Atomisation enthalpies

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 ... 

Evidently the most - practically the only - stable oxidation state of La in ionic compounds is III. Does this hold for the later members of the lanthanide series Fig. 5.1 suggests that the I oxidation state has little prospect of stability, given the high atomisation enthalpies and the relatively low second and third ionisation energies. The II oxidation state has better prospects, however. Consider the disproportionation ... 

Thus the higher I3 of Eu is mainly responsible for the limited stability of its II oxidation state, although the lower atomisation enthalpy helps as well. As shown on Fig. 5.2, the third ionisation energy follows the sequence ... 

The ionic model is of limited applicability for the heavier transition series (4d and 5d). Halides and oxides in the lower oxidation states tend to disproportionate, chiefly because of the very high atomisation enthalpies of the elemental substances. Many of the lower halides turn out to be cluster compounds, containing metal-metal bonds (see Section 8.5). However, the ionic model does help to rationalise the tendency for high oxidation states to dominate in the 4d and 5d series. As an example, we look at the fluorides MF3 and MF4 of the triad Ti, Zr and Hf. As might be expected, the reaction between fluorine gas and the elemental substances leads to the formation of the tetrafluorides MF4. We now investigate the stabilities of the trifluorides MF3 with respect to the disproportionation ... 

These points are well illustrated by comparing Cu, Ag and Au with respect to the relative stabilities of their oxidation states. Although few compounds formed by these elements can properly be described as ionic, the model can quite successfully rationalise the basic facts. The copper Group 1 Id is perhaps the untidiest in the Periodic Table. For Cu, II is the most common oxidation state Cu(I) compounds are quite numerous but have some tendency towards oxidation or disproportionation, and Cu(III) compounds are rare, being easily reduced. With silver, I is the dominant oxidation state the II oxidation state tends to disproportionate to I and III. For gold, III is the dominant state I tends to disproportionate and II is very rare. No clear trend can be discerned. The relevant quantities are the ionization energies Iu l2 and A the atomisation enthalpies of the metallic substances and the relative sizes of the atoms and their cations. These are collected below / and the atomisation enthalpies AH%tom are in kJ mol-1 and r, the metallic radii, are in pm. 

Since AH° is not very different from AG°, the entropy term is evidently quite small, and we are justified in concentrating on the enthalpy terms in our analysis. The atomisation enthalpy of the elemental substance, the relevant ionisation energies of the gaseous atomic substance and the hydration enthalpy of the cation are obviously the quantities to be compared when looking at different species. The last three steps in the analysis above amount to —439n kJ mol-1. 

To summarise E° values for redox couples of the type M(s)/M"+(aq) can largely be rationalised in terms of the atomisation enthalpies, ionisation energies and hydration enthalpies. The entropy terms can be neglected in most cases. 

Remembering that we want bond energies which, when added together, reproduce the experimental atomisation enthalpy of the molecular substance, 416 rather than 439 kJ mol-1 is appropriate for the C-H bonds in methane. However, some caution must be exercised in transferring this value to other situations. Consider, for example, CH3C1. From the atomisation enthalpy of CCl4(g), the mean C-Cl bond energy is found to... 

A better C-C bond energy is obtained from the atomisation enthalpies... 

Table 6.2 Mean bond energies from atomisation enthalpies at 25 °C of gaseous molecular substances, in kJ molRanges of values are not given in cases where the relevant data are available only for one or two molecules...

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)... 

The d block elemental substances have relatively high atomisation enthalpies. It is a fair approximation to regard these as having a narrow but dense d-band in the middle of a much broader, but less dense, s-band, as shown schematically below ... 

The atomisation enthalpies of the lanthanides as metallic elemental substances exhibit very different trends. From La to Eu, we see a steady decrease, followed by an abrupt increase at Gd. The atomisation enthalpies then decrease (not quite monotonically) to Yb, then increase at Lu. These trends may be rationalised as follows. According to magnetic studies, the lanthanide atoms in the elemental substances have the electronic configurations 6s25d14f" Eu and Yb are exceptions, discussed further below. The band structure is evidently complex and will not be described in detail. The atomisation enthalpy can be broken down for thermochemical purposes into two steps ... 

The Hg-C bond is very weak, partly on account of the large size of the Hg atom and the low polarity of the bond (the mercury dialkyls are molecular substances, with linear C—Hg-C skeletons). Despite the low atomisation enthalpy of mercury compared with other metallic elemental substances, reactions of this kind are usually exothermic. The mercury dialkyls are relatively insensitive to air and moisture and are therefore convenient to store and use. They are prepared from Grignard reagents ... 

---