Phosphorus standard state

Solid phosphorus is found in two forms white phosphorus (P4) and red phosphorus (P). White phosphorus is the standard state. 

The elements nitrogen and phosphorus of Group 15 have standard states composed of the discrete molecular species N7 and P4. 

Four allotropes of phosphorus are known, the hexagonal /(-white, stable only below —77°C, the cubic a-white trap 44.1°C), the violet, and the black (which is thermodynamically the most stable). The a-white form is usually taken as the standard state. The violet is obtained by continued heating at 500°C of a solution of phosphorus in lead. When a-white phosphorus is heated to 250 C in the absence of air, a red variety (rap 590CC) is obtained which is believed to consist of a mixture of the a-white and violet allotropes, although the studies of the violet component in the mixture have shown that at least four polymorphic forms of red (violet) phosphorus exist. 

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... 

There is one exception to this choice of standard state. The standard state of phosphorus is taken to he white phosphorus, rather than the more stable red or black form, because the latter are less well characterized and less reproducible in their properties. 

From the data in Appendix K, determine the fonn that represents the standard state for each of the following elements (a) oxygen (b) carbon (c) phosphorus (d) rubidium (e) calcium. 

Phosphorus exhibits complicated allotropy eleven forms have been reported, of which at least five are crystalline. Crystalline white phosphorus contains tetrahedral P4 molecules (Figure 14.3 a) in which the P—P distances (221 pm) are consistent with single bonds (r ov = 110 pm). White phosphorus is defined as the standard state of the element, but is actually... 

The thermodynamic measurements for nickel phosphides were recently summarised by Schlesinger [2002SCH] (in that reference, red phosphorus (V) was used as the standard state for phosphorus). Schlesinger concluded that the details of the Ni-P phase diagram remain controversial, and that the available data are insufficient to allow calculation of assessed thermodynamic values for any nickel phosphides. No values are selected in the present review. 

The standard state of zero Gibbs energy for phosphorus is solid white phosphorus, P4(s). At 25 °C,... 

Although outdated, this book still provides useful references to the older thermochemical literature. Tabulated are A.H and enthalpy of transition values for the elements and their compounds, with the data for carbon-containing compounds being terminated at two carbon atoms. It should be noted that the data pertain to a temperature of 18 C and to diamond, rather than graphite, as the standard state for carbon the yellow form is the reference state for phosphorus. The yellow form is thermochemically identical to the white form which is the reference state used in the NBS Thermochemical Tables [149]. The data upon which this book was based were used in preparing NBS Circular 500,.see item [131]. 

In the case of phosphorus, the most common form, white phosphorus (P4), is chosen as the standard state, even though red phosphorus is more stable at 1 atm and 298 K. 

A The main reason for the difference is that the boron atom is electron deficient, and forms structures based u]x>n icosahedral Bp clusters which allow each boron atom to have a share of more electrons. Multiple bonding in phosphorus is weak, and the element exerts its valency of three in the P tetrahedral clusters which form the standard state. 

The standard states are white phosphorous (cubic, P ) and hexagonal arsenic, antimony, and bismuth. The enthalpy of black phosphorus is —43 kJ mole". ... 

The standard state is generally the most thermodynamically stable form of the pure element that exists under standard thermodynamic conditions. For carbon it is graphite and for phosphorus it is white phosphorus, P4(s). (However, red phosphorus is more stable than white phosphorus.)... 

The standard state of phosphorus at 25 C is P4. This molecule has four equivalent P atoms, no double or triple bonds, and no expanded octets. Draw its Lewis structure. 

For radiocarbon, the standard ratio s is provided by the preindustrial atmosphere, for which 8 = 0. Cosmic rays interacting with atmospheric nitrogen were the main source of preindustrial radiocarbon. In the steady state, this source drsource is just large enough to generate an atmospheric delta value equal to zero. The source appears in equation 9 for atmospheric radiocarbon. Its value, specified in subroutine SPECS, I adjust to yield a steady-state atmospheric delta value of 0. The source balances the decay of radiocarbon in the atmosphere and in all of the oceanic reservoirs. Because radiocarbon has an overall source and sink—unlike the phosphorus, total carbon, 13C, and alkalinity in this simulation—the steady-state values of radiocarbon do not depend on the initial values. 

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