Thermodynamic conventions

By thermodynamic convention, l Hp < 0 for exothermic reactions, so that a negative sign is attached to the heat-generation term. When there are multiple reactions, the heat-generation term refers to the net effect of all reactions. Thus, the term is an implicit summation over all M reactions that... 

Sol id Sol utions. The aqueous concentrations of trace elements in natural waters are frequently much lower than would be expected on the basis of equilibrium solubility calculations or of supply to the water from various sources. It is often assumed that adsorption of the element on mineral surfaces is the cause for the depleted aqueous concentration of the trace element (97). However, Sposito (Chapter 11) shows that the methods commonly used to distinguish between solubility or adsorption controls are conceptually flawed. One of the important problems illustrated in Chapter 11 is the evaluation of the state of saturation of natural waters with respect to solid phases. Generally, the conclusion that a trace element is undersaturated is based on a comparison of ion activity products with known pure solid phases that contain the trace element. If a solid phase is pure, then its activity is equal to one by thermodynamic convention. However, when a trace cation is coprecipitated with another cation, the activity of the solid phase end member containing the trace cation in the coprecipitate wil 1 be less than one. If the aqueous phase is at equil ibrium with the coprecipitate, then the ion activity product wi 1 1 be 1 ess than the sol ubi 1 ity constant of the pure sol id phase containing the trace element. This condition could then lead to the conclusion that a natural water was undersaturated with respect to the pure solid phase and that the aqueous concentration of the trace cation was controlled by adsorption on mineral surfaces. While this might be true, Sposito points out that the ion activity product comparison with the solubility product does not provide any conclusive evidence as to whether an adsorption or coprecipitation process controls the aqueous concentration. 

The heat of explosion is then calculated as the difference between the sum of the heats of formation of the products, and the sum of the heats of formation of the reactants, using the usual thermodynamic convention that heat evolved is negative. 

The lattice energy U is defined as the energy released (U is therefore negative by thermodynamic convention) when a mole of the requisite free gaseous ions comes together from infinite interionic separation to make up the crystal. If N is Avogadro s number (6.0221 x 1023), we have the Bom-Lande formula ... 

Before we go on to define the acidity constant of a given organic acid in water, we need to introduce a thermodynamic convention for scaling such constants. We do this relative to H30+ in that we define the dissociation of H30+ in water to have a standard free-energy change ArG° = 0, which means that the equilibrium constant of this reaction is equal to 1 ... 

Electron affinity is conventionally defined as the energy released when an electron is added to the valence shell of an atom. Unfortunately, this is in contradiction to the universal thermodynamic convention that enthalpies of exothermic reactions shall be assigned negative signs. Since it seems impossible to overthrow the election affinity convention at this late date without undue confusion, one can adopt one of two viewpoints to minimize confusion. One is to let the electron affinities of the most active nonmetals be positive, even though in thermodynamic calculations the enthalpies are negative ... 

Note that the definition of electron affinity does not follow the usual thermodynamic convention in ihat a positive electron affinity is exothermic. See Chapter 2. 

In the past the electrostatic convention has often been called the European convention and the thermodynamic convention popularized by Luitmer (The Oxidation Potentials of the Elements and Their Values in Aqueous Solution Prenlicc-HBlI Englewood Cliffs. NJ, (952) the American convention. In an effort to reduce confusion, the IUPAC adopted the "Stockholm convention" in which electrode potentials refer to the electrostatic potential and end s refer to the thermodynamic quantity. Furthermore, the recommendation is that standard reduction potentials be listed as electrode potentials" to avoid the possibility of confusion over signs. 

Pro memoria in this book, in opposition to the thermodynamic convention, we consider all effects, which increase the temperature of the system as positive. Thus, the enthalpy has a minus sign. 

According to thermodynamic conventions for the assignment of emf values to electrode assemblies, summarized in Table s2.1, the emf across the silver-silver chloride electrode pair is given by the following equation ... 

These thermodynamic concepts denote the energy which is bound during the formation of a given compound from its constituent elements at constant volume (energy of formation) or at constant pressure (enthalpy of formation, which includes the mechanical work performed at the standard state44) (25 °C = 77°F and a pressure of 1 bar). The data are tabulated in accordance with thermodynamic convention if the formation of a compound from its elements is... 

Because two different conventions have been used, there is a good deal of confusion in the chemical literature about the signs for electron affinity values. Electron affinity has been defined in many textbooks as the energy released when an electron is added to a gaseous atom. This convention requires that a positive sign be attached to an exothermic addition of an electron to an atom, which opposes normal thermodynamic conventions. Therefore, in this... 

According to thermodynamic convention a spontaneous cell reaction has a negative free-energy change numerically equal to the electrical work (in joules) per unit of reaction as written. In general, the total free-energy change is... 

Because the concentration (activity) of water is essentially constant in dilute aqueous solutions (a mole fraction of unity), the hydration of the proton can be ignored in defining acid-base equilibria. Because the equilibrium activity of the proton and of H30 are not known separately, the thermodynamic convention sets the standard free energy change AG for reaction 10 equal to zero that is, Kt, = 1. In dealing with dilute solutions we can, because of this convention, represent the aquo hydrogen ion by H (aq), or more conveniently by H" that is. 

According to thermodynamic convention Ky = 1 Thus According to thermodynamic convention A j = 1 Thus ... 

Because of the more usual thermodynamic convention regarding the sign of changes in H and U, hj , and y are of opposite sign to and Trpy,... 

The minus sign appears in this equation because the same thermodynamic convention is followed as in Chapters 12 and 13—that work done by the system (here, an electrochemical cell) has a negative sign. Because work is measured in joules and charge in coulombs, has units of joules per coulomb. One volt (abbreviated V) is 1 joule per coulomb. Because the total charge Q is the current / multiplied by the time t in seconds during which the current is flowing, this equation for the electrical work can also be written... 

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