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CHAPTER
5 Standard

Thermodynamic

CHAPTER OUTLINE
Functions of
5.1 Standard States of Pure
Substances
5.2 Standard Enthalpy of Reaction
Reaction
5.3 Standard Enthalpy of
Formation

5.4 Determination of Standard
Enthalpies of Formation For the chemical reaction aA bB ∆ cC dD, we found the condition for reaction
and Reaction equilibrium to be am bm cm dm [Eq. (4.98)]. To effectively apply this con-
A
B
D
C
dition to reactions, we will need tables of thermodynamic properties (such as G, H,
5.5 Temperature Dependence and S) for individual substances. The main topic of this chapter is how one uses ex-
of Reaction Heats perimental data to construct such tables. In these tables, the properties are for sub-
stances in a certain state called the standard state, so this chapter begins by defining
5.6 Use of a Spreadsheet to the standard state (Sec. 5.1). From tables of standard-state thermodynamic properties,
Obtain a Polynomial Fit one can calculate the changes in standard-state enthalpy, entropy, and Gibbs energy for
chemical reactions. Chapters 6 and 11 show how equilibrium constants for reactions
5.7 Conventional Entropies can be calculated from such standard-state property changes.
and the Third Law
5.8 Standard Gibbs Energy 5.1 STANDARD STATES OF PURE SUBSTANCES
of Reaction
The standard state of a pure substance is defined as follows. For a pure solid or a pure
5.9 Thermodynamics Tables liquid, the standard state is defined as the state with pressure P 1 bar [Eq. (1.11)]
and temperature T, where T is some temperature of interest. Thus for each value of T
5.10 Estimation of there is a single standard state for a pure substance. The symbol for a standard state is
Thermodynamic Properties
a degree superscript (read as “naught,” “zero,” or “standard”), with the temperature
5.11 The Unattainability of written as a subscript. For example, the molar volume of a pure solid or liquid at 1 bar
Absolute Zero and 200 K is symbolized by V° m,200 , where the degree superscript indicates the standard
pressure of 1 bar and 200 stands for 200 K. For a pure gas, the standard state at tem-
5.12 Summary perature T is chosen as the state where P 1 bar and the gas behaves as an ideal gas.
Since real gases do not behave ideally at 1 bar, the standard state of a pure gas is a fic-
titious state. Calculation of properties of the gas in the fictitious standard state from
properties of the real gas is discussed in Sec. 5.4. Summarizing, the standard states for
pure substances are:
Solid or liquid: P 1 bar, T
(5.1)*
Gas: P 1 bar, T, gas ideal
The standard-state pressure is denoted by P°:
P° 1 bar (5.2)*
Standard states for components of solutions are discussed in Chapters 9 and 10.