, it is necessary to have information about the standard enthalpy change for a reaction as well as the standard entropies of the reactants and products to calculate the change in Gibbs function. At some temperature T, A// j can be obtained from Af/Z of each of the substances involved in the transformation. Data on the standard enthalpies of formation are tabulated in either of two ways. One method is to list Af/Z at some convenient temperature, such as 25°C, or at a series of temperatures. Tables 4.2 through 4.5 contain values of AfZ/ at 298.15 K. Values at temperatures not listed are calculated with the aid of heat capacity equations, whose coefficients are given in Table 4.8. 

The calculations on the ammonium nitrate reaction demonstrate the immense value of tables that list the enthalpies for various substances. The values at 25° C and 1 atm are called standard enthalpies. For elements, the standard enthalpy is defined as zero. For compounds, the values are called standard enthalpies of formation because the compounds are considered to be formed from elements in their standard state. 

Just as there are tables of standard enthalpies of formation, values of the standard Gibbs free energy of formation, AfG, are listed [5—9] for very many compounds and these may be combined in a way analogous to eqn. (10)... 

One real value of tables of standard enthalpies of formation is that they permit the calculation of the standard enthalpy of any reaction for which all the reactants and products are listed it is not necessary to do an experimental measurement. Based on Hess s law, the basic premise of the use of tables is that the enthalpy of reaction is the difference between the sum of the enthalpies of the formation of the products and the sum of the enthalpies of formation of the reactants. That is,... 

Values of AG°f at 25°C for some common substances are listed in Table 17.3, and additional values are given in Appendix B. Note that AG°f for an element in its most stable form at 25°C is defined to be zero. Thus, solid graphite has AG°f = 0 kj/mol, but diamond, a less stable form of solid carbon at 25°C, has AG°f = 2.9kJ/mol. As with standard enthalpies of formation, AH°f, a zero value of AG°f for elements in their most stable form establishes a thermochemical "sea level," or reference point, with respect to which the standard free energies of other substances are measured. We can t measure the absolute value of a substance s free energy (as we can the entropy), but that s not a problem because we are interested only in free-energy differences between reactants and products. 

From the enthalpies of formation listed in standard tabulations, you will quickly find that AH" at 25 °C for the reaction is +234 kJ mol-1, so that it is out of the question as a realistic preparative route. From the table of bond energies, you will be able to offer the rationalisation that although the C-F bond is stronger than C-H, this does not compensate for the much weaker H-H compared with H-F bond. A more detailed explanation will require analysis of the factors which determine the relative strengths of E-H and E-F bonds. 

Many tables of values of standard changes of enthalpy of formation list values for individual ions, particularly in aqueous solutions. In order to do so, an arbitrary definition must be introduced because the properties of individual ions in solution cannot be determined. We consider an electrolyte Mv+Av which is completely ionized in the infinitely dilute solution. We choose this solution to be the standard state for the enthalpy. The enthalpy of this solution per mole of solute, H, is given by... 

The following is a list of functions for the reactants with which the standard transformed enthalpies of formation can be calculated. It is shorter than the list of reactants for the transformed Gibbs energies because these is less information about standard enthalpies of formation of species. 

The input speciesmat is a matrix that gives the standard Gibbs energy of formation at 298.15 K, the standard enthalpy of formation at 298.15 K, the electric charge, and the number of hydrogen atoms in each species. There is a row in the matrix for each species of the reactant, gfnsp is alist of the functions for the species, hfnsp is a list of the functions for the enthalpies of the species. )... 

In the gas phase, monomeric SO3 has a >3h planar structure with bond length S-0 142 pm, as shown in Fig. 16.6.2(a). The cyclic trimer (803)3 occurs in colorless orthorhombic y-S03 (mp 16.9°C), and its structure is shown in Fig. 16.6.2(b). The helical chain structure of P-SO3 is shown in Fig. 16.6.2(c). A third and still more stable form, a-S03 (mp 62°C), involves cross-linkage between the chains to give a complex layer structure. The standard enthalpies of formation of the four forms of SO3 at 298 K are listed below ... 

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