Gibbs-Helmholtz

In modern separation design, a significant part of many phase-equilibrium calculations is the mathematical representation of pure-component and mixture enthalpies. Enthalpy estimates are important not only for determination of heat loads, but also for adiabatic flash and distillation computations. Further, mixture enthalpy data, when available, are useful for extending vapor-liquid equilibria to higher (or lower) temperatures, through the Gibbs-Helmholtz equation. ... 

The partial molar enthalpy for every component i is found from an appropriate form of the Gibbs-Helmholtz equation... 

Gibbs-Helmholtz equation This equation relates the heats and free energy changes which occur during a chemical reaction. For a reaction carried out at constant pressure... 

Helmholtz free energy The maximum amount of energy available to do work resulting from changes in a system at constant volume. See free energy and Gibbs-Helmholtz equation. 

Convert AS° to kJ/K using the relation 1J = 10-3 kj. Finally, calculate AG° from the Gibbs-Helmholtz equation with T = 298 K. 

The Gibbs-Helmholtz equation can be used to calculate the standard free energy of formation of a compound. This quantity, AGf, is analogous to the enthalpy of formation, AH . It is defined as the free energy change per mole when a compound is formed from the elements in their stable states at 1 atm. 

To find AG° at temperatures other than 25°C, use the Gibbs-Helmholtz equation. 

Strategy The first step is to write a balanced equation for the chemical reaction. Then calculate AH° using Table 8.3 (page 209) and AS° using Table 17.1 (page 456). Finally, use the Gibbs-Helmholtz equation to determine AG° at 230°C (3 sig. fig.)... 

As far as the Gibbs-Helmholtz equation is concerned, there is another reason for ignoring the temperature dependence of AH and AS. These two quantities always change in the same direction as the temperature changes (i.e., if AH becomes more positive, so does AS). Hence the two effects tend to cancel each other. 

From Example 17.5 and the preceding discussion, it should be clear that AG°, unlike AH° and AS°, is strongly dependent on temperature. This comes about, of course, because of the T in the Gibbs-Helmholtz equation ... 

A change in reaction conditions can, and often does, change the direction in which a reaction occurs spontaneously. The Gibbs-Helmholtz equation in the form... 

When the temperature of a reaction system is increased, the sign of AG°, and hence the direction in which the reaction proceeds spontaneously, may or may not change. Whether it does or does not depends on the signs of AH° and AS°. The four possible situations, deduced from the Gibbs-Helmholtz equation, are summarized in Table 17.2 (p. 464). 

Strategy This is another application of the Gibbs-Helmholtz equation. Setting AG° = 0 gives AH° = TAS° solving for T gives T = AH°/AS°. Since you know AH° and AS°, T is readily calculated. 

Clausius-Clapeyron, 230,303-305 Gibbs-Helmholtz, 459,461 Henderson-Hasselbalch, 79-80 mass relation, 62-63 Nemst, 493-494 net ionic, 79-80 nonmetals, 575q nuclear, 513,530-531q Planck s, 135 redox, balancing, 90-92 Schrodinger, 140 thermochemical, 204... 

Ghiorso, Albert, 515 Giauque, William, 174 Gibbs, J. Willard, 459 Gibbs-Helmholtz equation The relation AG = AH - TAS, 459,461,474q Gillespie, R. J., 175 Glucose... 

The relation between reaction free energy, temperature, cell voltage, and reversible heat in a galvanic cell is reflected by the Gibbs-Helmholtz equation [Eq. (31)]. 

There is a very important equation relating to the electromotive forces of reversible cells which was deduced independently by J. Willard Gibbs (1875) and H. von Helmholtz (1882), and is usually called the Gibbs-Helmholtz Equation. 

Thus E is, for fixed concentrations of the substances used up and produced in the reaction, determined by the value of the equilibrium constant K, at the given temperature. The change of E with temperature is given generally by the Gibbs-Helmholtz... 

Equation (3.42) is called the Gibbs-Helmholtz equation. We will find it to be a very useful relationship. A similar derivation would show that... 

We use the Gibbs-Helmholtz relationship to obtain the derivatives on the left... 

The EMF values of galvanic cells and the electrode potentials are usually determined isothermally, when all parts of the cell, particularly the two electrode-electrolyte interfaces, are at the same temperature. The EMF values will change when this temperature is varied. According to the well-known thermodynamic Gibbs-Helmholtz equation, which for electrochemical systems can be written as... 

The Gibbs-Helmholtz equation also links the temperature coefficient of Galvani potential for individual electrodes to energy effects or entropy changes of the electrode reactions occurring at these electrodes. However, since these parameters cannot be determined experimentally for an isolated electrode reaction (this is possible only for the full current-producing reaction), this equation cannot be used to calculate this temperature coefficient. 

The above equation is known as the Gibbs-Helmholtz equation. It enables the evaluation of the AH of a reaction from a knowledge of the free energy change (AG) and of its temperature coefficient [(8AG)/(8T)]P. 

Expressed in this form, the Gibbs-Helmholtz equation is widely used in experimental thermodynamics to determine AH, the enthalpy of a reaction, from the experimentally de-... 

The Gibbs-Helmholtz equation is applicable to closed systems of fixed composition undergoing isobaric (constant pressure) processes. 

The Van t Hoff isotherm establishes the relationship between the standard free energy change and the equilibrium constant. It is of interest to know how the equilibrium constant of a reaction varies with temperature. The Varft Hoff isochore allows one to calculate the effect of temperature on the equilibrium constant. It can be readily obtained by combining the Gibbs-Helmholtz equation with the Varft Hoffisotherm. The relationship that is obtained is... 

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