ENERGY AND CHEMICAL CHANGE 159 



think of all our materials as repositories of energy as well as of 

 matter, each of these two constituents being equally real and 

 equally important. A piece of the substance known as " iron " 

 must thus be held to contain so much iron matter and so much 

 internal energy. So ferrous sulphide contains sulphur matter, 

 iron matter, and internal energy. Thus, by a substance we mean 

 a distinct species of matter, simple or compound, with its appro- 

 priate proportion of internal energy. During the progress of a 

 chemical change, like the union of iron and sulphur, the internal 

 energy of the system also changes. The total energy which can 

 thus be made available as the result of a chemical action, and 

 converted (through, say, heat or electrical energy) into work, is 

 called the free energy of the reaction. 



In the course of this discussion it has become clear that it is 

 characteristic of chemical phenomena that, besides a change in the 

 nature of the matter, there is always an alteration in the amount 

 of internal energy in the system. This alteration involves the 

 production of internal energy from, or the transformation of 

 internal energy into some other form of energy. 



Energy and Chemical Activity. Other things being equal, 

 when the free energy of a reaction is large, the reaction proceeds 

 rapidly; that is to say, a large proportion of the reacting materials 

 are changed in the unit of time. Those reactions in which the 

 change of internal energy is small proceed more slowly. The 

 speed of a chemical change, and the quantity of energy available 

 because of it, are therefore closely related. Now, we are ac- 

 customed to speak of materials which, like iron and sulphur, 

 interact rapidly and with liberation of much energy as " chemic- 

 ally active," or as possessing great " chemical affinity " for one 

 another. Thus, relative chemical activity or affinity may be 

 estimated, (1) by observing the speed of a change or, in many 

 cases (2) by measuring the heat developed or (3) by ascertaining 

 the electromotive force of the current, when the materials are 



