Reference states of elements

As discussed in Chapter 7, the reference states of elements are chosen as the state that the element is stable in at 1.0 bar pressure and the temperature of interest. 

The value of AH is an approximate measure of the stability of a substance relative to the elements from which it is made. The standard enthalpies of formation of graphite, diamond, water, ethyne (acetylene, C2H2), ammonia and sodium chloride are shown in Fig. 13.6. The reference states of elements define an energy baseline or sea level . Compounds such as ethyne, for which AHf is positive, and which therefore possess a greater enthalpy than their constituent elements, appear above sea level and are called endothermic compounds. Compounds such as water, ammonia and sodium chloride, for which AHf is negative and which therefore possess a lower enthalpy than their constituent elements, appear below sea level and are called exothermic compounds. 

Phosphorus is an exception to the rule regarding reference states of elements. Although red phosphorus is the stable allotrope at 298.15 K, it is not well characterized. Instead, the reference state is white phosphorus (crystalline P4) at 1 bar. 

When ionic solutions form, the ions occur in pairs, so it is not possible to isolate the enthalpy of formation of a positive or negative ion. Hence it is not possible to obtain the heats of formation of ions with the usual reference state of elements in their standard state. The enthalpy of formation for ions is tabulated by defining AHf for as zero at all temperatures. Thus... 

Thermodynamics deals with processes and reactions and is rarely concerned with the absolute values of the internal energy or enthalpy of a system, for example, only with the changes in these quantities. Hence the energy changes must be well defined. It is often convenient to choose a reference state as an arbitrary zero. Often the reference state of a condensed element/compound is chosen to be at a pressure of 1 bar and in the most stable polymorph of that element/compound at the... 

Standard states. The standard or reference state of each of the elemental substances is taken to be that physical state (or one of them, if there are two or more) in which the element naturally exists at a pressure, or a fugacity, of one atmosphere and at a temperature of 18°. The isotopic composition of each element in its standard state is understood to be the naturally existing one. For the element carbon, we have selected its form as diamond, C (c, diamond), as the standard state because no other form of solid carbon is at present a reproducible and invariable one. 

The sum of bond enthalpies is therefore related (with change of sign) to AHf of the gaseous compound, but with an altered reference state of gaseous atoms El(g) [rather than the standard form El(std)] for each element El, ... 

In choosing a reference state, we are allowed to make a choice for each element, because elements cannot be transformed into each other by chemical means. The choice usually made for the reference state of an element is the form in which it is stable at temperature T and the standard pressure =1.0 bar. For example, at most temperatures, for 02 this would be gaseous diatomic molecules for iron, it would be the solid metal, and for bromine, it would be the diatomic in the liquid state below 59°C and in the gaseous state above 59°C. We call the standard enthalpy change of the reaction in which 1 mol of compound i is formed from its component elements in their reference states the heat of formation of compound i, A H°(T). The heat of reaction is related to heats of formation as... 

As mentioned, the reference-state for elemental bromine changes from liquid to gas at 59°C. Which of the following is discontinuous at 59°C Why ... 

The mass determination of ionic species (atomic or polyatomic ions) in mass spectrometry is always a comparative measurement, which means the mass of an ionic species is determined with respect to reference masses of elements (or substances) used for mass calibration. The reference mass is thus acquired from the mass unit (m = In = 1/12) of the mass of the neutral carbon isotope (m = 1.66 X 10 kg). A mass calibration is easy to perform in solid-state mass spectrometry if the sample contains carbon (using carbon cluster ions with whole masses, as discussed above). The so-called doublet method was apphed formerly, e.g., ions and doubly charged Mg + forming a doublet at the same nominal mass number 12 were considered, where they are slightly displaced with respect to one another. The doublet method is no longer of relevance in modern inorganic mass spectrometry. Orientation in the mass spectra can be carried out via the matrix, minor and trace elements after mass calibration and by comparing the measured isotopic pattern of elements with theoretical values. 

Thermodynamic properties taken from Robie, Hemingway, and Fisher are based on a reference state of the elements in their standard states at 1 bar (10 P = 0.987 atm). This change in reference pressure has a negligible effect on the tabulated values for the condensed phases. [For gas phases only data from NBS (reference state = 1 atm) are given.]... 

With the use of f and as in A the implication is that both the compound in question and its constituent elements arc in standard states and that the elements, moreover, are in their reference states for any given temperature the reference states of the elements will normally be those that are stable at the chosen standard-state pressure and at that temperature. A resulting feature of tabulations and 4 f G as functions of temperature for compounds is that discontinuous changes are sometimes to be seen these correspond to changes in the stable reference states of the elements, as phase-transition temperatures are passed. Thus, values of AfH" (S02Cl2,g) would show discontinuous changes at three temperatures corresponding to the transitions S(cr,I)->S(cr,II), S(cr,II) S(/), and S(/) l/2 82(g), where I refers to rhombic and II to monoclinic crystal forms. 

It should be emphasised that the use of superscript,", e.g., in Af//°, implies that the compound in question is in the standard state and that the elements are in their reference states. The reference states of the elements at the reference temperature cf. Section II.3.3) are listed in Table II-6. 

Eq.(11.48) applies also to AfG" - AfG , sinee the Gibbs energy of formation describes the formation process of a compound or complex from the reference states of the elements involved ... 

The layout of the tables and the functions quoted correspond to conventions which are also used in standard works such as the JANAF Tables and the Tables of the U.S. Bureau of Mines. The following thermochemical functions are tabulated heat capacity Cp, entropy S, Gibbs energy function —Gef = - [C-//(298.15)1 / 7] enthalpy H, enthalpy increment //-//(298.15), Gibbs energy G = H-TS, and the formation quantities AH(,AG and logA f. The formation reactions refer to the reference states of the elements, which are given in a separate table. 

The standard states of elements are denoted as reference phases. (See Section 4.5 and Chapter 10). 

An entry of 0.0 for AfH° for an element indicates the reference state of that element. See References 1 and 2 for further information on reference states. A blank means no value is available. 

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