Thermodynamics under standard conditions

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. 

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. ... 

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

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. 

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 ... 

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%. 

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 ... 

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 ... 

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. 

Under standard conditions at 25 °C, AVt = 1.481V. From thermodynamic considerations, electrolysis at AV < A tn proceeds with heat absorption from the environment, whereas the opposite is the case at AV> A tn- At AV=A t > no net exchange takes place between the cell and the environment and the term thermoneutral has been coined to emphasize such a situation. 

From the thermodynamic data of Appendix C, show that the product of the reaction of ammonia gas with oxygen would be nitrogen, rather than nitric oxide, under standard conditions and in the absence of kinetic control by, for example, specific catalysis of NO formation by platinum. (Assume the other product to be water vapor.)... 

Use the data of Appendix C to show that the aqueous hypochlorite ion is thermodynamically unstable with respect to exothermic decomposition (AH°= —60.1 kJ mol-1, AG°= —95.0 kJ mol-1) to aqueous chloride and oxygen gas under standard conditions. 

The ranges of Eh and pH over which a particular chemical species is thermodynamically expected to be dominant in a given aqueous system can be displayed graphically as stability fields in a Pourbaix diagram,10-14 These are constructed with the aid of the Nernst equation, together with the solubility products of any solid phases involved, for certain specified activities of the reactants. For example, the stability field of liquid water under standard conditions (partial pressures of H2 and 02 of 1 bar, at 25 °C) is delineated in Fig. 15.2 by... 

The energy released by hydrolysis of ATP and the other nucleoside triphosphates is accounted for by the structure of the triphosphate group. The bond between the ribose and the a phosphate is an ester linkage. The a, ft and ft,y linkages are phosphoanhydrides (Fig. 8-40). Hydrolysis of the ester linkage yields about 14 kJ/mol under standard conditions, whereas hydrolysis of each anhydride bond yields about 30 kJ/mol ATP hydrolysis often plays an important thermodynamic role in biosynthesis. When coupled to a reaction with a positive free-energy change, ATP hydrolysis shifts the equilibrium of the overall process to favor product forma-... 

A technical handbook contains tables of thermodynamic quantities for common reactions. If you want to know whether a certain reaction is spontaneous under standard conditions, which of the following properties would give you that information directly (on inspection) Which would not Explain your answers, (a) AGr° (b) AHr° (c) ASr° (d) AUt° ... 

The synthesis of chromium carbide required a high temperature owing to the competition between carbon and oxygen. Figure 14.1 shows that, under standard conditions, Cr203 is thermodynamically favoured at low temperatures. The oxide formation is probably due to water impurities present in the gas phase. The oxide layer is thicker than the usual passivation layer and this oxidation process at low oxygen pressure has already been explained earlier by Moreau and Benard.14... 

Thermodynamics is used to predict whether reactants have a spontaneous tendency to change into products. This tendency is associated with a decrease in the free energy or Gibbs energy of the system (G) to a minimum. As a consequence, the thermodynamic criterion for spontaneous change at constant temperature and pressure is AG < 0. Under standard conditions (concentrations = 1 M, and P = 1 atm), the standard Gibbs energy variation (AG°) is related with the equilibrium constant (A) by equation 11 ... 

Equations (47)-(50) indicate how thermodynamic quantities can be obtained from cell potentials measured under standard conditions. However, standard states are hypothetical states (e.g., infinitely dilute behavior at 1.0 m concentration), which cannot be prepared in the cell. As a result, an extrapolation procedure is used to find 8° from measured cell voltages as a function of concentration. From Eq. (47), we write the dependence of 8 on the concentration of the electrolyte in the form... 

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