Page 89 - Modeling of Chemical Kinetics and Reactor Design

Thermodynamics of Chemical Reactions 59

CHAPTER TWO

Thermodynamics

of Chemical Reations

INTRODUCTION

The two main principles involved in establishing conditions for
performing a reaction are chemical kinetics and thermodynamics.
Chemical kinetics is the study of rate and mechanism by which one
chemical species is converted to another. The rate is the mass in
moles of a product produced or reactant consumed per unit time. The
mechanism is the sequence of individual chemical reaction whose
overall result yields the observed reaction. Thermodynamics is a
fundamental of engineering having many applications to chemical
reactor design.
Some chemical reactions are reversible and, no matter how fast a
reaction takes place, it cannot proceed beyond the point of chemical
equilibrium in the reaction mixture at the specified temperature and
pressure. Thus, for any given conditions, the principle of chemical
equilibrium expressed as the equilibrium constant, K, determines how
far the reaction can proceed if adequate time is allowed for equilibrium
to be attained. Alternatively, the principle of chemical kinetics deter-
mines at what rate the reaction will proceed towards attaining the
maximum. If the equilibrium constant K is very large, for all practical
purposes the reaction is irreversible. In the case where a reaction is
irreversible, it is unnecessary to calculate the equilibrium constant and
check the position of equilibrium when high conversions are needed.
Both the principles of chemical reaction kinetics and thermodynamic
equilibrium are considered in choosing process conditions. Any com-
plete rate equation for a reversible reaction involves the equilibrium
constant, but quite often, complete rate equations are not readily
available to the engineer. Thus, the engineer first must determine the
temperature range in which the chemical reaction will proceed at a

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