Standard thermodynamic conditions

DMF as a useful polar solvent is produced industrially on a large technical scale (250000 tons/year) by carbonylation of dimethylamine in the presence of methanol [59]. Using Raney nickel as catalyst, the synthesis of DMF from dimethyl-amine, CO2, and hydrogen was first discovered by Farlow and Adkins [60]. The formation of DMF from dimethylamine, H2, and CO2 is thermodynamically favorable under standard conditions thermodynamic data are given for aqueous reactants and liquid products in eq. (6) [61]. The enthalpy of DMF production (Aff = -56.5 kJ mor, AG° = -0.75 kJ mol, = -119 kJ rnor K- ) is more... 

For pure organic materials, it is also possible to calculate the heating value starting from the heats of formation found in tables of thermodynamic data. The NHV is obtained using the general relation of thermochemistry applicable to standard conditions of pressure and temperature (1 bar and 25°C)) f 9j... 

The thermodynamics of electrochemical reactions can be understood by considering the standard electrode potential, the potential of a reaction under standard conditions of temperature and pressure where all reactants and products are at unit activity. Table 1 Hsts a variety of standard electrode potentials. The standard potential is expressed relative to the standard hydrogen reference electrode potential in units of volts. A given reaction tends to proceed in the anodic direction, ie, toward the oxidation reaction, if the potential of the reaction is positive with respect to the standard potential. Conversely, a movement of the potential in the negative direction away from the standard potential encourages a cathodic or reduction reaction. 

Corrosion occurs even if the two reactants involved are not at standard conditions. In this case the nonstandard equiUbrium potential for each reaction, often referred to as the reversible potential, can be calculated from the Nemst equation. Additional information on thermodynamic aspects of corrosion can be found in Reference 10. 

Table l.Estimated Thermodynamic Properties of Various Substances at Standard Conditions... 

If there are no standard conditions or in the case where it is not be possible to measure the standard potential, the value can be determined by thermodynamic calculations (see Sec. 1.3.2). 

Besides measuring the potential in the standard conditions, it is possible to calculate its value from thermodynamic data [9]. In addition one can determine the influence of changing pressure, temperature, concentration, etc. 

Estimate the temperature at which it is thermodynamically possible for carbon to reduce iron(III) oxide to iron under standard conditions by the endothermic reaction... 

We can use the electrochemical series to predict the thermodynamic tendency for a reaction to take place under standard conditions. A cell reaction that is spontaneous under standard conditions (that is, has K > 1) has AG° < 0 and therefore the corresponding cell has E° > 0. The standard emf is positive when ER° > Et that is, when the standard potential for the reduction half-reaction is more positive than that for the oxidation half-reaction. 

Still, the question has to be addressed as to which of the many modifications of Prl2 is thermodynamically stable under standard conditions. So far, it is clear that Prl2-IV must be a high-temperature phase as it is produced in pure and single-phase by annealing just below the peritectic temperature (with an excess of praseodymium metal in order to avoid the formation of Pr2ls) and rapid cooling to ambient temperature. 

The thermodynamics of nitrogen chemistry helps explain why N2 is so abundant in the atmosphere, and yet the element remains inaccessible to most life forms. Table 14-4 shows that most of the abundant elements react with O2 spontaneously under standard conditions. This is why many of the elements occur in the Earth s crust as their oxides. However, N2 is resistant to oxidation, as shown by the positive A Gj for NO2. ... 

Electrochemical cells can be constructed using an almost limitless combination of electrodes and solutions, and each combination generates a specific potential. Keeping track of the electrical potentials of all cells under all possible situations would be extremely tedious without a set of standard reference conditions. By definition, the standard electrical potential is the potential developed by a cell In which all chemical species are present under standard thermodynamic conditions. Recall that standard conditions for thermodynamic properties include concentrations of 1 M for solutes in solution and pressures of 1 bar for gases. Chemists use the same standard conditions for electrochemical properties. As in thermodynamics, standard conditions are designated with a superscript °. A standard electrical potential is designated E °. 

C19-0059. Use standard thermodynamic values to determine whether or not each of the following redox reactions is spontaneous under standard conditions ... 

To calculate the equilibrium composition of a mixture at a given temperature, we first need to calculate the equilibrium constant from thermodynamic data valid under the standard conditions of 298 K and 1 bar, as in Tab. 2.2. Differentiating Eq. (22) and using AG° = A - TAS° we obtain the Van t Hoff equation ... 

The factor Dg can either be determined from the dissociation energy and the ground state vibration energy or from thermodynamic data. The heat of formation of H atoms from H2 molecules can be found in the literature, but some care should be exercised in considering the total energy content of H atoms and H2 molecules under standard conditions. 

When the metal can form a stable carbide, the product of the carbothermic reduction of its oxide may be a carbide instead of the metal itself. The question as to whether a carbide or the metal forms under standard conditions when the oxide is reduced by carbon is not answered by the Ellingham diagram. To obtain an answer to this question, a more detailed consideration of the thermodynamic properties of the system is necessary. 

