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Standard states, conventional reactions

Enthalpies of reaction can also be calculated from individual enthalpies of formation (or heats of formation), AHf, for the reactants and products. Because the temperature, pressure, and state of the substance will cause these enthalpies to vary, it is common to use a standard state convention. For gases, the standard state is 1 atm pressure. For a substance in an aqueous solution, the standard state is 1 molar concentration. And for a pure substance (compound or element), the standard state is the most stable form at 1 atm pressure and 25°C. A degree symbol to the right of the H indicates a standard state, AH°. The standard enthalpy of formation of a substance (AHf) is the change in enthalpy when 1 mol of the substance is formed from its elements when all substances are in their standard states. These values are then tabulated and can be used in determining A//°rxn. [Pg.127]

Thus AG0 indicates a change for a reaction in which [H+] is standardised at 10 7 mol dm-3 to distinguish it from the case of AG° under normal standard state convention (when [H+] = 1 mol dm-3). [Pg.34]

The change in free energy of a reaction in the standard state (conventionally, all reactants and products at IM) is related to the equilibrium constant for the reaction by the following relation ... [Pg.38]

Conventions about standard states (the reference states introduced earlier) are necessary because otherwise the meaning of the standard free energy of a reaction would be ambigrious. We sunnnarize the principal ones ... [Pg.367]

Standard-state potentials are generally not tabulated for chemical reactions, but are calculated using the standard-state potentials for the oxidation, E°o, and reduction half-reactions, fi°red- By convention, standard-state potentials are only listed for reduction half-reactions, and E° for a reaction is calculated as... [Pg.147]

Standard Hydrogen Electrode The standard hydrogen electrode (SHE) is rarely used for routine analytical work, but is important because it is the reference electrode used to establish standard-state potentials for other half-reactions. The SHE consists of a Pt electrode immersed in a solution in which the hydrogen ion activity is 1.00 and in which H2 gas is bubbled at a pressure of 1 atm (Figure 11.7). A conventional salt bridge connects the SHE to the indicator half-cell. The shorthand notation for the standard hydrogen electrode is... [Pg.471]

The effect of pressure on AG° and AH0 depends on the choice of standard states employed. When the standard state of each component of the reaction system is taken at 1 atm pressure, whether the species in question is a gas, liquid, or solid, the values of AG° and AH0 refer to a process that starts and ends at 1 atm. For this choice of standard states, the values of AG° and AH0 are independent of the system pressure at which the reaction is actually carried out. It is important to note in this connection that we are calculating the enthalpy change for a hypothetical process, not for the actual process as it occurs in nature. This choice of standard states at 1 atm pressure is the convention that is customarily adopted in the analysis of chemical reaction equilibria. [Pg.8]

The notion of standard enthalpy of formation of pure substances (AfH°) as well as the use of these quantities to evaluate reaction enthalpies are covered in general physical chemistry courses [1]. Nevertheless, for sake of clarity, let us review this matter by using the example under discussion. The standard enthalpies of formation of C2H5OH(l), CH3COOH(l), and H20(1) at 298.15 K are, by definition, the enthalpies of reactions 2.3,2.4, and 2.5, respectively, where all reactants and products are in their standard states at 298.15 K and the elements are in their most stable physical states at that conventional temperature—the so-called reference states at 298.15 K. [Pg.9]

Jnst as free protons do not exist in solution in acid-base reactions, there are no free electrons in redox reactions. However it is possible to define the activity of electrons relative to a specified standard state and thereby treat electrons as discrete species in equilibrinm calcnlations in the same way as ions and molecules. The standard state of electron activity for this pnrpose is by convention defined with respect to the redox conple made by hydrogen ions and hydrogen gas ... [Pg.94]

The cell reaction for cells without liquid junction can be written as the sum of an oxidation reaction and a reduction reaction, the so-called half-cell reactions. If there are C oxidation reactions, and therefore C reduction reactions, there are C C — 1) possible cells. Not all such cells could be studied because of irreversible phenomena that would take place within the cell. Still, a large number of cells are possible. It is therefore convenient to consider half-cell reactions and to associate a potential with each such reaction or electrode. Because of Equation (12.88), there would be (C - 1) independent potentials. We can thus assign an arbitrary value to the potential associated with one half-cell reaction or electrode. By convention, and for aqueous solutions, the value of zero has been assigned to the hydrogen half-cell when the hydrogen gas and the hydrogen ion are in their standard states, independent both of the temperature and of the pressure on the solution. [Pg.347]

