chem1 virtual textbook

a reference text for General Chemistry

Stephen Lower

Simon Fraser University

 

 

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Introduction: why this stuff is important

The greater part of what we call chemistry is concerned with the different kinds of reactions that substances can undergo. The statement that “hydrogen fluoride is a stable molecule” is really a way of saying that the reaction HF → ½ H2 + ½ F2 has a negligible tendency to occur in the forward direction and an overwhelming tendency to occur in the reverse direction. More generally, we can predict how the composition of an arbitrary mixture of H2, F2, and HF will tend to change by comparing the values of the equilibrium constant K and the equilibrium quotient Q; in your study of equilibrium, you will recall that if Q/K > 1, the reaction will proceed to the left, whereas if Q/K < 1 it will proceed to the right. In either case, the system will undergo a change in composition until it reaches the equilibrium state where Q = K.

Clearly, the value of K is the crucial quantity that characterizes a chemical reaction, but what factors govern its value? In particular, is there any way that we can predict the value of the equilibrium constant of a reaction solely from information about the products and reactants themselves, without any knowledge at all about the mechanism or other details of the reaction? The answer is yes, and this turns out to be the central purpose of chemical thermodynamics:

The purpose of thermodynamics is to predict the equilibrium composition
of a system from the properties of its components.

Don’t let the significance of this pass you by; it means that we can say with complete certainty whether or not a given change is possible, and if it is possible, to what extent it will occur— without the need to study the particular reaction in question. To a large extent, this is what makes chemistry a science, rather than a mere cataloging of facts.

Some related links:

I am indebted to Frank Lambert's patient cajoling for setting me on the right path, and to Leonard Nash's 1971 book Chem-Thermo: A Statistical Approach to Classical Chemical Thermodynamics, which sustained me on the way.