Chemical elements standard state

For a free energy of fonnation, the preferred standard state of the element should be the thennodynamically stable (lowest chemical potential) fonn of it e.g. at room temperature, graphite for carbon, the orthorhombic crystal for sulfiir. 

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

The values given in the following table for the heats and free energies of formation of inorganic compounds are derived from a) Bichowsky and Rossini, Thermochemistry of the Chemical Substances, Reinhold, New York, 1936 (h) Latimer, Oxidation States of the Elements and Their Potentials in Aqueous Solution, Prentice-Hall, New York, 1938 (c) the tables of the American Petroleum Institute Research Project 44 at the National Bureau of Standards and (d) the tables of Selected Values of Chemical Thermodynamic Properties of the National Bureau of Standards. The reader is referred to the preceding books and tables for additional details as to methods of calculation, standard states, and so on. 

Enthalpy of Formation The ideal gas standard enthalpy (heat) of formation (AHJoqs) of chemical compound is the increment of enthalpy associated with the reaction of forming that compound in the ideal gas state from the constituent elements in their standard states, defined as the existing phase at a temperature of 298.15 K and one atmosphere (101.3 kPa). Sources for data are Refs. 15, 23, 24, 104, 115, and 116. The most accurate, but again complicated, estimation method is that of Benson et al. " A compromise between complexity and accuracy is based on the additive atomic group-contribution scheme of Joback his original units of kcal/mol have been converted to kj/mol by the conversion 1 kcal/mol = 4.1868 kJ/moL... 

Entropy of Formation The ideal gas standard entropy of formation (AS°298) of a chemical compound is the increment of entropy associated with the reaction of forming that compound in the ideal gas state from the constituent elements in their standard state definea as the existing phase at a temperature of 298.15 K and one atmosphere (101.325 kPa). Thus ... 

There are many ways to express the energy of a molecule. Most common to organic chemists is as a heat of formation, AHf. This is the heat of a hypothetical chemical reaction that creates a molecule from so-called standard states of each of its constituent elements. For example, AHf for methane would be the energy required to create CH4 from graphite and H2, the standard states of carbon and hydrogen, respectively. 

The standard heat of formation ( AH ) of a chemical compound is the standard heat of reaction corresponding to the chemical combination of its constituent elements to form one mole of the compound, each existing in its standard state at 1 atm and 25°C. It has units of cal/g-mole. 

List aU chemical species, both elements and compounds, that are believed to exist at equilibrium. By element we mean the predominant species at standard-state conditions, for example, O2 for oxygen at 1 bar and 298.15 K. 

The standard enthalpy of formation, A fH, of a compound at 0 K reflects the strength of the chemical bonds in the compound relative to those in the constituent elements in their standard state. The standard enthalpy of formation of a binary oxide such as CaO is thus the enthalpy change of the reaction... 

It is obvious from the definition of standard enthalpy of formation that these quantities do not represent the absolute enthalpic stability of compounds. They merely reflect their enthalpic stability relative to that of the chemical elements in standard reference states (to which AfH° = 0 has been arbitrarily assigned). It is thus unreasonable to state that a given substance is more stable than another just because it has a lower standard enthalpy of formation. We can only use AfH° values to make such direct comparisons when we are assessing the relative stability of isomers. 

It is, of course, not necessary to have an extensive list of heats of reaction to determine the heat absorbed or evolved in every possible chemical reaction. A more convenient and logical procedure is to list the standard heats of formation of chemical substances. The standard heat of formation is the enthalpy of a substance in its standard state referred to its elements in their standard states at the same temperature. From this definition it is obvious that heats of formation of the elements in their standard states are zero. 

As the Gibbs function is a thermodynamic property, values of AG do not depend on the intermediate chemical reactions that have been used to transform a set of reactants, under specified conditions, to a series of products. Thus, one can add known values of a Gibbs function to obtain values for reactions for which direct data are not available. The most convenient values to use are the functions for the formation of a compound in its standard state from the elements in their standard states, as given in Tables 7.2... 

