Reference state of an element

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... 

The formation reaction of a substance is the reaction in which the substance, at a given temperature and in a given physical state, is formed from the constituent elements in their reference states at the same temperature. The reference state of an element is usually chosen to be the standard state of the element in the allotropic form and physical state that is stable at the given temperature and the standard pressure. For instance, at 298.15 K and 1 bar the stable allotrope of carbon is crystalline graphite rather than diamond. 

The value of AfH° for a given substance depends only on T. By definition, AfH for the reference state of an element is zero. 

The problem with eqn 1.22 is that we have no way of knowing the absolute enthalpies of the substances. To avoid this problem, we can imagine the reaction as taking place by an indirect route, in which the reactants are first broken down into the elements and then the products are formed from the elements (Fig. 1.24). Specifically, the standard enthalpy of formation, AfH, of a substance is the standard enthalpy (per mole of the substance) for its formation from its elements in their reference states. The reference state of an element is its most stable form under the prevailing conditions (Table 1.7). Don t confuse reference state with standard state the reference state of carbon at 25 C is graphite (not diamond) the standard state of carbon is any specified phase of the element at 1 bar. For example, the standard enthalpy of formation of liquid water (at 25 C, as always in this text) is obtained from the thermochemical equation... 

Distinguish between (a) the standard state and the reference state of an element (b) endothermic and exothermic compounds. 

The general method of balancing electron-transfer equations requires that halfreaction equations be available. Short lists of common half-reactions, similar to Table 17-1, are given in most textbooks, and chemistry handbooks have extensive lists. However, no list can provide all possible half-reactions, and it is not practical to carry lists in your pocket for instant reference. The practical alternative is to learn to make your own half-reaction equations. There is only one prerequisite for this approach you must know the oxidation states of the oxidized and reduced forms of the substances involved in the electron-transfer reaction. In Chapter 8 you learned the charges on the ions of the most common elements now we review the method of determining the charge (the oxidation state) of an element when it is combined in a radical. 

The heat of formation (enthalpy of formation) of a compound is an important thermodynamic quantity, because a table of heats of formation of a limited number of compounds enables one to calculate the heats of reaction (reaction enthalpies) of a great many processes, that is, how exothermic or endothermic these reactions are. The heat of formation (enthalpy of formation) of a compound at a specified temperature T is defined [195] as the standard heat of reaction (standard reaction enthalpy) for formation of the compound at T from its elements in their standard states (their reference states). By the standard state of an element we mean the thermodynamically stablest state at 105 Pa (standard pressure, about normal atmospheric pressure), at the specified temperature (the exception is phosphorus, for... 

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. 

Note The term oxidation nunJjer is also referred to as oxidation state because an oxidation number represents an oxidation state of an element. 

The term oxidation originally referred to the combination of a substance with oxygen. This results in an increase in the oxidation state of an element in that substance. According to the original definition, the following reactions involve oxidation of the substance shown on the far left of each equation. Oxidation states are shown for one atom of the indicated kind. 

The reference form of an element for the purpose of specifying the formation reaction is usually the stablest form (physical state and allotrope) of the element under standard thermodynamic conditions. The reference form of oxygen at 25°C is 02(g) the reference form of carbon at 25°C is graphite. ... 

The standard state of an element is the pure substance at 1 bar (previously, 1 atm) and having the specified allotropic form, if necessary. Although there is no specified standard temperature, many references use 25.0°C as the designated temperature. 

The oxidation number of an element in a compound is a number assigned to that element that reflects how its electrons are involved in making up the compound. For example, if an atom of an element loses one electron and forms a +1 cation, the oxidation number of that element in a compound is +1. If an atom of an element gains one electron and forms a -1 ion, the oxidation number is -1. This is also often referred to as the oxidation state of an element. It is a sort of book-keeping concept that is used in some naming schemes, as well as to track electrons in redox reactions. Thus it is a concept of some importance. 

The chemical and physical state of an element affects the binding energy of its electrons, as mentioned in Chapter 2.1.2.3.3, p. 242. Energy level shifts can be followed with the help of X-ray transitions they are also directly measured by photoelectron spectroscopy. The subject was treated in [1, 5]. Shifts of inner levels may be observed as shown in various papers quoted in [2]. Energy level shift measurements and their application to surface state problems are discussed in [3]. As an example, Pd Mg level energy values from this reference are given below ... 

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... 

By this definition the enthalpy of formation of an element in its standard state is zero. In other words, elements in their standard states are taken as reference states in the tabulation of enthalpies of reaction, just as sea level is the reference point in measuring geographic heights. 

The values given in Table 3-5 were used for the enthalpies of the gases of atoms in their normal states (the reference states for the bond energies) relative to the standard states of the elements, to which the enthalpies of formation given in the Bureau of Standards compilation refer. Most of the values in Table 3-5 are taken from the Bureau of Standards compilation an important exception is the value for nitrogen, which has been shown by recent spectroscopic and thermochemical... 

A single reference temperature Tr must be defined, at which the standard-state enthalpy H°(Tr) of the elements (in their most stable form) are all defined to be zero, H°(Tr) = 0. This reference temperture is taken to be Tr = 298.15 K. The enthalpy of an element at a temperature other than Tr is nonzero, in general, that is, H°(T Tr) 0 (for an element). 

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, ... 

No elements are listed in Table 8.2 because, by definition, the most stable form of any element in its standard state has AH°f = 0 kj. (That is, the enthalpy change for formation of an element from itself is zero.) Defining AH°f as zero for all elements thus establishes a kind of thermochemical "sea level," or reference point, from which all enthalpy changes are measured. 

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