Energy changes reversibility

The enthalpy, entropy and free energy changes for an isothennal reaction near 0 K caimot be measured directly because of the impossibility of carrying out the reaction reversibly in a reasonable time. One can, however, by a suitable combination of measured values, calculate them indirectly. In particular, if the value of... 

Cell Volta.ge a.ndIts Components. The minimum voltage required for electrolysis to begin for a given set of cell conditions, such as an operational temperature of 95°C, is the sum of the cathodic and anodic reversible potentials and is known as the thermodynamic decomposition voltage, is related to the standard free energy change, AG°C, for the overall chemical reaction,... 

These reactions are reversible and ordinarily do not involve large energy changes. 

This interaction energy is reversible because removal of the wetting liquid from the surface only requires the disruption of these interaction sites. Solidification of the liquid into an adhesive changes the requirements for dewetting, however. 

But spontaneity depends on the concentrations of reactants and products. If the ratio [Bl YCA] is less than a certain value, the reaction is spontaneous in the forward direction if [Bl YCA] exceeds this value, the reaction is spontaneous in the reverse direction. Therefore, it is useful to define a standard free-energy change (AG°) which applies to a standard state where [A] = [B] = 1 M. 

The fixation of carbon dioxide to form hexose, the dark reactions of photosynthesis, requires considerable energy. The overall stoichiometry of this process (Eq. 22.3) involves 12 NADPH and 18 ATP. To generate 12 equivalents of NADPH necessitates the consumption of 48 Einsteins of light, minimally 170 kj each. However, if the preceding ratio of l ATP per NADPH were correct, insufficient ATP for COg fixation would be produced. Six additional Einsteins would provide the necessary two additional ATP. Prom 54 Einsteins, or 9180 kJ, one mole of hexose would be synthesized. The standard free energy change, AG°, for hexose formation from carbon dioxide and water (the exact reverse of cellular respiration) is +2870 kj/mol. 

The net free energy change, AG°, for this conversion is —37.7 kj/mol. The consumption of a total of six nucleoside triphosphates drives this process forward. If glycolysis were merely reversed to achieve the net synthesis of glucose from pyruvate, the net reaction would be... 

The driving force of the reaction is the free energy change AC which is related to the reversible or equilibrium e.m.f. of the cell by the relationship... 

An increased yield stress is required [62] in order to reverse the unfavourable conformations of the molecular chains that develop during annealing. This explanation is supported by the energy changes observed in annealed polymers. The enthalpy difference, as determined by DSC was AH = 1.8 J/g (Sect. 4.2), whereas the additional work required for yielding in an annealed sample was... 

The half-cell potential and the half-cell free energy change are related by the following relationship for reversible conditions ... 

One main advantage of such a power source is the direct transformation of the chemical energy of methanol combustion into electrical energy. Hence, the reversible cell potential, can be calculated from the Gibbs energy change, AG, associated with the overall combustion reaction of methanol (1), by the equation ... 

For a system at equilibrium, the free energy change, AG, at constant temperature and pressure is zero. Let two phases, a and (3, of the same substance, in equilibrium at a temperature T and pressure P, be considered. If the two phases are reversibly converted into one another (a (3) by withdrawing or supplying heat, the system is always in equilibrium, and AG is zero. It readily follows that the molar free energy of the substance in the two phases in equilibrium is the same... 

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