Enthalpy of elements

The atomisation enthalpy of elemental sodium Afl%tom, the first ionisation energy of atomic sodium Iu the dissociation enthalpy D of gaseous chlorine, the electron attachment energy Ex of atomic chlorine and the enthalpy of formation A//)1 of crystalline sodium chloride can all be taken from standard tabulations of experimental data. An experimental lattice energy UL is thus given by ... 

A satisfactory theory of metallic bonding must account for the characteristic properties of high electrical and thermal conductivity, metallic lustre, ductility and the complex magnetic properties of metals which imply the presence of unpaired electrons. The theory should also rationalise the enthalpies of atomisation A/f tom of metallic elemental substances. A/f tom is a measure of the cohesive energy within the solid, and we saw in Chapter 5 how it plays an important part in the thermochemistry of ions in solids and solutions. The atomisation enthalpies of elemental substances (metallic and nonmetallic) are collected in Table 7.1. There is a fair correlation between A/Z tom an(J physical properties such as hardness and melting/boiling points. 

Table 7.1 Atomisation enthalpies of elemental substances (kJ mol 25 °C), or enthalpies of formation of gaseous monatomic substances E(g)... 

Studies related to entropy changes revealed that reduction in temperature leads to decrease in entropy change for all processes. It was therefore, postulated that for a process occurring at absolute zero temperature the entropy change would be zero. This has led to a basis from which absolute values of entropy can be determined, taking entropy at absolute zero of temperature to be zero. Thus, unlike i/and F whose changes can be accurately measured but not the absolute value, the absolute value of entropy can indeed be measured. We take, for calculation purposes, enthalpy of elements in a defined standard state to be zero, but that assumption is only for convenience, no molecule or atom can have zero heat content at ambient conditions. On the other hand, a fully ordered (crystalline) solid at absolute zero temperature will have zero entropy. 

Because element 114 behaves to some extent like a closed-shell atom, and element 115 as if a new group of elements had been started, we give in Fig. 18 the results of Eichler (77) from the extrapolation of the standard enthalpy of elements 112 to 117. The relatively small value for 114 and the relatively large value for 115 may be taken as another indication that the interpretation given here, arising from the atomic calculations with the relativistic effects, will be important as regards many physical quantities as well as in the chemical interpretation of these elements. 

Calorimetric Functions of State in Chemical Processes 3.9.1 Standard Enthalpies of Elements... 

In this work the following convention is employed the standard enthalpies of elements in their reference phases at p = 1 bar and T = 298.15 K are set to Zero,... 

The standard enthalpy of formation is zero for elements in reference phases. Convention employed in this work the enthalpy of elements in their reference phases at 298.15 K and 1 bar is zero. Standard entropy of the substance at 7= 298.15 K (p= 1 bar). 

The energy of a substance under constant atmospheric pressure (such as in an open test tube or beaker) is called its enthalpy (symbol, H). The enthalpy of elements and compounds cannot be measured or calculated, but differences in enthalpy (enthalpy changes), symbolised AH, are easily measured in the laboratory. If a chemical or physical change is carried out at constant pressure, the amount of heat energy absorbed or produced in the change equals the change in enthalpy that has taken place. 

Free enthalpy characterizes maximum useful work, which is performed between atoms and molecules on the account of potential energy of only electrons. In this coimection it may be assumed that the free enthalpy of elements per se is equal to zero. Then the values of median (molar) free enthalpy and of any compounds may be characterized as energy of its formation in the amoimt of 1 mole. In order to compare free enthalpy of the formation of different compounds, it is necessary to measure the energy of their formation from individual elements under some identical conditions. 

The enthalpies of elements and compounds at temperature T are calculated by the equation ... 

The enthalpy H of a liquidus alloy consisting of A and B is not equal to the sum of (enthalpies of elements A and B, respectively). During the formation of a liquid, mixing of different atoms leads to energy absorption or release (mixing enthalpy, If it is assumed... 

Once again, this reaction reads 1 mol of carbon and 1 mol of oxygen react to form 1 mol of carbon dioxide. The enthalpies of reactants and products are printed above and below, respectively, the symbols in the reaction, and the enthalpy change for the reaction, A/, placed above the arrow. Note that the enthalpies of elements in their standard states (pure, 1 atm) at 25°C have arbitrarily been set to zero. The enthalpy change accompanying this reaction is the standard heat of formation and is given by... 

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