Free reaction enthalpy

The consumed electrical energy is 17 7 (U, cell voltage [V], 7, cell current [A]). The part U I — AH, that exceeds the enthalpy of the cell reaction AH = AG + T AS (AG, reaction free enthalpy (Gibbs energy)... 

The course of a pyrolytic reaction may be determined kinetically or thermodynamically. When the reaction is able to reach equilibrium, thermodynamic factors control the reaction outcome. The calculation of a reaction free enthalpy using rel. (2.2.25a) allows the prediction of the reaction course. Since reactions are displaced toward the formation of products when AG° < 0, the calculation of the temperature for which AG° =... 

As in a spontaneous irreversible reactions free enthalpy may only decrease, AZ. has negative value, < AZ. ... 

To deduce further information on the missing step reaction, density functional theory (DFT) calculations have been applied toward estimating reaction enthalpies, A//r, and reaction free enthalpies, AGr, at 298.15K for several of the 1 + 4 processes. The AGr value provides a measure of the driving force behind the individual reaction channels. Although such calculations for small molecules are... 

Reaction enthalpies, AHr, and reaction free enthalpies, ACr, at 298.15 K estimated via DFT calculations (LIB3LYP, 6-3lG(d,p)) for the addition of an ethyl radical to ethyl dithiobenzoate, for several radical-radical combination reactions, and for the missing step reactions of the four cross-combination products (4a, 4b, 4d, and 4 ) with an ethyl radical 1. The reaction enthalpy and reaction free enthalpy values are given in units of kj mol . ... 

Free energy changes for reactions, like enthalpy or entropy changes, are additive. That is, if Reaction 3 = Reaction 1 + Reaction 2 then AG3 — AG] + A G2... 

For the electrochemical cell reaction, the reaction free energy AG is the utilizable electrical energy. The reaction enthalpy AH is the theoretical available energy, which is increased or reduced by an amount TAS. The product of the temperature and the entropy describes the amount of heat consumed or released reversibly during the reaction. With tabulated values for the enthalpy and the entropy it is possible to obtain AG. ... 

The reversible reaction heat of the cell is defined as the reaction entropy multiplied by the temperature [Eq. (15)]. For an electrochemical cell it is also called the Peltier effect and can be described as the difference between the reaction enthalpy AH and the reaction free energy AG. If the difference between the reaction free energy AG and the reaction enthalpy AH is below zero, the cell becomes warmer. On the other hand, for a difference larger than zero, it cools down. The reversible heat W of the electrochemical cell is therefore ... 

A remarkable feature of the metathesis reaction is that the enthalpy difference between products and reactants (AHr) is virtually zero, because the total number and the types of the chemical bonds are equal before and after the reaction. Hence, ideally, the free enthalpy of the reac-... 

Of course, even in the case of acyclic alkenes reaction enthalpy is not exactly zero, and therefore the product distribution is never completely statistically determined. Table V gives equilibrium data for the metathesis of some lower alkenes, where deviations of the reaction enthalpy from zero are relatively large. In this table the ratio of the contributions of the reaction enthalpy and the reaction entropy to the free enthalpy of the reaction, expressed as AHr/TASr, is given together with the equilibrium distribution. It can be seen that for the metathesis of the lower linear alkenes the equilibrium distribution is determined predominantly by the reaction entropy, whereas in the case of the lower branched alkenes the reaction enthalpy dominates. If the reaction enthalpy deviates substantially from zero, the influence of the temperature on the equilibrium distribution will be considerable, since the high temperature limit will always be a 2 1 1 distribution. Typical examples of the influence of the temperature are given in Tables VI and VII. 

Calculate the standard reaction entropy, enthalpy, and Gibbs free energy for each of the following reactions from data found in Appendix 2A ... 

Table 3 contains the enthalpies, zero point energies, entropies and free enthalpies of the activation and reaction steps (3)—(5). The enthalpies are the pure differences of the enthalpies of formation calculated by MINDO/3 at T = 298 K in the gas phase. The free enthalpies were calculated with the help of enthalpies corrected by the zero point energies and of the entropies given in Table 3. 

Table 3. Enthalpies, free enthalpies, zero point energies (ZPE) (all in kJ mol-1) and entropies (J K 1 mol-1) of the activation and reaction steps for Eqs. (3)-(5)...

Although carbonylation of the 2-norbomyl ion at or below room temperature leads to exclusive formation of the 2-ea o-norbomyloxo-carbonium ion, reactions at higher temperatures have shown that the 2-cwdo-norbornyloxocarbonium ion is just as stable as the exo-isomer (Hogeveen and Roobeek, 1969). This means that at low temperatures the carbonylation is kineticaUy controlled, and at high temperatures thermodynatnically controlled. The detailed free-enthalpy diagram in... 

It is of substantial interest to note that, c.s the temperature of the reaction mixture is increased to —33-5°, ion 19 is converted quantitatively back to 18. At that temperature the first-order rate constant for the reversion has been calculated to be 8-0 x 10 sec , which corresponds to a free enthalpy barrier AG ) of 17-4 kcal/mol. 

For a redox reaction in an electrochemical cell the decrease in free enthalpy (- AG) is in accordance with the energy delivered by the transfer of electrons through an external circuit if this takes place in a reversible way, i.e., at a rate slow enough to allow complete attainment of equilibrium, the conversion of 1 gram mole will deliver an electrical energy of - AG = z FE. In total cell reaction mred, + n ox2 m ox, + nred2, where m81 = nS2 electrons are transfered (<5, and S2 represent the respective valence differences of the two redox systems), we have... 

The standard-potential, E°, shows a temperature dependence called the "zero shift , according to its direct relationship with the free enthalpy for the standard conditions chosen, - AG° = RTIn K (eqn. 2.37), and the Arrhenius equation for the reaction rate,... 

In order to obtain a definite breakthrough of current across an electrode, a potential in excess of its equilibrium potential must be applied any such excess potential is called an overpotential. If it concerns an ideal polarizable electrode, i.e., an electrode whose surface acts as an ideal catalyst in the electrolytic process, then the overpotential can be considered merely as a diffusion overpotential (nD) and yields (cf., Section 3.1) a real diffusion current. Often, however, the electrode surface is not ideal, which means that the purely chemical reaction concerned has a free enthalpy barrier especially at low current density, where the ion diffusion control of the electrolytic conversion becomes less pronounced, the thermal activation energy (AG°) plays an appreciable role, so that, once the activated complex is reached at the maximum of the enthalpy barrier, only a fraction a (the transfer coefficient) of the electrical energy difference nF(E ml - E ) = nFtjt is used for conversion. 

When the free enthalpy of reaction AG for the transformation of the structure of a compound to any other structure is positive, then this structure is thermodynamically stable. Since AG depends on the transition enthalpy AH and the transition entropy AS, and AH and AS in turn depend on pressure and temperature, a structure can be stable only within a certain range of pressures and temperatures. By variation of the pressure and/or the temperature, AG will eventually become negative relative to some other structure and a phase transition will occur. This may be a phase transition from a solid to another solid modification, or it may be a transition to another aggregate state. 

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