Standard stale

For the convenience of tabulation and compulation of thermodynamic data, it is essential lo present them in a commonly accepted form relative to a single standard slate of reference. At all lemperatures, the standard stale for a pure liquid or solid is the condensed phase under a pressure of I atmosphere. The standard stale for a gas is the hypothetical ideal gas at anil fugarity (equivalent til a perfect gas" stale), in which state the enthalpy is that of the real gas at the same temperature when the pressure approaches aero. Values of thermodynamic quantities for standard-state conditions are identified by a superscriptQ. and Hn. for instance, is the enthalpy change of a reaction when reactants and products are in the standard state. 

The standard enthalpy of formation, A H (also represented by A or simply H ). of a substance at a given temperature is by definition, the enthalpy change when I mole of the substance in its standard state is formed, isothermally, at the indicated temperature from the elements, each in its standard stale. Usual units are kiloealories/mole. For all elements in their stable form at 25 C (298.15 K), the enthalpy of formation is zero If solid substances have more than one crystalline form, the most stable one is taken as the standard state, and the others have slightly different enthalpies. This convention about zero enthalpy is arbitrary hut universally accepted, and it may be compared to the arbitrary choice of zero lor terrestrial altitudes. The combination of enthalpies of formation, enthalpies of transition, and heat capacities makes possible the calculation of the enthalpy of a substance, in a given state at a given temperature, relative to a commonly accepted reference. 

T" 0,1 MPh or 101,32, kPa x mole fraction, m molality, mean ionic molality White I and Sn arc ihc Standard Stale solid phases lor these twit chemical elements. 

For a chosen value of the log activity parameter taken as the independent variable, and under the assumption that all soliil phases are in their Standard Stales, the solid that produces the largest value of the logarithm of the activity... 

Enthalply Reference ftmperalure = Tr> 298.15 K Standard Stale Pressure = aiMPa... 

Enthalply Reference ftinperalure = Tr 298.1SK Standard Stale Pressure p >0.1 MPa... 

Hence, the anme activity coefficient for y,. The activity and the activity coefficient, like the fbgaciiy coefficient, are normalized fogacities. However, unlike the normalizing factor xfr in the normalizing factors in j end y, contain property information, for pure i (when ff = ff) or for the mixture of which i is a component (when ff = JCU). Moreover, the numerical valnea of d, and y , unlike those of are arbitrary to tha extent that the choice of standard slate is arbitrary. Activities are identically equal to mole fractions for ideal solutions. For a real solution, d, approuches V, in en appropriate composition limit. Thus, for a Raoult s Law standard stale. ... 

For each of the formulations I. 2. aed 3. there are further choices one can make, For example, in Eq. (1.2-63) the choice of standard stales for the activity coefficients has been leh open. 

Equations (1.2-59), (1.2-60). and (l.4- )-(. 4-4) are valid regardless of the standard-stale conventions adopted for the components of the mixture, If Raonlt s Law staedard stales are used for all components (the usual procedure), then... 

When it is obvious that the solubility of a solid depends on intemiolecular forces between solute and solvent, it is pethaps not as obvious that this solubility also depends on how far the system temperature is removed from the solid s melting point and on the solute s anthalpy of fusion. These dependencies become clear when we relate the standard-stale fogacity of the solute in the solid phase 10 that in die liquid phase, as indicated below. 

The values 716 and 217.5 are the standaid entha ies of fonnation of gaseous carbon and hydrogen atoms from the elements in thmr standard stales. 

Chemists define a set of conditions, caled standard stales, in order to compare heats of different reactions. Each substance has a standard heat of formation (AHf), the heat of reaction when the substance is formed from its elements under these conditions. AH values are used to calculate the standard heat of reaction (AH ). 

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