Note 73 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE

This is in agreement with the thermochemical data, because if all the substances be taken in the gaseous state (for sulphur the heat of fusion is 0·3, and the heat of vaporisation 2·3) we have H2 + S = 4·7; H2 + Cl2 = 44; H2 + Br2 = 24, and H2 + I2 = -3 thousand heat units; hence the formation of H2S gives less heat than that of HCl and HBr, but more than that of HI. In dilute solutions H2 + S + Aq = 9·3, and consequently less than the formation of all the halogen acids, as H2S evolves but little heat with water, and therefore in dilute solutions chlorine, bromine, and iodine decompose hydrogen sulphide.