The Foundations of Chemistry

If the substance exists in several
different forms, we take the form that
is most stable at 25°C and 1 atm as the
standard state.

602 CHAPTER 15: Chemical Thermodynamics

Hg(
); sodium, a silvery white solid metal, Na(s); and carbon, a grayish black solid called

graphite, C(graphite). We use C(graphite) instead of C(s) to distinguish it from other solid

forms of carbon, such as C(diamond). The reaction C(diamond) nC(graphite) would be

exothermicby 1.897 kJ/mol rxn; C(graphite) is thus more stable than C(diamond). Exam-

ples of standard states of compounds include ethanol (ethyl alcohol or grain alcohol), a

liquid, C 2 H 5 OH(
); water, a liquid, H 2 O(
); calcium carbonate, a solid, CaCO 3 (s); and

carbon dioxide, a gas, CO 2 (g). Keep in mind the following conventions for thermochem-

ical standard states.

1.For a puresubstance in the liquid or solid phase, the standard state is the pure

liquid or solid.

2.For a gas, the standard state is the gas at a pressure of one atmosphere;in a mixture

of gases, its partial pressure must be one atmosphere.

3.For a substance in solution, the standard state refers to one-molarconcentration.

For ease of comparison and tabulation, we often refer to thermochemical or thermo-

dynamic changes “at standard states” or, more simply, to a standard change.To indicate a

change at standard pressure, we add a superscript zero. If some temperature other than

standard temperature of 25°C (298 K) is specified, we indicate it with a subscript; if no

subscript appears, a temperature of 25°C (298 K) is implied.

The standard enthalpy change, H^0 rxn,for reaction

reactants88nproducts

refers to the 
Hwhen the specified number of moles of reactants, all at standard

states, are converted completelyto the specified number of moles of products, all at

standard states.

We allow a reaction to take place, with changes in temperature or pressure if necessary;

when the reaction is complete, we return the products to the same conditions of temper-

ature and pressure that we started with, keeping track of energy or enthalpy changesas we do

so. When we describe a process as taking place “at constant Tand P,” we mean that the

initial and final conditions are the same. Because we are dealing with changes in state

functions, the net change is the same as the change we would have obtained hypotheti-

cally with Tand Pactually held constant.

STANDARD MOLAR ENTHALPIES OF FORMATION, Hf^0

It is not possible to determine the total enthalpy content of a substance on an absolute

scale. We need to describe only changesin this state function, however, so we can define

an arbitrary scaleas follows.

The standard molar enthalpy of formation, H^0 f,of a substance is the enthalpy

change for the reaction in which one moleof the substance in a specified state is

formed from its elements in their standard states. By convention, the 
Hf^0 value for

any element in its standard stateis zero.

See the Saunders Interactive 15-7
General Chemistry CD-ROM,
Screen 6.17, Standard Enthalpy of
Formation.

We can think of
H^0 fas the enthalpy
content of each substance, in its
standard state, relative to the enthalpy
content of the elements, in their
standard states. This is why
H^0 ffor an
element in its standard state is zero.

This is sometimes referred to as the
standard heat of reaction.