In this equation, the standard chemical potential MAgjS 6e used because the deviation of 6 from stoichiometry is much less than 1. According to the definition of the standard potential, = 0, is the standard Gibbs energy of the element in the standard state AG . Then... 

In Fig. 1, various elements involved with the development of detailed chemical kinetic mechanisms are illustrated. Generally, the objective of this effort is to predict macroscopic phenomena, e.g., species concentration profiles and heat release in a chemical reactor, from the knowledge of fundamental chemical and physical parameters, together with a mathematical model of the process. Some of the fundamental chemical parameters of interest are the thermochemistry of species, i.e., standard state heats of formation (A//f(To)), and absolute entropies (S(Tq)), and temperature-dependent specific heats (Cp(7)), and the rate parameter constants A, n, and E, for the associated elementary reactions (see Eq. (1)). As noted above, evaluated compilations exist for the determination of these parameters. Fundamental physical parameters of interest may be the Lennard-Jones parameters (e/ic, c), dipole moments (fi), polarizabilities (a), and rotational relaxation numbers (z ,) that are necessary for the calculation of transport parameters such as the viscosity (fx) and the thermal conductivity (k) of the mixture and species diffusion coefficients (Dij). These data, together with their associated uncertainties, are then used in modeling the macroscopic behavior of the chemically reacting system. The model is then subjected to sensitivity analysis to identify its elements that are most important in influencing predictions. 

Figure 10,1 (A) Activity-molar concentration plot. Trace element concentration range is shown as a zone of constant slope where Henry s law is obeyed. Dashed lines and question marks at high dilution in some circumstances Henry s law has a limit also toward inhnite dilution. The intercept of Henry s law slope with ordinate axis defines Henry s law standard state chemical potential. (B) Deviations from Nernst s law behavior in a logarithmic plot of normalized trace/carrier distribution between solid phase s and ideal aqueous solution aq. Reproduced with modifications from liyama (1974), Bullettin de la Societee Francaise de Mineralogie et Cristallographie, 97, 143-151, by permission from Masson S.A., Paris, France. A in part A and log A in part B have the same significance, because both represent the result of deviations from Henry s law behavior in solid.

The free-energy change in forming a compound at temperature T in its standard state from its elements in their standard states (also at temperature T) is defined as AG°p the standard free energy of formation for a chemical compound. The standard state for a gaseous species is a pressure of 1 bar, denoted p°. 

THERMOCHEMISTRY. That aspect of chemistry which deals with die heat changes which accompany chemical reactions and processes, the heal produced by them, and die influence of temperature and odier thermal quantities upon them. Tt is closely related to chemical thermodynamics. The heat of formation of a compound is the heat absorbed when it is formed from its elements in their standard states. An exothermic reaction evolves heat and endothermic reaction requires heat for initiation. 

There s no point in trying to synthesize a substance from its elements under standard-state conditions if the substance has a positive value of AG°f. Such a substance would have to be prepared at other temperatures and /or pressures, or it would have to be made from alternative starting materials using a reaction that has a negative free-energy change. Thus, a knowledge of thermodynamics can save considerable time in chemical synthesis. 

One of the most fundamental concepts of chemistry is the distinction between kinetic and thermodynamic factors nonetheless, such arguments are frequently ignored, or at best only tacitly considered, in wider discussions of reactivity. Chemical thermodynamics is concerned with the energetic relationships between chemical species. The most useful parameter is the Gibbs free energy, G, which, like all thermodynamic terms, is based on an arbitrary scale placing a value of zero upon pure elements in their stable standard states at 298 K and 1 atmosphere pressure. Differences between free energies are denoted by AG, as shown in Eq. (1.1). 

Table 16-1 compiles some data for S°, the molar entropy, and AGp the free energy of formation from the elements. All values in Table 16-1 are presented at 25°C and at standard states. Notice that the units of entropy and free energy are stated per mole, mol-1. This means that the moles used to balance a chemical reaction are included by the multiplication of the coefficient (mol in balanced equation) and the value from the table so that unit mol cancels. This is also the way in which we handled calculations involving AH values. 

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