THERMOCHEMISTRY is the name given to that branch of theoretical chemistry which seeks to trace the connection be tween the heat evolved or absorbed during a chemical reaction and the nature and course of the reaction. Chemical reactions which are accompanied by a great evolution of heat are familiar ; the combustion of coal or gas, the reduction of iron ores by coke in the blast furnace, and the slaking of lime are common examples. All explosives are unstable compounds or mixtures of compounds, the gaseous reaction products of which are raised to a very high temperature by the great heat evolved by the explosion ; the pro pulsive force is due to the great pressure exerted by the gaseous products, owing to the high temperature and to the small volume they occupy before expansion. Such reactions are familiar because their effects are so obvious. They take place so rapidly that the heat evolved cannot be dissipated without raising the products of the reaction to a high temperature. It is the effects of the high temperature that are noticed, rather than the fact that large amounts of heat are evolved. In the rusting of iron we have an example of a chemical reaction which is also accompanied by the evolution of much heat ; but this fact escapes ordinary notice since the rusting usually takes place so slowly that the heat has time to dissipate without perceptibly raising the temperature of the metal. If, however, finely divided iron filings are dropped into pure oxygen, the reaction takes place so suddenly that there is no time for the heat to get away before the particles get white hot.
Confronted with so many every-day examples of chemical re actions accompanied by evolution of heat, it is natural to assume a close connection between the energy changes and the material changes. For a long time, however, the interest of chemists was mainly occupied by material changes only. Following the general recognition of the law of conservation of matter and the gradual acceptance of Dalton's atomic theory in the early part of the 19th century, there was a great development of knowledge of the properties and composition of different chemical substances and of their action on each other. The real development of modern thermochemistry may be said to start from the recognition of the law of conservation of energy in the middle of the century. Some
what earlier Thomas Andrews and Hess had systematically studied the thermal effects of chemical reactions taking place in solution, and Hess, as a result of his work, had formulated a law which is one of the consequences of the conservation of energy, namely, that the thermal effect of a chemical reaction is the same however it takes place.
(a) Heat is necessary to convert water at its boiling point into steam at the same temperature. A small part of this heat goes to perform work through expansion against atmospheric pressure; the major part is transferred into internal energy of steam mole cules. The internal energy of unit mass of water vapour is there fore considerably greater than that of unit mass of liquid water at the same temperature. When steam is condensed again to water, exactly the same amount of heat is evolved as was absorbed when it was formed.
(b) A chemical reaction, such as the combustion of petrol, can be made to take place without performance of useful work, e.g., by allowing it to take place in a closed vessel. The heat evolved by the combustion of unit weight to carbon dioxide and water vapour under these conditions can be accurately measured. When petrol is burnt in an internal combustion engine, the power out put (and therefore the useful work performed by combustion of unit weight), the loss of heat to the cylinder walls and the residual heat in the exhaust gases can all be measured. The sum of the heat loss to the cylinder walls, and the residual heat in the ex haust gas is always less than the total heat of combustion by an amount which is the equivalent of the work done.