The sulfur dioxide enters the reactor with an initial concentration of 10% by volume, the remainder being air. At the exit of the first bed, the temperature is 620°C. Assume ideal gas behavior, the reactor operates at 1 bar and R = 8.3145 kJ-Kr -lonoD1. Assume air to be 21% 02 and 79% N2. Thermodynamic data at standard conditions at 298.15 K are given in Table 6.186. 

Table 6.18 Thermodynamic data at standard conditions and 298.15 K for sulfuric acid production.

From the above equations, it is seen that the value of K is related to the value of AG° and E° of the cell, but not AG and E of the cell. E°, AG° and K are indicators of the thermodynamic tendency of an oxidation-reduction reaction to occur under standard conditions. 

The enthalpy change of a reaction, AH, is the heat energy change when the reaction is carried out at constant pressure. It is necessary to express these values under standard conditions. For enthalpy changes measured under standard conditions, the symbol AH is used. Thermodynamic standard conditions are ... 

Initially, both electrodes are at equilibrium. Since the anode has accumulated electrons and the cathode has depleted electrons, electrons begin to flow from electrode from the anode to the cathode. The thermodynamic driving force for the electron flow is the electrode potential difference, which for the fuel cell reaction is 1.23 Y at standard conditions. In addition to electron flow, H + ions produced at the anode diffuse through the bulk solution and react at the cathode. The reaction is able to continue as long as H2 is fed at the anode and 02 at the cathode. Hence, the cell is not at equilibrium. The shift in electrode potential from equilibrium is called the overpotential (>/). 

A fascinating point, especially to physical chemists, is the potential theoretical efficiency of fuel cells. Conventional combustion machines principally transfer energy from hot parts to cold parts of the machine and, thus, convert some of the energy to mechanical work. The theoretical efficiency is given by the so-called Carnot cycle and depends strongly on the temperature difference, see Fig. 13.3. In fuel cells, the maximum efficiency is given by the relation of the useable free reaction enthalpy G to the enthalpy H (AG = AH - T AS). For hydrogen-fuelled cells the reaction takes place as shown in Eq. (13.1a). With A//R = 241.8 kJ/mol and AGr = 228.5 under standard conditions (0 °C andp = 100 kPa) there is a theoretical efficiency of 95%. If the reaction results in condensed H20, the thermodynamic values are A//R = 285.8 kJ/ mol and AGR = 237.1 and the efficiency can then be calculated as 83%. 

To investigate these two questions, a parametric model of the Jacobian of human erythrocytes was constructed, based on the earlier explicit kinetic model of Schuster and Holzhiitter [119]. The model consists of 30 metabolites and 31 reactions, thus representing a metabolic network of reasonable complexity. Parameters and intervals were defined as described in Section VIII, with approximately 90 saturation parameters encoding the (unknown) dependencies on substrates and products and 10 additional saturation parameters encoding the (unknown) allosteric regulation. The metabolic state is described by the concentration and fluxes given in Ref. [119] for standard conditions and is consistent with thermodynamic constraints. 

In general, the study of the variation of the formal electrode potential of a redox process with temperature has thermodynamic implications. Hence, one is interested in the measurement of AG°, AS° and AH° for the electron transfer process. It is recalled from thermodynamics that, under standard conditions, AE° is directly proportional to the free energy of the redox reaction according to the equation ... 

Monsanto developed the rhodium-catalysed process for the carbonylation of methanol to produce acetic acid in the late sixties. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. At standard conditions the reaction is thermodynamically allowed,... 

Reaction (9) generates methyl iodide for the oxidative addition, and reaction (10) converts the reductive elimination product acetyl iodide into the product and it regenerates hydrogen iodide. There are, however, a few distinct differences [2,9] between the two processes. The thermodynamics of the acetic anhydride formation are less favourable and the process is operated much closer to equilibrium. (Thus, before studying the catalysis of carbonylations and carboxylations it is always worthwhile to look up the thermodynamic data ) Under standard conditions the AG values are approximately ... 

Thermodynamics of hydroformylation and hydrogenation of propene at standard conditions are as follows ... 

The insertion of CO is in many instances thermodynamically unfavourable the thermodynamically most favourable product in hydroformylation and carbonylation reactions of the present type is always the formation of low or high-molecular weight alkanes or alkenes, if chain termination occurs via (3-hydride elimination). The decomposition of 3-pentanone into butane and carbon monoxide shows the thermodynamic data for this reaction under standard conditions. Higher pressures of CO will push the equilibrium somewhat to the left. 

AG <0, thermodynamically spontaneous (energy released, often irreversible) AG >0, thermodynamically nonspontaneous (energy required) AG = 0, reaction at equilibrium (freely reversible) AG = energy involved under standardized conditions Decrease energy of activation, AG ... 

The results obtained under standard conditions can be used to predict thermodynamic behavior at other concentrations and temperatures. To derive the necessary equations, consider the general redox reaction. 

From thermodynamic principles, chemists have demonstrated that the free energy change at nonstandard conditions, Ac , is related to the free energy change imder standard conditions, Ac °, by... 

---