Concise Physical Chemistry

c04 JWBS043-Rogers September 13, 2010 11:24 Printer Name: Yet to Come

62 THERMOCHEMISTRY

database with a variety of entries (webbook.nist.gov) and why the most meticulous

care is exercised to be sure that the data entered are accurate.

All of the previous examples are consistent with the equations

rU^298 =

∑

fU^298 (products)−

∑

fU^298 (reactants)

and

rH^298 =

∑

fH^298 (products)−

∑

fH^298 (reactants)

where it is to be understood that products and reactants are multiplied by their

stoichiometric coefficients and that they are in their standard states. We shall soon

see that all thermodynamic properties follow analogous equations, which is why

thermodynamics is so useful.

Although combustion thermochemistry on C(gr), CH 4 (methane), and so on, is the

prime source of
fH^298 data, other reactions can contribute as well. For example,

suppose we know that
fH^298 (ethane)=− 83. 8 ± 0 .4kJmol−^1 but we don’t know


fH^298 (ethene). Measurement of the enthalpy of the hydrogenation reaction ethene

→ethane gives− 136. 3 ± 0 .2kJmol−^1 ; hence
fH^298 (ethene) must be higher in

enthalpy than
fH^298 (ethane) by just that amount. By this reasoning,

fH^298 (ethene)=− 83. 8 ± 0. 4 + 136. 3 ± 0. 2 = 52. 5 ± 0 .4kJmol−^1

The tabulated value is 52. 5 ± 0 .4kJmol−^1.

Molar enthalpies of physical processes like phase changes (vaporization and melt-

ing), solution of a solute in a solvent, mixing of miscible solvents, and dilution are

treated by slight modifications of the methods shown, always in accord with the first

law of thermodynamics, which has never been violated in a controlled, reproducible

experiment.

4.6 GROUP ADDITIVITY

Upon scanning a series of heats of combustion
cH^298 , one notices a regular increase

with molecular weight. For example, for the alkanes in their standard states (g), we

have the following:

Methane Ethane Propane n-Butane

cH^298 (g) −890.8 −1560.7 −2219.2 −2877.6

Difference −669.9 −658.5 −658.4