Page 151 - Modern physical chemistry

7

Relationships among

Reactants

Z 1 Introduction

A CHEMICAL REACTION GENERALLY INVOLVES rearrangement of the electron
structures about atomic cores. Bonds between atoms may be broken and reformed. One
or more elementary processes may be involved.
Traditionally, a reaction is studied in a macroscopic system. A state of a homogeneous
region of the system is characterized by the intensive variables ( a) pressure P, (b) tem-
perature T, and (c) activity a j of eachj constituent. As the reaction proceeds, these vary.
Furthermore, the region would possess the extensive properties volume V, an internal
energy E, an enthalpy H, an entropy S, a Gibbs free energy G, a Helmholtz free energy A.
The pressure P, temperature T, volume V, and activities all ... , an may be determined
in the conventional ways. With energy capacity measurements from a calorimeter and the
third law, the entropies of pure substances can be determined. The heat q for a reaction
at a given temperature and pressure or volume can be determined with a calorimeter.
Thus, the change in G for reactants under certain initial conditions going to products
under certain final conditions can be determined. And the equilibrium constant for the
reaction can be related to calorimetric measurements.
Alternatively, molecular parameters can be found from spectroscopic measurements.
From these, entropies can be calculated for the gas phase and the calorimetric entropies
checked. Furthermore, equilibrium constants for the gas phase can be calculated. Since
statistical mechanics is employed here, these calculations will not be developed at this
stage of our studies.

Z2 Conventions

Thermodynamic data are customarily reported for specified standard states. For a
liquid or solid substance, the standard state is the pure material at 1 bar pressure and
the given temperature. For a gaseous substance, the standard state is the pure material
in its hypothetical ideal gas condition at 1 bar pressure and the given temperature.
For a solvent, the standard state is the material at unit mole fraction under a pres-
sure of 1 bar at the given temperature. For a solute, the standard state is the hypotheti-
cal ideal solution at unit molality (unit activity) under a pressure of 1 bar and the given
temperature.

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