Concise Physical Chemistry

c19 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come

STRUCTURE 307

Elements

Gaseous Atoms

Compounds

-1153

-1174

-21

FIGURE 19.2 Bond enthalpies calculated in CH 2 , from the reference state of gaseous atoms

(top), and relative to elements in their standard state (H 2 (g) and C(graphite)). Not to scale.

standard state of the elements in their most stable form at 298 K. To translate from one

reference state to the other, we need to know their enthalpy differences. These values

have been measured with care (and some difficulty). They are
atomizationH^298 (H 2 )=

436 kJ mol−^1 and
atomizationH^298 (Cgr)=717 kJ mol−^1. These larger enthalpies must

be subtracted from the bond enthalpies relative to the atoms to find – 1174 – (–1153)

=–21 kJ mol−^1 , the sum of bond enthalpies in CH 2 relative to the thermodynamic

standard state of H 2 (g) and C(graphite) (Fig. 19.2).

This result, relative to a new standard state, is the same as the one used with

success in Section 19.1. The rationale for what seems to be a rather exotic standard

state will be clearer in the next chapter. It is also important to remember that without

correction factors for molecular deformations of the real molecule relative to the

additive model we have postulated here, either summing appropriate bond enthalpies

or summing simple group enthalpies gives astrainlessmolecule.

19.3 STRUCTURE

Given that the hydrogen atoms in methane repel one another, the only reasonable

structure we can assign to methane is that of a tetrahedron in which four bound H

atoms achieve maximum separation in 3-space about the central C atom:

Using this simple structural symmetry and an arbitrary bond length (the length

of a wooden peg, perhaps), we can construct a unique “ball-and-stick” model of

methane and, by extension, very many models of alkanes (Fig. 19.3). By the nature