for 25.0°C. To indicate that the energy change is meant to imply standard con-
ditions, a ° superscript is attached to the symbol. We therefore speak ofrxnH°,
rxnU°, etc. Temperatures are usually specified as well.
Although we have defined rxnHfor a chemical process, values ofrxnHare
not determined by evaluating the difference HfHi.This is because absolute val-
ues for enthalpy cannot be determined. Only relative values,changesin enthalpy,
can be measured. What we need is a set of chemical reactions whose rxnHval-
ues can serve as standards against which all other rxnHvalues can be measured.
The method for determining rxnHfor chemical processes is based on the
ideas of the chemist Germain Henri Hess (1802–1850), who was born in
Switzerland but spent most of his life in Russia. Hess can be considered the
founder of the subtopic of thermodynamics called thermochemistry. Hess stud-
ied the energy changes (in terms of heat, mostly) of chemical reactions.
Ultimately, he realized that several key ideas are important in studying the
energy changes that accompany chemical reactions. In a modern form (Hess
lived before the field of thermodynamics was fully established), they are:- Specific chemical changes are accompanied by a characteristic change in
energy. - New chemical changes can be devised by combining known chemical
changes. This is done algebraically. - The change in energy of the combined chemical reaction is the equiva-
lent algebraic combination of the energy changes of the component
chemical reaction.
The above ideas are collectively known as Hess’s lawand are the fundamen-
tal basis of thermodynamics as applied to chemical reactions. Because we are
treating chemical equations algebraically, we need to keep the following two
thoughts in mind as we combine their energy changes algebraically. - When a reaction is reversed, the energy change of the reaction reverses
sign. This is a consequence of enthalpy being a state function. - When multiples of a reaction are considered, the same multiple of the
energy change must be used. This applies to fractional as well as whole-
number multiples. This is a consequence of enthalpy being an extensive
property.
Hess’s law means that we can take the measured changes in energy for re-
actions and combine them in whatever way we need, and the change in energy
for the overall reaction is just some algebraic sum of the known energies.
Measured energy changes for chemical reactions can be tabulated, and for the
appropriate combination of chemical reactions, we need only consult the ta-
bles and perform the proper algebra. Hess’s law is a direct consequence of en-
thalpy being a state function.
The question now is, what reactions should we tabulate? There is an inex-
haustible supply of possible chemical reactions. Do we tabulate the energy
changes of all of them? Or of only a selected few? And which few?
Enthalpy changes of only one kind of chemical reaction need to be tabulated
(although it is not uncommon to see tables of enthalpy changes of other reac-
tions, like combustion reactions). A formation reactionis any reaction making
1 mole of a product using, as reactants, the product’s constituent elements in
their standard states.* We use the symbol fHto stand for the enthalpy change
54 CHAPTER 2 The First Law of Thermodynamics
*The standard state of an element is the pure substance at 1 bar (previously, 1 atm) and
having the specified allotropic form, if necessary. Although there is no specified standard
temperature, many references use 25.0°C as the designated temperature.