In the second case the standard hydrogen electrode is placed on the right-hand side of the representation of the cell, and the other electrode would be placed on the left-hand side. The emf of the cell would then be written as = i a — ij/c. The value of if/f is defined to be zero and the potential of the electrode on the left, t a, is the emf of the cell. The symbol ip is called the oxidation potential. When all of the reacting substances of the electrode are in their standard states, then // would become ip° and would be called the standard oxidation potential. This terminology is that of Latimer and emphasizes the nature of the reaction taking place at the electrode. We present it here for completeness, knowing that reduction potentials are now the standard convention, but that some of the older literature used oxidation potentials. [Pg.348]

With the establishment of conventions for the Standard State and for the reference zero value of the chemical potential, it is possible to develop fully the thermodynamic description of chemical reactions. This development relies on the concept of thermodynamic activity, introduced in Section 1.2, and on the condition for chemical equilibrium in a reaction 1,15... [Pg.25]

While absolute entropy values can now be determined absolute values of Internal Energy and Enthalpy cannot be conceived. For ease of calculation, related especially to metallurgical reactions (constant pressure processes), a suitable reference point of enthalpy is conventionally chosen and that is - for pure elements, the enthalpy is zero when in Standard State . Standard... [Pg.57]

The heat of formation is always given on a per mole basis, and its units are (cal)/(g mole), (kcal)/(g mole), or (Btu)/(lb mole). By convention, the heat of formation of all the elements in their normal state of aggregation in the standard state is zero. Table 9.1 lists the standard heats of formation of a number of inorganic compounds. Table 9.2 lists the standard heats of formation of a number of explosive compounds and common explosive reaction products. [Pg.118]

A convention has been adopted to simplify free-energy calculations for biochemical reactions. The standard state is defined as having a pH of 7. Consequently, when H+ is a reactant, its activity has the value 1 (corresponding to a pH of 7) in equations 1 and 4 (below). The activity of water also is taken to be 1 in these equations. The standardfree-energy... [Pg.309]

In natural waters, other surface reactions will be occurring simultaneously. These include protonation and deprotonation of the >FeOH site at the inner o-plane and complexation of other cations and anions to either the inner (o) or outer (IS) surface planes. Expressions similar to Equation (5) above can be written for each of these reactions. In most studies, the activity coefficients of surface species are assumed to be equal to unity thus, the activities of the surface sites and surface species are equal to their concentrations. Different standard states for the activities of surface sites and species have been defined either explicitly or implicitly in different studies (Sverjensky, 2003). Sveijensky (2003) notes that the use of a hypothetical 1.0 M standard state or similar convention for the activities of surface sites and surface species leads to surface-complexation constants that are directly dependent on the site density and surface area of the sorbent. He defines a standard state for surfaces sites and species that is based on site occupancy and produces equilibrium constants independent of these properties of the solids. For more details about the properties of the electrical double layer, methods to calculate surface specia-tion and alternative models for activity coefficients for surface sites, the reader should refer to the reference cited above and other works cited therein. [Pg.4763]

The An correction term to account for the change in the number of moles in the stoechiometric equation for the reaction is necessary because the thermochemical quantities are tabulated for a standard state at 1 atm., while the rate constants are expressed in concentration units (see also rel. 2.2.24). Because of this convention two constants are necessary, R = 1.987 kcal mol, and R = 0.082 L mol deg. ... [Pg.102]

To sum up, a formation reaction is understood by convention to be a reaction that forms 1 mole of compound from the elements that make it up. Standard heats of reaction at 25°C and 1 atm for any reaction can be calculated from the tabulated (or experimental) standard heats of formation values by using Eq. (4.34), because the standard heat of formation is a state (point) function, as illustrated in the examples below. [Pg.440]


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See also in sourсe #XX -- [ Pg.8 ]




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