Enthalpy of atomisation

A/i the dissociation or bond energy of hydrogen (it is also, by definition, twice the enthalpy of atomisation two gram atoms being produced). 

A/i, the enthalpy of atomisation of chlorine, which is also half the bond dissociation enthalpy. 

Ah second ionisation energy for sodium (additional) +4561 A/13 enthalpy of atomisation of chlorine, x 2 (since two... 

Table 11.1 and Table 11.2 (p. 314) give some of the physical properties of the common halogens. Figure 11.1 shows graphically some of the properties given in Table 11.1, together with enthalpies of atomisation. 

In the case of solids, the enthalpy of atomisation includes the enthalpies of fusion and vaporisation in the case of liquids, it includes the enthalpy of vaporisation. 

Compare this definition with that for the standard enthalpy of atomisation of an element. 

The number of P-H bonds on the right-hand side of the equation is the same as on the left-hand side, and unless there is good reason to suppose that the bonds in PH2 are significantly different from those in PH3, we can safely assume that the P-H bonding is unaffected by the disproportionation. But there are bonds in elemental phosphorus, whose enthalpy of atomisation is 315 kJ mol-1, and this will tend to drive the disproportionation of PH2. 

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. 

Consider now a metallic assembly of n Be atoms, using for the meanwhile only the 2s orbitals. As for Li(s), we obtain a band of n MOs but we now have In electrons, so that the band should be filled. Not only should this crystal have no metallic properties but it should also have zero cohesive energy since the occupancy of antibonding MOs should cancel out the effect of the occupied bonding MOs. But Be(s) does crystallise as a metallic solid, whose enthalpy of atomisation is twice as great as that of... 

Fig Standard enthalpies of atomisation for elements of the three series. 

IGOR/FES] is likely to be in error. The enthalpy of formation of SmSe(g) was ealcu-lated from the enthalpy of atomisation in [74NAG/SHI] (corrected to 298.15 K by adding 3 kJ mol , estimated by the review), the selected value of the enthalpy of formation of Se(g), and the enthalpy of formation of Sm(g) in [82WAG/EVA] yielding Af//° (SmSe, g, 298.15 K) = (112.2 + 15.6) kJ-moP. Since the accuracy of the estimated thermal data cannot be ascertained the review adopts a rounded value for the enthalpy of formation of SmSe(g) with increased error bars... 

Several evaluations of Af/7° (HCN, g, 298.15 K) from spectroscopic determinations of the enthalpy of atomisation of HCN(g) and from quantum mechanical ab initio calculations of enthalpies of reactions are available. Martin [96MAR] calculated Af//° (HCN, g, 298.15 K) = (128.24 2.54) kJ-moP from experimental spectroscopic data. From ab initio calculations of the enthalpy of atomisation of HCN(g) at 0 K and the enthalpy of the reaction 2HCN(g) (CN)2(g) + N2(g) he obtained with auxiliary... 

J Enthalpy of atomisation, AH is the enthalpy change for the production of one mole of atoms in the gas phase from the element in its standard state. It is always endothermic. 

This relates the enthalpy of formation of an Ionic solid to the enthalpies of atomisation of the elements concerned, the ionisation... 

The following data, in kj moT , should be used in this question. Enthalpies of atomisation calcium +193 chlorine +121. Ionisation energies for calcium 1st +590 2nd +1150 Electron affinity for Cl(g) -364... 

Note that the enthalpy of atomisation of chlorine is for the reaction 7jClj(g) - Cl(g). 

---