Introduction

The study of the energetics of chemical reactions introduces the topics of thermodynamics and thermochemistry. Thermodynamics is a study of matter in bulk that is concerned with energy changes in systems, chemical or mechanical, whereas thermochemistry is the study of heat changes in chemical reactions and is, thus, a branch of thermodynamics. Thermodynamics allows us to predict whether or not a reaction, say A→B, will occur spontaneously, that is, with a decrease in the energy of the system. It does not indicate the speed of the reaction, or even whether it will take place at all without the assistance of heat, irradiation or a catalyst; for this information we shall study reaction kinetics in a later chapter.

General laws of thermodynamics

Thermodynamics rests upon three fundamental principles, the laws of thermodynamics. The laws apply, in the most general sense, to the universe as a whole, and may be given in the following form.

The energy of the universe is constant, whatever processes take place in it.

The entropy of the universe increases, whatever processes take place in it, and must, therefore, tend towards a maximum value.

The absolute scale of temperature has a minimum value, absolute zero, and the entropy of any perfect, crystalline substance is zero at that temperature.

The development of the subject is based on these laws and we shall investigate how they are used in understanding reactions in chemistry.